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[Emacs-diffs] Changes to emacs-lisp-intro.texi
|
From: |
Glenn Morris |
|
Subject: |
[Emacs-diffs] Changes to emacs-lisp-intro.texi |
|
Date: |
Thu, 06 Sep 2007 04:02:56 +0000 |
CVSROOT: /sources/emacs
Module name: emacs
Changes by: Glenn Morris <gm> 07/09/06 04:02:56
Index: emacs-lisp-intro.texi
===================================================================
RCS file: emacs-lisp-intro.texi
diff -N emacs-lisp-intro.texi
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ emacs-lisp-intro.texi 6 Sep 2007 04:02:55 -0000 1.1
@@ -0,0 +1,22721 @@
+\input texinfo @c -*-texinfo-*-
address@hidden %**start of header
address@hidden ../info/eintr
address@hidden setfilename emacs-lisp-intro.info
address@hidden sethtmlfilename emacs-lisp-intro.html
address@hidden Programming in Emacs Lisp
address@hidden vr cp
address@hidden fn cp
address@hidden odd
address@hidden
+
address@hidden ---------
address@hidden <<<< For hard copy printing, this file is now
address@hidden set for smallbook, which works for all sizes
address@hidden of paper, and with Postscript figures >>>>
address@hidden
address@hidden largebook
address@hidden print-postscript-figures
address@hidden set largebook
address@hidden clear print-postscript-figures
address@hidden ---------
+
address@hidden %**end of header
+
address@hidden edition-number 3.07
address@hidden update-date 9 November 2006
+
address@hidden
+ ## Summary of shell commands to create various output formats:
+
+ pushd /usr/local/src/emacs/lispintro/
+ ## pushd /u/intro/
+
+ ## Info output
+ makeinfo --paragraph-indent=0 --verbose emacs-lisp-intro.texi
+
+ ## ;; (progn (when (bufferp (get-buffer "*info*")) (kill-buffer
"*info*")) (info "/usr/local/src/emacs/info/eintr"))
+
+ ## DVI output
+ texi2dvi emacs-lisp-intro.texi
+
+ ## xdvi -margins 24pt -topmargin 4pt -offsets 24pt -geometry 760x1140 -s
5 -useTeXpages -mousemode 1 emacs-lisp-intro.dvi &
+
+ ## HTML output
+ makeinfo --html --no-split --verbose emacs-lisp-intro.texi
+
+ ## galeon emacs-lisp-intro.html
+
+ ## Plain text output
+ makeinfo --fill-column=70 --no-split --paragraph-indent=0 \
+ --verbose --no-headers --output=emacs-lisp-intro.txt emacs-lisp-intro.texi
+
+ popd
+
+# as user `root'
+# insert thumbdrive
+ mtusb # mount -v -t ext3 /dev/sda /mnt
+ cp -v /u/intro/emacs-lisp-intro.texi /mnt/backup/intro/emacs-lisp-intro.texi
+ umtusb # umount -v /mnt
+# remove thumbdrive
+
+ ## Other shell commands
+
+ pushd /usr/local/src/emacs/lispintro/
+ ## pushd /u/intro/
+
+ ## PDF
+ texi2dvi --pdf emacs-lisp-intro.texi
+ # xpdf emacs-lisp-intro.pdf &
+
+ ## DocBook -- note file extension
+ makeinfo --docbook --no-split --paragraph-indent=0 \
+ --verbose --output=emacs-lisp-intro.docbook emacs-lisp-intro.texi
+
+ ## XML with a Texinfo DTD -- note file extension
+ makeinfo --xml --no-split --paragraph-indent=0 \
+ --verbose --output=emacs-lisp-intro.texinfoxml emacs-lisp-intro.texi
+
+ ## PostScript (needs DVI)
+ # gv emacs-lisp-intro.ps &
+ # Create DVI if we lack it
+ # texi2dvi emacs-lisp-intro.texi
+ dvips emacs-lisp-intro.dvi -o emacs-lisp-intro.ps
+
+ ## RTF (needs HTML)
+ # Use OpenOffice to view RTF
+ # Create HTML if we lack it
+ # makeinfo --no-split --html emacs-lisp-intro.texi
+ /usr/local/src/html2rtf.pl emacs-lisp-intro.html
+
+ ## LaTeX (needs RTF)
+ /usr/bin/rtf2latex emacs-lisp-intro.rtf
+
+ popd
+
address@hidden ignore
+
address@hidden ================ Included Figures ================
+
address@hidden Set print-postscript-figures if you print PostScript figures.
address@hidden If you clear this, the ten figures will be printed as ASCII
diagrams.
address@hidden (This is not relevant to Info, since Info only handles ASCII.)
address@hidden Your site may require editing changes to print PostScript; in
this
address@hidden case, search for `print-postscript-figures' and make appropriate
changes.
+
address@hidden ================ How to Create an Info file ================
+
address@hidden If you have `makeinfo' installed, run the following command
+
address@hidden makeinfo emacs-lisp-intro.texi
+
address@hidden or, if you want a single, large Info file, and no paragraph
indents:
address@hidden makeinfo --no-split --paragraph-indent=0 --verbose
emacs-lisp-intro.texi
+
address@hidden After creating the Info file, edit your Info `dir' file, if the
address@hidden `dircategory' section below does not enable your system to
address@hidden install the manual automatically.
address@hidden (The `dir' file is often in the `/usr/local/share/info/'
directory.)
+
address@hidden ================ How to Create an HTML file ================
+
address@hidden To convert to HTML format
address@hidden makeinfo --html --no-split --verbose emacs-lisp-intro.texi
+
address@hidden ================ How to Print a Book in Various Sizes
================
+
address@hidden This book can be printed in any of three different sizes.
address@hidden In the above header, set @-commands appropriately.
+
address@hidden 7 by 9.25 inches:
address@hidden @smallbook
address@hidden @clear largebook
+
address@hidden 8.5 by 11 inches:
address@hidden @c smallbook
address@hidden @set largebook
+
address@hidden European A4 size paper:
address@hidden @c smallbook
address@hidden @afourpaper
address@hidden @set largebook
+
address@hidden ================ How to Typeset and Print ================
+
address@hidden If you do not include PostScript figures, run either of the
address@hidden following command sequences, or similar commands suited to your
address@hidden system:
+
address@hidden texi2dvi emacs-lisp-intro.texi
address@hidden lpr -d emacs-lisp-intro.dvi
+
address@hidden or else:
+
address@hidden tex emacs-lisp-intro.texi
address@hidden texindex emacs-lisp-intro.??
address@hidden tex emacs-lisp-intro.texi
address@hidden lpr -d emacs-lisp-intro.dvi
+
address@hidden If you include the PostScript figures, and you have old software,
address@hidden you may need to convert the .dvi file to a .ps file before
address@hidden printing. Run either of the following command sequences, or one
address@hidden similar:
address@hidden
address@hidden dvips -f < emacs-lisp-intro.dvi > emacs-lisp-intro.ps
address@hidden
address@hidden or else:
address@hidden
address@hidden postscript -p < emacs-lisp-intro.dvi > emacs-lisp-intro.ps
address@hidden
+
address@hidden (Note: if you edit the book so as to change the length of the
address@hidden table of contents, you may have to change the value of `pageno'
below.)
+
address@hidden ================ End of Formatting Sections ================
+
address@hidden For next or subsequent edition:
address@hidden create function using with-output-to-temp-buffer
address@hidden create a major mode, with keymaps
address@hidden run an asynchronous process, like grep or diff
+
address@hidden For 8.5 by 11 inch format: do not use such a small amount of
address@hidden whitespace between paragraphs as smallbook format
address@hidden largebook
address@hidden
+\global\parskip 6pt plus 1pt
address@hidden tex
address@hidden ifset
+
address@hidden For all sized formats: print within-book cross
address@hidden reference with ``...'' rather than [...]
+
address@hidden This works with the texinfo.tex file, version 2003-05-04.08,
address@hidden in the Texinfo version 4.6 of the 2003 Jun 13 distribution.
+
address@hidden
+\if \xrefprintnodename
+ \global\def\xrefprintnodename#1{\unskip, ``#1''}
+ \else
+ \global\def\xrefprintnodename#1{ ``#1''}
+\fi
+% \global\def\xrefprintnodename#1{, ``#1''}
address@hidden tex
+
address@hidden ----------------------------------------------------
+
address@hidden Emacs
address@hidden
+* Emacs Lisp Intro: (eintr).
+ A simple introduction to Emacs Lisp programming.
address@hidden direntry
+
address@hidden
+This is an @cite{Introduction to Programming in Emacs Lisp}, for
+people who are not programmers.
address@hidden 1
+Edition @value{edition-number}, @value{update-date}
address@hidden 1
+Copyright @copyright{} 1990, 1991, 1992, 1993, 1994, 1995, 1997, 2001,
+ 2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
address@hidden 1
+
address@hidden
+Published by the:@*
+
+GNU Press, @hfill @uref{http://address@hidden
+a division of the @hfill General: @email{press@@address@hidden
+Free Software Foundation, Inc. @hfill Orders:@w{ }
@email{sales@@address@hidden
+51 Franklin Street, Fifth Floor @hfill Tel: +1 (617) address@hidden
+Boston, MA 02110-1301 USA @hfill Fax: +1 (617) address@hidden
address@hidden iftex
+
address@hidden
+Published by the:
+
address@hidden
+GNU Press, Website: http://www.gnupress.org
+a division of the General: press@@gnu.org
+Free Software Foundation, Inc. Orders: sales@@gnu.org
+51 Franklin Street, Fifth Floor Tel: +1 (617) 542-5942
+Boston, MA 02110-1301 USA Fax: +1 (617) 542-2652
address@hidden example
address@hidden ifnottex
+
address@hidden 1
address@hidden Printed copies are available for $30 address@hidden
+ISBN 1-882114-43-4
+
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; there
+being no Invariant Section, with the Front-Cover Texts being ``A GNU
+Manual'', and with the Back-Cover Texts as in (a) below. A copy of
+the license is included in the section entitled ``GNU Free
+Documentation License''.
+
+(a) The FSF's Back-Cover Text is: ``You have freedom to copy and
+modify this GNU Manual, like GNU software. Copies published by the
+Free Software Foundation raise funds for GNU development.''
address@hidden copying
+
address@hidden half title; two lines here, so do not use `shorttitlepage'
address@hidden
+{\begingroup%
+ \hbox{}\vskip 1.5in \chaprm \centerline{An Introduction to}%
+ \endgroup}%
+{\begingroup\hbox{}\vskip 0.25in \chaprm%
+ \centerline{Programming in Emacs Lisp}%
+ \endgroup\page\hbox{}\page}
address@hidden tex
+
address@hidden
address@hidden 6
address@hidden @titlefont{An Introduction to}
address@hidden 2
address@hidden @titlefont{Programming in Emacs Lisp}
address@hidden 2
address@hidden Revised Third Edition
address@hidden 4
address@hidden by Robert J. Chassell
+
address@hidden
address@hidden 0pt plus 1filll
address@hidden
address@hidden titlepage
+
address@hidden
address@hidden off
address@hidden @thispage @| @| @thischapter
address@hidden @thissection @| @| @thispage
address@hidden iftex
+
address@hidden
address@hidden Keep T.O.C. short by tightening up for largebook
address@hidden largebook
address@hidden
+\global\parskip 2pt plus 1pt
+\global\advance\baselineskip by -1pt
address@hidden tex
address@hidden ifset
address@hidden ifnothtml
+
address@hidden
address@hidden
+
address@hidden
address@hidden Top, Preface, (dir), (dir)
address@hidden An Introduction to Programming in Emacs Lisp
+
address@hidden
+
+This master menu first lists each chapter and index; then it lists
+every node in every chapter.
address@hidden ifnottex
+
address@hidden >>>> Set pageno appropriately <<<<
+
address@hidden The first page of the Preface is a roman numeral; it is the first
address@hidden right handed page after the Table of Contents; hence the
following
address@hidden setting must be for an odd negative number.
+
address@hidden
address@hidden@pageno = -11
address@hidden iftex
+
address@hidden
+* Preface:: What to look for.
+* List Processing:: What is Lisp?
+* Practicing Evaluation:: Running several programs.
+* Writing Defuns:: How to write function definitions.
+* Buffer Walk Through:: Exploring a few buffer-related functions.
+* More Complex:: A few, even more complex functions.
+* Narrowing & Widening:: Restricting your and Emacs attention to
+ a region.
+* car cdr & cons:: Fundamental functions in Lisp.
+* Cutting & Storing Text:: Removing text and saving it.
+* List Implementation:: How lists are implemented in the computer.
+* Yanking:: Pasting stored text.
+* Loops & Recursion:: How to repeat a process.
+* Regexp Search:: Regular expression searches.
+* Counting Words:: A review of repetition and regexps.
+* Words in a defun:: Counting words in a @code{defun}.
+* Readying a Graph:: A prototype graph printing function.
+* Emacs Initialization:: How to write a @file{.emacs} file.
+* Debugging:: How to run the Emacs Lisp debuggers.
+* Conclusion:: Now you have the basics.
+* the-the:: An appendix: how to find reduplicated words.
+* Kill Ring:: An appendix: how the kill ring works.
+* Full Graph:: How to create a graph with labelled axes.
+* Free Software and Free Manuals::
+* GNU Free Documentation License::
+* Index::
+* About the Author::
+
address@hidden
+ --- The Detailed Node Listing ---
+
+Preface
+
+* Why:: Why learn Emacs Lisp?
+* On Reading this Text:: Read, gain familiarity, pick up habits....
+* Who You Are:: For whom this is written.
+* Lisp History::
+* Note for Novices:: You can read this as a novice.
+* Thank You::
+
+List Processing
+
+* Lisp Lists:: What are lists?
+* Run a Program:: Any list in Lisp is a program ready to run.
+* Making Errors:: Generating an error message.
+* Names & Definitions:: Names of symbols and function definitions.
+* Lisp Interpreter:: What the Lisp interpreter does.
+* Evaluation:: Running a program.
+* Variables:: Returning a value from a variable.
+* Arguments:: Passing information to a function.
+* set & setq:: Setting the value of a variable.
+* Summary:: The major points.
+* Error Message Exercises::
+
+Lisp Lists
+
+* Numbers Lists:: List have numbers, other lists, in them.
+* Lisp Atoms:: Elemental entities.
+* Whitespace in Lists:: Formatting lists to be readable.
+* Typing Lists:: How GNU Emacs helps you type lists.
+
+The Lisp Interpreter
+
+* Complications:: Variables, Special forms, Lists within.
+* Byte Compiling:: Specially processing code for speed.
+
+Evaluation
+
+* How the Interpreter Acts:: Returns and Side Effects...
+* Evaluating Inner Lists:: Lists within lists...
+
+Variables
+
+* fill-column Example::
+* Void Function:: The error message for a symbol
+ without a function.
+* Void Variable:: The error message for a symbol without a value.
+
+Arguments
+
+* Data types:: Types of data passed to a function.
+* Args as Variable or List:: An argument can be the value
+ of a variable or list.
+* Variable Number of Arguments:: Some functions may take a
+ variable number of arguments.
+* Wrong Type of Argument:: Passing an argument of the wrong type
+ to a function.
+* message:: A useful function for sending messages.
+
+Setting the Value of a Variable
+
+* Using set:: Setting values.
+* Using setq:: Setting a quoted value.
+* Counting:: Using @code{setq} to count.
+
+Practicing Evaluation
+
+* How to Evaluate:: Typing editing commands or @kbd{C-x C-e}
+ causes evaluation.
+* Buffer Names:: Buffers and files are different.
+* Getting Buffers:: Getting a buffer itself, not merely its name.
+* Switching Buffers:: How to change to another buffer.
+* Buffer Size & Locations:: Where point is located and the size of
+ the buffer.
+* Evaluation Exercise::
+
+How To Write Function Definitions
+
+* Primitive Functions::
+* defun:: The @code{defun} special form.
+* Install:: Install a function definition.
+* Interactive:: Making a function interactive.
+* Interactive Options:: Different options for @code{interactive}.
+* Permanent Installation:: Installing code permanently.
+* let:: Creating and initializing local variables.
+* if:: What if?
+* else:: If--then--else expressions.
+* Truth & Falsehood:: What Lisp considers false and true.
+* save-excursion:: Keeping track of point, mark, and buffer.
+* Review::
+* defun Exercises::
+
+Install a Function Definition
+
+* Effect of installation::
+* Change a defun:: How to change a function definition.
+
+Make a Function Interactive
+
+* Interactive multiply-by-seven:: An overview.
+* multiply-by-seven in detail:: The interactive version.
+
address@hidden
+
+* Prevent confusion::
+* Parts of let Expression::
+* Sample let Expression::
+* Uninitialized let Variables::
+
+The @code{if} Special Form
+
+* if in more detail::
+* type-of-animal in detail:: An example of an @code{if} expression.
+
+Truth and Falsehood in Emacs Lisp
+
+* nil explained:: @code{nil} has two meanings.
+
address@hidden
+
+* Point and mark:: A review of various locations.
+* Template for save-excursion::
+
+A Few Buffer--Related Functions
+
+* Finding More:: How to find more information.
+* simplified-beginning-of-buffer:: Shows @code{goto-char},
+ @code{point-min}, and @code{push-mark}.
+* mark-whole-buffer:: Almost the same as @code{beginning-of-buffer}.
+* append-to-buffer:: Uses @code{save-excursion} and
+ @code{insert-buffer-substring}.
+* Buffer Related Review:: Review.
+* Buffer Exercises::
+
+The Definition of @code{mark-whole-buffer}
+
+* mark-whole-buffer overview::
+* Body of mark-whole-buffer:: Only three lines of code.
+
+The Definition of @code{append-to-buffer}
+
+* append-to-buffer overview::
+* append interactive:: A two part interactive expression.
+* append-to-buffer body:: Incorporates a @code{let} expression.
+* append save-excursion:: How the @code{save-excursion} works.
+
+A Few More Complex Functions
+
+* copy-to-buffer:: With @code{set-buffer},
@code{get-buffer-create}.
+* insert-buffer:: Read-only, and with @code{or}.
+* beginning-of-buffer:: Shows @code{goto-char},
+ @code{point-min}, and @code{push-mark}.
+* Second Buffer Related Review::
+* optional Exercise::
+
+The Definition of @code{insert-buffer}
+
+* insert-buffer code::
+* insert-buffer interactive:: When you can read, but not write.
+* insert-buffer body:: The body has an @code{or} and a @code{let}.
+* if & or:: Using an @code{if} instead of an @code{or}.
+* Insert or:: How the @code{or} expression works.
+* Insert let:: Two @code{save-excursion} expressions.
+* New insert-buffer::
+
+The Interactive Expression in @code{insert-buffer}
+
+* Read-only buffer:: When a buffer cannot be modified.
+* b for interactive:: An existing buffer or else its name.
+
+Complete Definition of @code{beginning-of-buffer}
+
+* Optional Arguments::
+* beginning-of-buffer opt arg:: Example with optional argument.
+* beginning-of-buffer complete::
+
address@hidden with an Argument
+
+* Disentangle beginning-of-buffer::
+* Large buffer case::
+* Small buffer case::
+
+Narrowing and Widening
+
+* Narrowing advantages:: The advantages of narrowing
+* save-restriction:: The @code{save-restriction} special form.
+* what-line:: The number of the line that point is on.
+* narrow Exercise::
+
address@hidden, @code{cdr}, @code{cons}: Fundamental Functions
+
+* Strange Names:: An historical aside: why the strange names?
+* car & cdr:: Functions for extracting part of a list.
+* cons:: Constructing a list.
+* nthcdr:: Calling @code{cdr} repeatedly.
+* nth::
+* setcar:: Changing the first element of a list.
+* setcdr:: Changing the rest of a list.
+* cons Exercise::
+
address@hidden
+
+* Build a list::
+* length:: How to find the length of a list.
+
+Cutting and Storing Text
+
+* Storing Text:: Text is stored in a list.
+* zap-to-char:: Cutting out text up to a character.
+* kill-region:: Cutting text out of a region.
+* copy-region-as-kill:: A definition for copying text.
+* Digression into C:: Minor note on C programming language macros.
+* defvar:: How to give a variable an initial value.
+* cons & search-fwd Review::
+* search Exercises::
+
address@hidden
+
+* Complete zap-to-char:: The complete implementation.
+* zap-to-char interactive:: A three part interactive expression.
+* zap-to-char body:: A short overview.
+* search-forward:: How to search for a string.
+* progn:: The @code{progn} special form.
+* Summing up zap-to-char:: Using @code{point} and @code{search-forward}.
+
address@hidden
+
+* Complete kill-region:: The function definition.
+* condition-case:: Dealing with a problem.
+* Lisp macro::
+
address@hidden
+
+* Complete copy-region-as-kill:: The complete function definition.
+* copy-region-as-kill body:: The body of @code{copy-region-as-kill}.
+
+The Body of @code{copy-region-as-kill}
+
+* last-command & this-command::
+* kill-append function::
+* kill-new function::
+
+Initializing a Variable with @code{defvar}
+
+* See variable current value::
+* defvar and asterisk::
+
+How Lists are Implemented
+
+* Lists diagrammed::
+* Symbols as Chest:: Exploring a powerful metaphor.
+* List Exercise::
+
+Yanking Text Back
+
+* Kill Ring Overview::
+* kill-ring-yank-pointer:: The kill ring is a list.
+* yank nthcdr Exercises:: The @code{kill-ring-yank-pointer} variable.
+
+Loops and Recursion
+
+* while:: Causing a stretch of code to repeat.
+* dolist dotimes::
+* Recursion:: Causing a function to call itself.
+* Looping exercise::
+
address@hidden
+
+* Looping with while:: Repeat so long as test returns true.
+* Loop Example:: A @code{while} loop that uses a list.
+* print-elements-of-list:: Uses @code{while}, @code{car}, @code{cdr}.
+* Incrementing Loop:: A loop with an incrementing counter.
+* Incrementing Loop Details::
+* Decrementing Loop:: A loop with a decrementing counter.
+
+Details of an Incrementing Loop
+
+* Incrementing Example:: Counting pebbles in a triangle.
+* Inc Example parts:: The parts of the function definition.
+* Inc Example altogether:: Putting the function definition together.
+
+Loop with a Decrementing Counter
+
+* Decrementing Example:: More pebbles on the beach.
+* Dec Example parts:: The parts of the function definition.
+* Dec Example altogether:: Putting the function definition together.
+
+Save your time: @code{dolist} and @code{dotimes}
+
+* dolist::
+* dotimes::
+
+Recursion
+
+* Building Robots:: Same model, different serial number ...
+* Recursive Definition Parts:: Walk until you stop ...
+* Recursion with list:: Using a list as the test whether to recurse.
+* Recursive triangle function::
+* Recursion with cond::
+* Recursive Patterns:: Often used templates.
+* No Deferment:: Don't store up work ...
+* No deferment solution::
+
+Recursion in Place of a Counter
+
+* Recursive Example arg of 1 or 2::
+* Recursive Example arg of 3 or 4::
+
+Recursive Patterns
+
+* Every::
+* Accumulate::
+* Keep::
+
+Regular Expression Searches
+
+* sentence-end:: The regular expression for @code{sentence-end}.
+* re-search-forward:: Very similar to @code{search-forward}.
+* forward-sentence:: A straightforward example of regexp search.
+* forward-paragraph:: A somewhat complex example.
+* etags:: How to create your own @file{TAGS} table.
+* Regexp Review::
+* re-search Exercises::
+
address@hidden
+
+* Complete forward-sentence::
+* fwd-sentence while loops:: Two @code{while} loops.
+* fwd-sentence re-search:: A regular expression search.
+
address@hidden: a Goldmine of Functions
+
+* forward-paragraph in brief:: Key parts of the function definition.
+* fwd-para let:: The @code{let*} expression.
+* fwd-para while:: The forward motion @code{while} loop.
+
+Counting: Repetition and Regexps
+
+* Why Count Words::
+* count-words-region:: Use a regexp, but find a problem.
+* recursive-count-words:: Start with case of no words in region.
+* Counting Exercise::
+
+The @code{count-words-region} Function
+
+* Design count-words-region:: The definition using a @code{while} loop.
+* Whitespace Bug:: The Whitespace Bug in
@code{count-words-region}.
+
+Counting Words in a @code{defun}
+
+* Divide and Conquer::
+* Words and Symbols:: What to count?
+* Syntax:: What constitutes a word or symbol?
+* count-words-in-defun:: Very like @code{count-words}.
+* Several defuns:: Counting several defuns in a file.
+* Find a File:: Do you want to look at a file?
+* lengths-list-file:: A list of the lengths of many definitions.
+* Several files:: Counting in definitions in different files.
+* Several files recursively:: Recursively counting in different files.
+* Prepare the data:: Prepare the data for display in a graph.
+
+Count Words in @code{defuns} in Different Files
+
+* lengths-list-many-files:: Return a list of the lengths of defuns.
+* append:: Attach one list to another.
+
+Prepare the Data for Display in a Graph
+
+* Data for Display in Detail::
+* Sorting:: Sorting lists.
+* Files List:: Making a list of files.
+* Counting function definitions::
+
+Readying a Graph
+
+* Columns of a graph::
+* graph-body-print:: How to print the body of a graph.
+* recursive-graph-body-print::
+* Printed Axes::
+* Line Graph Exercise::
+
+Your @file{.emacs} File
+
+* Default Configuration::
+* Site-wide Init:: You can write site-wide init files.
+* defcustom:: Emacs will write code for you.
+* Beginning a .emacs File:: How to write a @code{.emacs file}.
+* Text and Auto-fill:: Automatically wrap lines.
+* Mail Aliases:: Use abbreviations for email addresses.
+* Indent Tabs Mode:: Don't use tabs with @TeX{}
+* Keybindings:: Create some personal keybindings.
+* Keymaps:: More about key binding.
+* Loading Files:: Load (i.e., evaluate) files automatically.
+* Autoload:: Make functions available.
+* Simple Extension:: Define a function; bind it to a key.
+* X11 Colors:: Colors in X.
+* Miscellaneous::
+* Mode Line:: How to customize your mode line.
+
+Debugging
+
+* debug:: How to use the built-in debugger.
+* debug-on-entry:: Start debugging when you call a function.
+* debug-on-quit:: Start debugging when you quit with @kbd{C-g}.
+* edebug:: How to use Edebug, a source level debugger.
+* Debugging Exercises::
+
+Handling the Kill Ring
+
+* What the Kill Ring Does::
+* current-kill::
+* yank:: Paste a copy of a clipped element.
+* yank-pop:: Insert element pointed to.
+* ring file::
+
+The @code{current-kill} Function
+
+* Understanding current-kill::
+
address@hidden in Outline
+
+* Body of current-kill::
+* Digression concerning error:: How to mislead humans, but not computers.
+* Determining the Element::
+
+A Graph with Labelled Axes
+
+* Labelled Example::
+* print-graph Varlist:: @code{let} expression in @code{print-graph}.
+* print-Y-axis:: Print a label for the vertical axis.
+* print-X-axis:: Print a horizontal label.
+* Print Whole Graph:: The function to print a complete graph.
+
+The @code{print-Y-axis} Function
+
+* print-Y-axis in Detail::
+* Height of label:: What height for the Y axis?
+* Compute a Remainder:: How to compute the remainder of a division.
+* Y Axis Element:: Construct a line for the Y axis.
+* Y-axis-column:: Generate a list of Y axis labels.
+* print-Y-axis Penultimate:: A not quite final version.
+
+The @code{print-X-axis} Function
+
+* Similarities differences:: Much like @code{print-Y-axis}, but not exactly.
+* X Axis Tic Marks:: Create tic marks for the horizontal axis.
+
+Printing the Whole Graph
+
+* The final version:: A few changes.
+* Test print-graph:: Run a short test.
+* Graphing words in defuns:: Executing the final code.
+* lambda:: How to write an anonymous function.
+* mapcar:: Apply a function to elements of a list.
+* Another Bug:: Yet another bug @dots{} most insidious.
+* Final printed graph:: The graph itself!
+
address@hidden detailmenu
address@hidden menu
+
address@hidden Preface, List Processing, Top, Top
address@hidden node-name, next, previous, up
address@hidden Preface
+
+Most of the GNU Emacs integrated environment is written in the programming
+language called Emacs Lisp. The code written in this programming
+language is the software---the sets of instructions---that tell the
+computer what to do when you give it commands. Emacs is designed so
+that you can write new code in Emacs Lisp and easily install it as an
+extension to the editor.
+
+(GNU Emacs is sometimes called an ``extensible editor'', but it does
+much more than provide editing capabilities. It is better to refer to
+Emacs as an ``extensible computing environment''. However, that
+phrase is quite a mouthful. It is easier to refer to Emacs simply as
+an editor. Moreover, everything you do in Emacs---find the Mayan date
+and phases of the moon, simplify polynomials, debug code, manage
+files, read letters, write books---all these activities are kinds of
+editing in the most general sense of the word.)
+
address@hidden
+* Why:: Why learn Emacs Lisp?
+* On Reading this Text:: Read, gain familiarity, pick up habits....
+* Who You Are:: For whom this is written.
+* Lisp History::
+* Note for Novices:: You can read this as a novice.
+* Thank You::
address@hidden menu
+
address@hidden Why, On Reading this Text, Preface, Preface
address@hidden
address@hidden Why Study Emacs Lisp?
address@hidden ifnottex
+
+Although Emacs Lisp is usually thought of in association only with Emacs,
+it is a full computer programming language. You can use Emacs Lisp as
+you would any other programming language.
+
+Perhaps you want to understand programming; perhaps you want to extend
+Emacs; or perhaps you want to become a programmer. This introduction to
+Emacs Lisp is designed to get you started: to guide you in learning the
+fundamentals of programming, and more importantly, to show you how you
+can teach yourself to go further.
+
address@hidden On Reading this Text, Who You Are, Why, Preface
address@hidden node-name, next, previous, up
address@hidden On Reading this Text
+
+All through this document, you will see little sample programs you can
+run inside of Emacs. If you read this document in Info inside of GNU
+Emacs, you can run the programs as they appear. (This is easy to do and
+is explained when the examples are presented.) Alternatively, you can
+read this introduction as a printed book while sitting beside a computer
+running Emacs. (This is what I like to do; I like printed books.) If
+you don't have a running Emacs beside you, you can still read this book,
+but in this case, it is best to treat it as a novel or as a travel guide
+to a country not yet visited: interesting, but not the same as being
+there.
+
+Much of this introduction is dedicated to walk-throughs or guided tours
+of code used in GNU Emacs. These tours are designed for two purposes:
+first, to give you familiarity with real, working code (code you use
+every day); and, second, to give you familiarity with the way Emacs
+works. It is interesting to see how a working environment is
+implemented.
+Also, I
+hope that you will pick up the habit of browsing through source code.
+You can learn from it and mine it for ideas. Having GNU Emacs is like
+having a dragon's cave of treasures.
+
+In addition to learning about Emacs as an editor and Emacs Lisp as a
+programming language, the examples and guided tours will give you an
+opportunity to get acquainted with Emacs as a Lisp programming
+environment. GNU Emacs supports programming and provides tools that
+you will want to become comfortable using, such as @kbd{M-.} (the key
+which invokes the @code{find-tag} command). You will also learn about
+buffers and other objects that are part of the environment.
+Learning about these features of Emacs is like learning new routes
+around your home town.
+
address@hidden
+In addition, I have written several programs as extended examples.
+Although these are examples, the programs are real. I use them.
+Other people use them. You may use them. Beyond the fragments of
+programs used for illustrations, there is very little in here that is
+`just for teaching purposes'; what you see is used. This is a great
+advantage of Emacs Lisp: it is easy to learn to use it for work.
address@hidden ignore
+
+Finally, I hope to convey some of the skills for using Emacs to
+learn aspects of programming that you don't know. You can often use
+Emacs to help you understand what puzzles you or to find out how to do
+something new. This self-reliance is not only a pleasure, but an
+advantage.
+
address@hidden Who You Are, Lisp History, On Reading this Text, Preface
address@hidden node-name, next, previous, up
address@hidden For Whom This is Written
+
+This text is written as an elementary introduction for people who are
+not programmers. If you are a programmer, you may not be satisfied with
+this primer. The reason is that you may have become expert at reading
+reference manuals and be put off by the way this text is organized.
+
+An expert programmer who reviewed this text said to me:
+
address@hidden
address@hidden prefer to learn from reference manuals. I ``dive into'' each
+paragraph, and ``come up for air'' between paragraphs.}
+
address@hidden I get to the end of a paragraph, I assume that that subject is
+done, finished, that I know everything I need (with the
+possible exception of the case when the next paragraph starts talking
+about it in more detail). I expect that a well written reference manual
+will not have a lot of redundancy, and that it will have excellent
+pointers to the (one) place where the information I want is.}
address@hidden quotation
+
+This introduction is not written for this person!
+
+Firstly, I try to say everything at least three times: first, to
+introduce it; second, to show it in context; and third, to show it in a
+different context, or to review it.
+
+Secondly, I hardly ever put all the information about a subject in one
+place, much less in one paragraph. To my way of thinking, that imposes
+too heavy a burden on the reader. Instead I try to explain only what
+you need to know at the time. (Sometimes I include a little extra
+information so you won't be surprised later when the additional
+information is formally introduced.)
+
+When you read this text, you are not expected to learn everything the
+first time. Frequently, you need only make, as it were, a `nodding
+acquaintance' with some of the items mentioned. My hope is that I have
+structured the text and given you enough hints that you will be alert to
+what is important, and concentrate on it.
+
+You will need to ``dive into'' some paragraphs; there is no other way
+to read them. But I have tried to keep down the number of such
+paragraphs. This book is intended as an approachable hill, rather than
+as a daunting mountain.
+
+This introduction to @cite{Programming in Emacs Lisp} has a companion
+document,
address@hidden
address@hidden GNU Emacs Lisp Reference Manual}.
address@hidden iftex
address@hidden
address@hidden, , The GNU Emacs Lisp Reference Manual, elisp, The GNU
+Emacs Lisp Reference Manual}.
address@hidden ifnottex
+The reference manual has more detail than this introduction. In the
+reference manual, all the information about one topic is concentrated
+in one place. You should turn to it if you are like the programmer
+quoted above. And, of course, after you have read this
address@hidden, you will find the @cite{Reference Manual} useful
+when you are writing your own programs.
+
address@hidden Lisp History, Note for Novices, Who You Are, Preface
address@hidden Lisp History
address@hidden Lisp history
+
+Lisp was first developed in the late 1950s at the Massachusetts
+Institute of Technology for research in artificial intelligence. The
+great power of the Lisp language makes it superior for other purposes as
+well, such as writing editor commands and integrated environments.
+
address@hidden Maclisp
address@hidden Common Lisp
+GNU Emacs Lisp is largely inspired by Maclisp, which was written at MIT
+in the 1960s. It is somewhat inspired by Common Lisp, which became a
+standard in the 1980s. However, Emacs Lisp is much simpler than Common
+Lisp. (The standard Emacs distribution contains an optional extensions
+file, @file{cl.el}, that adds many Common Lisp features to Emacs Lisp.)
+
address@hidden Note for Novices, Thank You, Lisp History, Preface
address@hidden node-name, next, previous, up
address@hidden A Note for Novices
+
+If you don't know GNU Emacs, you can still read this document
+profitably. However, I recommend you learn Emacs, if only to learn to
+move around your computer screen. You can teach yourself how to use
+Emacs with the on-line tutorial. To use it, type @kbd{C-h t}. (This
+means you press and release the @key{CTRL} key and the @kbd{h} at the
+same time, and then press and release @kbd{t}.)
+
+Also, I often refer to one of Emacs' standard commands by listing the
+keys which you press to invoke the command and then giving the name of
+the command in parentheses, like this: @kbd{M-C-\}
+(@code{indent-region}). What this means is that the
address@hidden command is customarily invoked by typing
address@hidden (You can, if you wish, change the keys that are typed to
+invoke the command; this is called @dfn{rebinding}. @xref{Keymaps, ,
+Keymaps}.) The abbreviation @kbd{M-C-\} means that you type your
address@hidden key, @key{CTRL} key and @key{\} key all at the same time.
+(On many modern keyboards the @key{META} key is labelled
address@hidden)
+Sometimes a combination like this is called a keychord, since it is
+similar to the way you play a chord on a piano. If your keyboard does
+not have a @key{META} key, the @key{ESC} key prefix is used in place
+of it. In this case, @kbd{M-C-\} means that you press and release your
address@hidden key and then type the @key{CTRL} key and the @key{\} key at
+the same time. But usually @kbd{M-C-\} means press the @key{CTRL} key
+along with the key that is labelled @key{ALT} and, at the same time,
+press the @key{\} key.
+
+In addition to typing a lone keychord, you can prefix what you type
+with @kbd{C-u}, which is called the `universal argument'. The
address@hidden keychord passes an argument to the subsequent command.
+Thus, to indent a region of plain text by 6 spaces, mark the region,
+and then type @address@hidden 6 M-C-\}}. (If you do not specify a number,
+Emacs either passes the number 4 to the command or otherwise runs the
+command differently than it would otherwise.) @xref{Arguments, ,
+Numeric Arguments, emacs, The GNU Emacs Manual}.
+
+If you are reading this in Info using GNU Emacs, you can read through
+this whole document just by pressing the space bar, @key{SPC}.
+(To learn about Info, type @kbd{C-h i} and then select Info.)
+
+A note on terminology: when I use the word Lisp alone, I often am
+referring to the various dialects of Lisp in general, but when I speak
+of Emacs Lisp, I am referring to GNU Emacs Lisp in particular.
+
address@hidden Thank You, , Note for Novices, Preface
address@hidden node-name, next, previous, up
address@hidden Thank You
+
+My thanks to all who helped me with this book. My especial thanks to
address@hidden Blandy}, @r{Noah Friedman}, @w{Jim Kingdon}, @r{Roland
+McGrath}, @w{Frank Ritter}, @w{Randy Smith}, @w{Richard M.@:
+Stallman}, and @w{Melissa Weisshaus}. My thanks also go to both
address@hidden Johnson} and @w{David Stampe} for their patient
+encouragement. My mistakes are my own.
+
address@hidden
+Robert J. Chassell
address@hidden flushright
+
address@hidden ================ Beginning of main text ================
+
address@hidden Start main text on right-hand (verso) page
+
address@hidden
+\par\vfill\supereject
+\headings off
+\ifodd\pageno
+ \par\vfill\supereject
+\else
+ \par\vfill\supereject
+ \page\hbox{}\page
+ \par\vfill\supereject
+\fi
address@hidden tex
+
address@hidden
address@hidden off
address@hidden @thispage @| @| @thischapter
address@hidden @thissection @| @| @thispage
address@hidden@pageno = 1
address@hidden iftex
+
address@hidden List Processing, Practicing Evaluation, Preface, Top
address@hidden node-name, next, previous, up
address@hidden List Processing
+
+To the untutored eye, Lisp is a strange programming language. In Lisp
+code there are parentheses everywhere. Some people even claim that
+the name stands for `Lots of Isolated Silly Parentheses'. But the
+claim is unwarranted. Lisp stands for LISt Processing, and the
+programming language handles @emph{lists} (and lists of lists) by
+putting them between parentheses. The parentheses mark the boundaries
+of the list. Sometimes a list is preceded by a single apostrophe or
+quotation mark, @samp{'address@hidden single apostrophe or quotation
+mark is an abbreviation for the function @code{quote}; you need not
+think about functions now; functions are defined in @ref{Making
+Errors, , Generate an Error Message}.} Lists are the basis of Lisp.
+
address@hidden
+* Lisp Lists:: What are lists?
+* Run a Program:: Any list in Lisp is a program ready to run.
+* Making Errors:: Generating an error message.
+* Names & Definitions:: Names of symbols and function definitions.
+* Lisp Interpreter:: What the Lisp interpreter does.
+* Evaluation:: Running a program.
+* Variables:: Returning a value from a variable.
+* Arguments:: Passing information to a function.
+* set & setq:: Setting the value of a variable.
+* Summary:: The major points.
+* Error Message Exercises::
address@hidden menu
+
address@hidden Lisp Lists, Run a Program, List Processing, List Processing
address@hidden node-name, next, previous, up
address@hidden Lisp Lists
address@hidden Lisp Lists
+
+In Lisp, a list looks like this: @code{'(rose violet daisy buttercup)}.
+This list is preceded by a single apostrophe. It could just as well be
+written as follows, which looks more like the kind of list you are likely
+to be familiar with:
+
address@hidden
address@hidden
+'(rose
+ violet
+ daisy
+ buttercup)
address@hidden group
address@hidden smallexample
+
address@hidden
+The elements of this list are the names of the four different flowers,
+separated from each other by whitespace and surrounded by parentheses,
+like flowers in a field with a stone wall around them.
address@hidden Flowers in a field
+
address@hidden
+* Numbers Lists:: List have numbers, other lists, in them.
+* Lisp Atoms:: Elemental entities.
+* Whitespace in Lists:: Formatting lists to be readable.
+* Typing Lists:: How GNU Emacs helps you type lists.
address@hidden menu
+
address@hidden Numbers Lists, Lisp Atoms, Lisp Lists, Lisp Lists
address@hidden
address@hidden Numbers, Lists inside of Lists
address@hidden ifnottex
+
+Lists can also have numbers in them, as in this list: @code{(+ 2 2)}.
+This list has a plus-sign, @samp{+}, followed by two @samp{2}s, each
+separated by whitespace.
+
+In Lisp, both data and programs are represented the same way; that is,
+they are both lists of words, numbers, or other lists, separated by
+whitespace and surrounded by parentheses. (Since a program looks like
+data, one program may easily serve as data for another; this is a very
+powerful feature of Lisp.) (Incidentally, these two parenthetical
+remarks are @emph{not} Lisp lists, because they contain @samp{;} and
address@hidden as punctuation marks.)
+
address@hidden 1200
+Here is another list, this time with a list inside of it:
+
address@hidden
+'(this list has (a list inside of it))
address@hidden smallexample
+
+The components of this list are the words @samp{this}, @samp{list},
address@hidden, and the list @samp{(a list inside of it)}. The interior
+list is made up of the words @samp{a}, @samp{list}, @samp{inside},
address@hidden, @samp{it}.
+
address@hidden Lisp Atoms, Whitespace in Lists, Numbers Lists, Lisp Lists
address@hidden node-name, next, previous, up
address@hidden Lisp Atoms
address@hidden Lisp Atoms
+
+In Lisp, what we have been calling words are called @dfn{atoms}. This
+term comes from the historical meaning of the word atom, which means
+`indivisible'. As far as Lisp is concerned, the words we have been
+using in the lists cannot be divided into any smaller parts and still
+mean the same thing as part of a program; likewise with numbers and
+single character symbols like @samp{+}. On the other hand, unlike an
+ancient atom, a list can be split into parts. (@xref{car cdr & cons,
+, @code{car} @code{cdr} & @code{cons} Fundamental Functions}.)
+
+In a list, atoms are separated from each other by whitespace. They can be
+right next to a parenthesis.
+
address@hidden @samp{empty list} defined
+Technically speaking, a list in Lisp consists of parentheses surrounding
+atoms separated by whitespace or surrounding other lists or surrounding
+both atoms and other lists. A list can have just one atom in it or
+have nothing in it at all. A list with nothing in it looks like this:
address@hidden()}, and is called the @dfn{empty list}. Unlike anything else, an
+empty list is considered both an atom and a list at the same time.
+
address@hidden Symbolic expressions, introduced
address@hidden @samp{expression} defined
address@hidden @samp{form} defined
+The printed representation of both atoms and lists are called
address@hidden expressions} or, more concisely, @dfn{s-expressions}.
+The word @dfn{expression} by itself can refer to either the printed
+representation, or to the atom or list as it is held internally in the
+computer. Often, people use the term @dfn{expression}
+indiscriminately. (Also, in many texts, the word @dfn{form} is used
+as a synonym for expression.)
+
+Incidentally, the atoms that make up our universe were named such when
+they were thought to be indivisible; but it has been found that physical
+atoms are not indivisible. Parts can split off an atom or it can
+fission into two parts of roughly equal size. Physical atoms were named
+prematurely, before their truer nature was found. In Lisp, certain
+kinds of atom, such as an array, can be separated into parts; but the
+mechanism for doing this is different from the mechanism for splitting a
+list. As far as list operations are concerned, the atoms of a list are
+unsplittable.
+
+As in English, the meanings of the component letters of a Lisp atom
+are different from the meaning the letters make as a word. For
+example, the word for the South American sloth, the @samp{ai}, is
+completely different from the two words, @samp{a}, and @samp{i}.
+
+There are many kinds of atom in nature but only a few in Lisp: for
+example, @dfn{numbers}, such as 37, 511, or 1729, and @dfn{symbols}, such
+as @samp{+}, @samp{foo}, or @samp{forward-line}. The words we have
+listed in the examples above are all symbols. In everyday Lisp
+conversation, the word ``atom'' is not often used, because programmers
+usually try to be more specific about what kind of atom they are dealing
+with. Lisp programming is mostly about symbols (and sometimes numbers)
+within lists. (Incidentally, the preceding three word parenthetical
+remark is a proper list in Lisp, since it consists of atoms, which in
+this case are symbols, separated by whitespace and enclosed by
+parentheses, without any non-Lisp punctuation.)
+
address@hidden 1250
+In addition, text between double quotation marks---even sentences or
+paragraphs---is an atom. Here is an example:
address@hidden Text between double quotation marks
+
address@hidden
+'(this list includes "text between quotation marks.")
address@hidden smallexample
+
address@hidden @samp{string} defined
address@hidden
+In Lisp, all of the quoted text including the punctuation mark and the
+blank spaces is a single atom. This kind of atom is called a
address@hidden (for `string of characters') and is the sort of thing that
+is used for messages that a computer can print for a human to read.
+Strings are a different kind of atom than numbers or symbols and are
+used differently.
+
address@hidden Whitespace in Lists, Typing Lists, Lisp Atoms, Lisp Lists
address@hidden node-name, next, previous, up
address@hidden Whitespace in Lists
address@hidden Whitespace in lists
+
address@hidden 1200
+The amount of whitespace in a list does not matter. From the point of view
+of the Lisp language,
+
address@hidden
address@hidden
+'(this list
+ looks like this)
address@hidden group
address@hidden smallexample
+
address@hidden 800
address@hidden
+is exactly the same as this:
+
address@hidden
+'(this list looks like this)
address@hidden smallexample
+
+Both examples show what to Lisp is the same list, the list made up of
+the symbols @samp{this}, @samp{list}, @samp{looks}, @samp{like}, and
address@hidden in that order.
+
+Extra whitespace and newlines are designed to make a list more readable
+by humans. When Lisp reads the expression, it gets rid of all the extra
+whitespace (but it needs to have at least one space between atoms in
+order to tell them apart.)
+
+Odd as it seems, the examples we have seen cover almost all of what Lisp
+lists look like! Every other list in Lisp looks more or less like one
+of these examples, except that the list may be longer and more complex.
+In brief, a list is between parentheses, a string is between quotation
+marks, a symbol looks like a word, and a number looks like a number.
+(For certain situations, square brackets, dots and a few other special
+characters may be used; however, we will go quite far without them.)
+
address@hidden Typing Lists, , Whitespace in Lists, Lisp Lists
address@hidden node-name, next, previous, up
address@hidden GNU Emacs Helps You Type Lists
address@hidden Help typing lists
address@hidden Formatting help
+
+When you type a Lisp expression in GNU Emacs using either Lisp
+Interaction mode or Emacs Lisp mode, you have available to you several
+commands to format the Lisp expression so it is easy to read. For
+example, pressing the @key{TAB} key automatically indents the line the
+cursor is on by the right amount. A command to properly indent the
+code in a region is customarily bound to @kbd{M-C-\}. Indentation is
+designed so that you can see which elements of a list belong to which
+list---elements of a sub-list are indented more than the elements of
+the enclosing list.
+
+In addition, when you type a closing parenthesis, Emacs momentarily
+jumps the cursor back to the matching opening parenthesis, so you can
+see which one it is. This is very useful, since every list you type
+in Lisp must have its closing parenthesis match its opening
+parenthesis. (@xref{Major Modes, , Major Modes, emacs, The GNU Emacs
+Manual}, for more information about Emacs' modes.)
+
address@hidden Run a Program, Making Errors, Lisp Lists, List Processing
address@hidden node-name, next, previous, up
address@hidden Run a Program
address@hidden Run a program
address@hidden Program, running one
+
address@hidden @samp{evaluate} defined
+A list in Lisp---any list---is a program ready to run. If you run it
+(for which the Lisp jargon is @dfn{evaluate}), the computer will do one
+of three things: do nothing except return to you the list itself; send
+you an error message; or, treat the first symbol in the list as a
+command to do something. (Usually, of course, it is the last of these
+three things that you really want!)
+
address@hidden use code for the single apostrophe, not samp.
+The single apostrophe, @code{'}, that I put in front of some of the
+example lists in preceding sections is called a @dfn{quote}; when it
+precedes a list, it tells Lisp to do nothing with the list, other than
+take it as it is written. But if there is no quote preceding a list,
+the first item of the list is special: it is a command for the computer
+to obey. (In Lisp, these commands are called @emph{functions}.) The list
address@hidden(+ 2 2)} shown above did not have a quote in front of it, so Lisp
+understands that the @code{+} is an instruction to do something with the
+rest of the list: add the numbers that follow.
+
address@hidden 1250
+If you are reading this inside of GNU Emacs in Info, here is how you can
+evaluate such a list: place your cursor immediately after the right
+hand parenthesis of the following list and then type @kbd{C-x C-e}:
+
address@hidden
+(+ 2 2)
address@hidden smallexample
+
address@hidden use code for the number four, not samp.
address@hidden
+You will see the number @code{4} appear in the echo area. (In the
+jargon, what you have just done is ``evaluate the list.'' The echo area
+is the line at the bottom of the screen that displays or ``echoes''
+text.) Now try the same thing with a quoted list: place the cursor
+right after the following list and type @kbd{C-x C-e}:
+
address@hidden
+'(this is a quoted list)
address@hidden smallexample
+
address@hidden
+You will see @code{(this is a quoted list)} appear in the echo area.
+
address@hidden Lisp interpreter, explained
address@hidden Interpreter, Lisp, explained
+In both cases, what you are doing is giving a command to the program
+inside of GNU Emacs called the @dfn{Lisp interpreter}---giving the
+interpreter a command to evaluate the expression. The name of the Lisp
+interpreter comes from the word for the task done by a human who comes
+up with the meaning of an expression---who ``interprets'' it.
+
+You can also evaluate an atom that is not part of a list---one that is
+not surrounded by parentheses; again, the Lisp interpreter translates
+from the humanly readable expression to the language of the computer.
+But before discussing this (@pxref{Variables}), we will discuss what the
+Lisp interpreter does when you make an error.
+
address@hidden Making Errors, Names & Definitions, Run a Program, List
Processing
address@hidden node-name, next, previous, up
address@hidden Generate an Error Message
address@hidden Generate an error message
address@hidden Error message generation
+
+Partly so you won't worry if you do it accidentally, we will now give
+a command to the Lisp interpreter that generates an error message.
+This is a harmless activity; and indeed, we will often try to generate
+error messages intentionally. Once you understand the jargon, error
+messages can be informative. Instead of being called ``error''
+messages, they should be called ``help'' messages. They are like
+signposts to a traveller in a strange country; deciphering them can be
+hard, but once understood, they can point the way.
+
+The error message is generated by a built-in GNU Emacs debugger. We
+will `enter the debugger'. You get out of the debugger by typing @code{q}.
+
+What we will do is evaluate a list that is not quoted and does not
+have a meaningful command as its first element. Here is a list almost
+exactly the same as the one we just used, but without the single-quote
+in front of it. Position the cursor right after it and type @kbd{C-x
+C-e}:
+
address@hidden
+(this is an unquoted list)
address@hidden smallexample
+
address@hidden
+What you see depends on which version of Emacs you are running. GNU
+Emacs version 22 provides more information than version 20 and before.
+First, the more recent result of generating an error; then the
+earlier, version 20 result.
+
address@hidden 1250
address@hidden
+In GNU Emacs version 22, a @file{*Backtrace*} window will open up and
+you will see the following in it:
+
address@hidden
address@hidden
+---------- Buffer: *Backtrace* ----------
+Debugger entered--Lisp error: (void-function this)
+ (this is an unquoted list)
+ eval((this is an unquoted list))
+ eval-last-sexp-1(nil)
+ eval-last-sexp(nil)
+ call-interactively(eval-last-sexp)
+---------- Buffer: *Backtrace* ----------
address@hidden group
address@hidden smallexample
+
address@hidden 1200
address@hidden
+Your cursor will be in this window (you may have to wait a few seconds
+before it becomes visible). To quit the debugger and make the
+debugger window go away, type:
+
address@hidden
+q
address@hidden smallexample
+
address@hidden
+Please type @kbd{q} right now, so you become confident that you can
+get out of the debugger. Then, type @kbd{C-x C-e} again to re-enter
+it.
+
address@hidden @samp{function} defined
+Based on what we already know, we can almost read this error message.
+
+You read the @file{*Backtrace*} buffer from the bottom up; it tells
+you what Emacs did. When you typed @kbd{C-x C-e}, you made an
+interactive call to the command @code{eval-last-sexp}. @code{eval} is
+an abbreviation for `evaluate' and @code{sexp} is an abbreviation for
+`symbolic expression'. The command means `evaluate last symbolic
+expression', which is the expression just before your cursor.
+
+Each line above tells you what the Lisp interpreter evaluated next.
+The most recent action is at the top. The buffer is called the
address@hidden buffer because it enables you to track Emacs
+backwards.
+
address@hidden 800
+At the top of the @file{*Backtrace*} buffer, you see the line:
+
address@hidden
+Debugger entered--Lisp error: (void-function this)
address@hidden smallexample
+
address@hidden
+The Lisp interpreter tried to evaluate the first atom of the list, the
+word @samp{this}. It is this action that generated the error message
address@hidden this}.
+
+The message contains the words @samp{void-function} and @samp{this}.
+
address@hidden @samp{function} defined
+The word @samp{function} was mentioned once before. It is a very
+important word. For our purposes, we can define it by saying that a
address@hidden is a set of instructions to the computer that tell the
+computer to do something.
+
+Now we can begin to understand the error message: @samp{void-function
+this}. The function (that is, the word @samp{this}) does not have a
+definition of any set of instructions for the computer to carry out.
+
+The slightly odd word, @samp{void-function}, is designed to cover the
+way Emacs Lisp is implemented, which is that when a symbol does not
+have a function definition attached to it, the place that should
+contain the instructions is `void'.
+
+On the other hand, since we were able to add 2 plus 2 successfully, by
+evaluating @code{(+ 2 2)}, we can infer that the symbol @code{+} must
+have a set of instructions for the computer to obey and those
+instructions must be to add the numbers that follow the @code{+}.
+
address@hidden 1250
+In GNU Emacs version 20, and in earlier versions, you will see only
+one line of error message; it will appear in the echo area and look
+like this:
+
address@hidden
+Symbol's function definition is void:@: this
address@hidden smallexample
+
address@hidden
+(Also, your terminal may beep at you---some do, some don't; and others
+blink. This is just a device to get your attention.) The message goes
+away as soon as you type another key, even just to move the cursor.
+
+We know the meaning of the word @samp{Symbol}. It refers to the first
+atom of the list, the word @samp{this}. The word @samp{function}
+refers to the instructions that tell the computer what to do.
+(Technically, the symbol tells the computer where to find the
+instructions, but this is a complication we can ignore for the
+moment.)
+
+The error message can be understood: @samp{Symbol's function
+definition is void:@: this}. The symbol (that is, the word
address@hidden) lacks instructions for the computer to carry out.
+
address@hidden Names & Definitions, Lisp Interpreter, Making Errors, List
Processing
address@hidden node-name, next, previous, up
address@hidden Symbol Names and Function Definitions
address@hidden Symbol names
+
+We can articulate another characteristic of Lisp based on what we have
+discussed so far---an important characteristic: a symbol, like
address@hidden, is not itself the set of instructions for the computer to
+carry out. Instead, the symbol is used, perhaps temporarily, as a way
+of locating the definition or set of instructions. What we see is the
+name through which the instructions can be found. Names of people
+work the same way. I can be referred to as @samp{Bob}; however, I am
+not the letters @samp{B}, @samp{o}, @samp{b} but am, or was, the
+consciousness consistently associated with a particular life-form.
+The name is not me, but it can be used to refer to me.
+
+In Lisp, one set of instructions can be attached to several names.
+For example, the computer instructions for adding numbers can be
+linked to the symbol @code{plus} as well as to the symbol @code{+}
+(and are in some dialects of Lisp). Among humans, I can be referred
+to as @samp{Robert} as well as @samp{Bob} and by other words as well.
+
+On the other hand, a symbol can have only one function definition
+attached to it at a time. Otherwise, the computer would be confused as
+to which definition to use. If this were the case among people, only
+one person in the world could be named @samp{Bob}. However, the function
+definition to which the name refers can be changed readily.
+(@xref{Install, , Install a Function Definition}.)
+
+Since Emacs Lisp is large, it is customary to name symbols in a way
+that identifies the part of Emacs to which the function belongs.
+Thus, all the names for functions that deal with Texinfo start with
address@hidden and those for functions that deal with reading mail
+start with @samp{rmail-}.
+
address@hidden Lisp Interpreter, Evaluation, Names & Definitions, List
Processing
address@hidden node-name, next, previous, up
address@hidden The Lisp Interpreter
address@hidden Lisp interpreter, what it does
address@hidden Interpreter, what it does
+
+Based on what we have seen, we can now start to figure out what the
+Lisp interpreter does when we command it to evaluate a list.
+First, it looks to see whether there is a quote before the list; if
+there is, the interpreter just gives us the list. On the other
+hand, if there is no quote, the interpreter looks at the first element
+in the list and sees whether it has a function definition. If it does,
+the interpreter carries out the instructions in the function definition.
+Otherwise, the interpreter prints an error message.
+
+This is how Lisp works. Simple. There are added complications which we
+will get to in a minute, but these are the fundamentals. Of course, to
+write Lisp programs, you need to know how to write function definitions
+and attach them to names, and how to do this without confusing either
+yourself or the computer.
+
address@hidden
+* Complications:: Variables, Special forms, Lists within.
+* Byte Compiling:: Specially processing code for speed.
address@hidden menu
+
address@hidden Complications, Byte Compiling, Lisp Interpreter, Lisp Interpreter
address@hidden
address@hidden Complications
address@hidden ifnottex
+
+Now, for the first complication. In addition to lists, the Lisp
+interpreter can evaluate a symbol that is not quoted and does not have
+parentheses around it. The Lisp interpreter will attempt to determine
+the symbol's value as a @dfn{variable}. This situation is described
+in the section on variables. (@xref{Variables}.)
+
address@hidden Special form
+The second complication occurs because some functions are unusual and do
+not work in the usual manner. Those that don't are called @dfn{special
+forms}. They are used for special jobs, like defining a function, and
+there are not many of them. In the next few chapters, you will be
+introduced to several of the more important special forms.
+
+The third and final complication is this: if the function that the
+Lisp interpreter is looking at is not a special form, and if it is part
+of a list, the Lisp interpreter looks to see whether the list has a list
+inside of it. If there is an inner list, the Lisp interpreter first
+figures out what it should do with the inside list, and then it works on
+the outside list. If there is yet another list embedded inside the
+inner list, it works on that one first, and so on. It always works on
+the innermost list first. The interpreter works on the innermost list
+first, to evaluate the result of that list. The result may be
+used by the enclosing expression.
+
+Otherwise, the interpreter works left to right, from one expression to
+the next.
+
address@hidden Byte Compiling, , Complications, Lisp Interpreter
address@hidden Byte Compiling
address@hidden Byte compiling
+
+One other aspect of interpreting: the Lisp interpreter is able to
+interpret two kinds of entity: humanly readable code, on which we will
+focus exclusively, and specially processed code, called @dfn{byte
+compiled} code, which is not humanly readable. Byte compiled code
+runs faster than humanly readable code.
+
+You can transform humanly readable code into byte compiled code by
+running one of the compile commands such as @code{byte-compile-file}.
+Byte compiled code is usually stored in a file that ends with a
address@hidden extension rather than a @file{.el} extension. You will
+see both kinds of file in the @file{emacs/lisp} directory; the files
+to read are those with @file{.el} extensions.
+
+As a practical matter, for most things you might do to customize or
+extend Emacs, you do not need to byte compile; and I will not discuss
+the topic here. @xref{Byte Compilation, , Byte Compilation, elisp,
+The GNU Emacs Lisp Reference Manual}, for a full description of byte
+compilation.
+
address@hidden Evaluation, Variables, Lisp Interpreter, List Processing
address@hidden node-name, next, previous, up
address@hidden Evaluation
address@hidden Evaluation
+
+When the Lisp interpreter works on an expression, the term for the
+activity is called @dfn{evaluation}. We say that the interpreter
+`evaluates the expression'. I've used this term several times before.
+The word comes from its use in everyday language, `to ascertain the
+value or amount of; to appraise', according to @cite{Webster's New
+Collegiate Dictionary}.
+
address@hidden
+* How the Interpreter Acts:: Returns and Side Effects...
+* Evaluating Inner Lists:: Lists within lists...
address@hidden menu
+
address@hidden How the Interpreter Acts, Evaluating Inner Lists, Evaluation,
Evaluation
address@hidden
address@hidden How the Lisp Interpreter Acts
address@hidden ifnottex
+
address@hidden @samp{returned value} explained
+After evaluating an expression, the Lisp interpreter will most likely
address@hidden the value that the computer produces by carrying out the
+instructions it found in the function definition, or perhaps it will
+give up on that function and produce an error message. (The interpreter
+may also find itself tossed, so to speak, to a different function or it
+may attempt to repeat continually what it is doing for ever and ever in
+what is called an `infinite loop'. These actions are less common; and
+we can ignore them.) Most frequently, the interpreter returns a value.
+
address@hidden @samp{side effect} defined
+At the same time the interpreter returns a value, it may do something
+else as well, such as move a cursor or copy a file; this other kind of
+action is called a @dfn{side effect}. Actions that we humans think are
+important, such as printing results, are often ``side effects'' to the
+Lisp interpreter. The jargon can sound peculiar, but it turns out that
+it is fairly easy to learn to use side effects.
+
+In summary, evaluating a symbolic expression most commonly causes the
+Lisp interpreter to return a value and perhaps carry out a side effect;
+or else produce an error.
+
address@hidden Evaluating Inner Lists, , How the Interpreter Acts, Evaluation
address@hidden node-name, next, previous, up
address@hidden Evaluating Inner Lists
address@hidden Inner list evaluation
address@hidden Evaluating inner lists
+
+If evaluation applies to a list that is inside another list, the outer
+list may use the value returned by the first evaluation as information
+when the outer list is evaluated. This explains why inner expressions
+are evaluated first: the values they return are used by the outer
+expressions.
+
address@hidden 1250
+We can investigate this process by evaluating another addition example.
+Place your cursor after the following expression and type @kbd{C-x C-e}:
+
address@hidden
+(+ 2 (+ 3 3))
address@hidden smallexample
+
address@hidden
+The number 8 will appear in the echo area.
+
+What happens is that the Lisp interpreter first evaluates the inner
+expression, @code{(+ 3 3)}, for which the value 6 is returned; then it
+evaluates the outer expression as if it were written @code{(+ 2 6)}, which
+returns the value 8. Since there are no more enclosing expressions to
+evaluate, the interpreter prints that value in the echo area.
+
+Now it is easy to understand the name of the command invoked by the
+keystrokes @kbd{C-x C-e}: the name is @code{eval-last-sexp}. The
+letters @code{sexp} are an abbreviation for `symbolic expression', and
address@hidden is an abbreviation for `evaluate'. The command means
+`evaluate last symbolic expression'.
+
+As an experiment, you can try evaluating the expression by putting the
+cursor at the beginning of the next line immediately following the
+expression, or inside the expression.
+
address@hidden 800
+Here is another copy of the expression:
+
address@hidden
+(+ 2 (+ 3 3))
address@hidden smallexample
+
address@hidden
+If you place the cursor at the beginning of the blank line that
+immediately follows the expression and type @kbd{C-x C-e}, you will
+still get the value 8 printed in the echo area. Now try putting the
+cursor inside the expression. If you put it right after the next to
+last parenthesis (so it appears to sit on top of the last parenthesis),
+you will get a 6 printed in the echo area! This is because the command
+evaluates the expression @code{(+ 3 3)}.
+
+Now put the cursor immediately after a number. Type @kbd{C-x C-e} and
+you will get the number itself. In Lisp, if you evaluate a number, you
+get the number itself---this is how numbers differ from symbols. If you
+evaluate a list starting with a symbol like @code{+}, you will get a
+value returned that is the result of the computer carrying out the
+instructions in the function definition attached to that name. If a
+symbol by itself is evaluated, something different happens, as we will
+see in the next section.
+
address@hidden Variables, Arguments, Evaluation, List Processing
address@hidden node-name, next, previous, up
address@hidden Variables
address@hidden Variables
+
+In Emacs Lisp, a symbol can have a value attached to it just as it can
+have a function definition attached to it. The two are different.
+The function definition is a set of instructions that a computer will
+obey. A value, on the other hand, is something, such as number or a
+name, that can vary (which is why such a symbol is called a variable).
+The value of a symbol can be any expression in Lisp, such as a symbol,
+number, list, or string. A symbol that has a value is often called a
address@hidden
+
+A symbol can have both a function definition and a value attached to
+it at the same time. Or it can have just one or the other.
+The two are separate. This is somewhat similar
+to the way the name Cambridge can refer to the city in Massachusetts
+and have some information attached to the name as well, such as
+``great programming center''.
+
address@hidden
+(Incidentally, in Emacs Lisp, a symbol can have two
+other things attached to it, too: a property list and a documentation
+string; these are discussed later.)
address@hidden ignore
+
+Another way to think about this is to imagine a symbol as being a chest
+of drawers. The function definition is put in one drawer, the value in
+another, and so on. What is put in the drawer holding the value can be
+changed without affecting the contents of the drawer holding the
+function definition, and vice-verse.
+
address@hidden
+* fill-column Example::
+* Void Function:: The error message for a symbol
+ without a function.
+* Void Variable:: The error message for a symbol without a value.
address@hidden menu
+
address@hidden fill-column Example, Void Function, Variables, Variables
address@hidden
address@hidden @code{fill-column}, an Example Variable
address@hidden ifnottex
+
address@hidden fill-column, @r{an example variable}
address@hidden Example variable, @code{fill-column}
address@hidden Variable, example of, @code{fill-column}
+The variable @code{fill-column} illustrates a symbol with a value
+attached to it: in every GNU Emacs buffer, this symbol is set to some
+value, usually 72 or 70, but sometimes to some other value. To find the
+value of this symbol, evaluate it by itself. If you are reading this in
+Info inside of GNU Emacs, you can do this by putting the cursor after
+the symbol and typing @kbd{C-x C-e}:
+
address@hidden
+fill-column
address@hidden smallexample
+
address@hidden
+After I typed @kbd{C-x C-e}, Emacs printed the number 72 in my echo
+area. This is the value for which @code{fill-column} is set for me as I
+write this. It may be different for you in your Info buffer. Notice
+that the value returned as a variable is printed in exactly the same way
+as the value returned by a function carrying out its instructions. From
+the point of view of the Lisp interpreter, a value returned is a value
+returned. What kind of expression it came from ceases to matter once
+the value is known.
+
+A symbol can have any value attached to it or, to use the jargon, we can
address@hidden the variable to a value: to a number, such as 72; to a
+string, @code{"such as this"}; to a list, such as @code{(spruce pine
+oak)}; we can even bind a variable to a function definition.
+
+A symbol can be bound to a value in several ways. @xref{set & setq, ,
+Setting the Value of a Variable}, for information about one way to do
+this.
+
address@hidden Void Function, Void Variable, fill-column Example, Variables
address@hidden node-name, next, previous, up
address@hidden Error Message for a Symbol Without a Function
address@hidden Symbol without function error
address@hidden Error for symbol without function
+
+When we evaluated @code{fill-column} to find its value as a variable,
+we did not place parentheses around the word. This is because we did
+not intend to use it as a function name.
+
+If @code{fill-column} were the first or only element of a list, the
+Lisp interpreter would attempt to find the function definition
+attached to it. But @code{fill-column} has no function definition.
+Try evaluating this:
+
address@hidden
+(fill-column)
address@hidden smallexample
+
address@hidden 1250
address@hidden
+In GNU Emacs version 22, you will create a @file{*Backtrace*} buffer
+that says:
+
address@hidden
address@hidden
+---------- Buffer: *Backtrace* ----------
+Debugger entered--Lisp error: (void-function fill-column)
+ (fill-column)
+ eval((fill-column))
+ eval-last-sexp-1(nil)
+ eval-last-sexp(nil)
+ call-interactively(eval-last-sexp)
+---------- Buffer: *Backtrace* ----------
address@hidden group
address@hidden smallexample
+
address@hidden
+(Remember, to quit the debugger and make the debugger window go away,
+type @kbd{q} in the @file{*Backtrace*} buffer.)
+
address@hidden
address@hidden 800
+In GNU Emacs 20 and before, you will produce an error message that says:
+
address@hidden
+Symbol's function definition is void:@: fill-column
address@hidden smallexample
+
address@hidden
+(The message will go away as soon as you move the cursor or type
+another key.)
address@hidden ignore
+
address@hidden Void Variable, , Void Function, Variables
address@hidden node-name, next, previous, up
address@hidden Error Message for a Symbol Without a Value
address@hidden Symbol without value error
address@hidden Error for symbol without value
+
+If you attempt to evaluate a symbol that does not have a value bound to
+it, you will receive an error message. You can see this by
+experimenting with our 2 plus 2 addition. In the following expression,
+put your cursor right after the @code{+}, before the first number 2,
+type @kbd{C-x C-e}:
+
address@hidden
+(+ 2 2)
address@hidden smallexample
+
address@hidden 1500
address@hidden
+In GNU Emacs 22, you will create a @file{*Backtrace*} buffer that
+says:
+
address@hidden
address@hidden
+---------- Buffer: *Backtrace* ----------
+Debugger entered--Lisp error: (void-variable +)
+ eval(+)
+ eval-last-sexp-1(nil)
+ eval-last-sexp(nil)
+ call-interactively(eval-last-sexp)
+---------- Buffer: *Backtrace* ----------
address@hidden group
address@hidden smallexample
+
address@hidden
+(As with the other times we entered the debugger, you can quit by
+typing @kbd{q} in the @file{*Backtrace*} buffer.)
+
+This backtrace is different from the very first error message we saw,
+which said, @samp{Debugger entered--Lisp error: (void-function this)}.
+In this case, the function does not have a value as a variable; while
+in the other error message, the function (the word `this') did not
+have a definition.
+
+In this experiment with the @code{+}, what we did was cause the Lisp
+interpreter to evaluate the @code{+} and look for the value of the
+variable instead of the function definition. We did this by placing the
+cursor right after the symbol rather than after the parenthesis of the
+enclosing list as we did before. As a consequence, the Lisp interpreter
+evaluated the preceding s-expression, which in this case was the
address@hidden by itself.
+
+Since @code{+} does not have a value bound to it, just the function
+definition, the error message reported that the symbol's value as a
+variable was void.
+
address@hidden
address@hidden 800
+In GNU Emacs version 20 and before, your error message will say:
+
address@hidden
+Symbol's value as variable is void:@: +
address@hidden example
+
address@hidden
+The meaning is the same as in GNU Emacs 22.
address@hidden ignore
+
address@hidden Arguments, set & setq, Variables, List Processing
address@hidden node-name, next, previous, up
address@hidden Arguments
address@hidden Arguments
address@hidden Passing information to functions
+
+To see how information is passed to functions, let's look again at
+our old standby, the addition of two plus two. In Lisp, this is written
+as follows:
+
address@hidden
+(+ 2 2)
address@hidden smallexample
+
+If you evaluate this expression, the number 4 will appear in your echo
+area. What the Lisp interpreter does is add the numbers that follow
+the @code{+}.
+
address@hidden @samp{argument} defined
+The numbers added by @code{+} are called the @dfn{arguments} of the
+function @code{+}. These numbers are the information that is given to
+or @dfn{passed} to the function.
+
+The word `argument' comes from the way it is used in mathematics and
+does not refer to a disputation between two people; instead it refers to
+the information presented to the function, in this case, to the
address@hidden In Lisp, the arguments to a function are the atoms or lists
+that follow the function. The values returned by the evaluation of
+these atoms or lists are passed to the function. Different functions
+require different numbers of arguments; some functions require none at
address@hidden is curious to track the path by which the word `argument'
+came to have two different meanings, one in mathematics and the other in
+everyday English. According to the @cite{Oxford English Dictionary},
+the word derives from the Latin for @samp{to make clear, prove}; thus it
+came to mean, by one thread of derivation, `the evidence offered as
+proof', which is to say, `the information offered', which led to its
+meaning in Lisp. But in the other thread of derivation, it came to mean
+`to assert in a manner against which others may make counter
+assertions', which led to the meaning of the word as a disputation.
+(Note here that the English word has two different definitions attached
+to it at the same time. By contrast, in Emacs Lisp, a symbol cannot
+have two different function definitions at the same time.)}
+
address@hidden
+* Data types:: Types of data passed to a function.
+* Args as Variable or List:: An argument can be the value
+ of a variable or list.
+* Variable Number of Arguments:: Some functions may take a
+ variable number of arguments.
+* Wrong Type of Argument:: Passing an argument of the wrong type
+ to a function.
+* message:: A useful function for sending messages.
address@hidden menu
+
address@hidden Data types, Args as Variable or List, Arguments, Arguments
address@hidden node-name, next, previous, up
address@hidden Arguments' Data Types
address@hidden Data types
address@hidden Types of data
address@hidden Arguments' data types
+
+The type of data that should be passed to a function depends on what
+kind of information it uses. The arguments to a function such as
address@hidden must have values that are numbers, since @code{+} adds numbers.
+Other functions use different kinds of data for their arguments.
+
address@hidden 1250
address@hidden concat
+For example, the @code{concat} function links together or unites two or
+more strings of text to produce a string. The arguments are strings.
+Concatenating the two character strings @code{abc}, @code{def} produces
+the single string @code{abcdef}. This can be seen by evaluating the
+following:
+
address@hidden
+(concat "abc" "def")
address@hidden smallexample
+
address@hidden
+The value produced by evaluating this expression is @code{"abcdef"}.
+
+A function such as @code{substring} uses both a string and numbers as
+arguments. The function returns a part of the string, a substring of
+the first argument. This function takes three arguments. Its first
+argument is the string of characters, the second and third arguments are
+numbers that indicate the beginning and end of the substring. The
+numbers are a count of the number of characters (including spaces and
+punctuations) from the beginning of the string.
+
address@hidden 800
+For example, if you evaluate the following:
+
address@hidden
+(substring "The quick brown fox jumped." 16 19)
address@hidden smallexample
+
address@hidden
+you will see @code{"fox"} appear in the echo area. The arguments are the
+string and the two numbers.
+
+Note that the string passed to @code{substring} is a single atom even
+though it is made up of several words separated by spaces. Lisp counts
+everything between the two quotation marks as part of the string,
+including the spaces. You can think of the @code{substring} function as
+a kind of `atom smasher' since it takes an otherwise indivisible atom
+and extracts a part. However, @code{substring} is only able to extract
+a substring from an argument that is a string, not from another type of
+atom such as a number or symbol.
+
address@hidden Args as Variable or List, Variable Number of Arguments, Data
types, Arguments
address@hidden node-name, next, previous, up
address@hidden An Argument as the Value of a Variable or List
+
+An argument can be a symbol that returns a value when it is evaluated.
+For example, when the symbol @code{fill-column} by itself is evaluated,
+it returns a number. This number can be used in an addition.
+
address@hidden 1250
+Position the cursor after the following expression and type @kbd{C-x
+C-e}:
+
address@hidden
+(+ 2 fill-column)
address@hidden smallexample
+
address@hidden
+The value will be a number two more than what you get by evaluating
address@hidden alone. For me, this is 74, because my value of
address@hidden is 72.
+
+As we have just seen, an argument can be a symbol that returns a value
+when evaluated. In addition, an argument can be a list that returns a
+value when it is evaluated. For example, in the following expression,
+the arguments to the function @code{concat} are the strings
address@hidden@code{"The "}} and @address@hidden" red foxes."}} and the list
address@hidden(number-to-string (+ 2 fill-column))}.
+
address@hidden For GNU Emacs 22, need number-to-string
address@hidden
+(concat "The " (number-to-string (+ 2 fill-column)) " red foxes.")
address@hidden smallexample
+
address@hidden
+If you evaluate this expression---and if, as with my Emacs,
address@hidden evaluates to address@hidden"The 74 red foxes."} will
+appear in the echo area. (Note that you must put spaces after the
+word @samp{The} and before the word @samp{red} so they will appear in
+the final string. The function @code{number-to-string} converts the
+integer that the addition function returns to a string.
address@hidden is also known as @code{int-to-string}.)
+
address@hidden Variable Number of Arguments, Wrong Type of Argument, Args as
Variable or List, Arguments
address@hidden node-name, next, previous, up
address@hidden Variable Number of Arguments
address@hidden Variable number of arguments
address@hidden Arguments, variable number of
+
+Some functions, such as @code{concat}, @code{+} or @code{*}, take any
+number of arguments. (The @code{*} is the symbol for multiplication.)
+This can be seen by evaluating each of the following expressions in
+the usual way. What you will see in the echo area is printed in this
+text after @address@hidden, which you may read as `evaluates to'.
+
address@hidden 1250
+In the first set, the functions have no arguments:
+
address@hidden
address@hidden
+(+) @result{} 0
+
+(*) @result{} 1
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+In this set, the functions have one argument each:
+
address@hidden
address@hidden
+(+ 3) @result{} 3
+
+(* 3) @result{} 3
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+In this set, the functions have three arguments each:
+
address@hidden
address@hidden
+(+ 3 4 5) @result{} 12
+
+(* 3 4 5) @result{} 60
address@hidden group
address@hidden smallexample
+
address@hidden Wrong Type of Argument, message, Variable Number of Arguments,
Arguments
address@hidden node-name, next, previous, up
address@hidden Using the Wrong Type Object as an Argument
address@hidden Wrong type of argument
address@hidden Argument, wrong type of
+
+When a function is passed an argument of the wrong type, the Lisp
+interpreter produces an error message. For example, the @code{+}
+function expects the values of its arguments to be numbers. As an
+experiment we can pass it the quoted symbol @code{hello} instead of a
+number. Position the cursor after the following expression and type
address@hidden C-e}:
+
address@hidden
+(+ 2 'hello)
address@hidden smallexample
+
address@hidden
+When you do this you will generate an error message. What has happened
+is that @code{+} has tried to add the 2 to the value returned by
address@hidden'hello}, but the value returned by @code{'hello} is the symbol
address@hidden, not a number. Only numbers can be added. So @code{+}
+could not carry out its addition.
+
address@hidden 1250
+In GNU Emacs version 22, you will create and enter a
address@hidden buffer that says:
+
address@hidden
address@hidden
address@hidden
+---------- Buffer: *Backtrace* ----------
+Debugger entered--Lisp error:
+ (wrong-type-argument number-or-marker-p hello)
+ +(2 hello)
+ eval((+ 2 (quote hello)))
+ eval-last-sexp-1(nil)
+ eval-last-sexp(nil)
+ call-interactively(eval-last-sexp)
+---------- Buffer: *Backtrace* ----------
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+As usual, the error message tries to be helpful and makes sense after you
+learn how to read address@hidden@code{(quote hello)} is an expansion of
+the abbreviation @code{'hello}.}
+
+The first part of the error message is straightforward; it says
address@hidden type argument}. Next comes the mysterious jargon word
address@hidden@samp{number-or-marker-p}}. This word is trying to tell you what
+kind of argument the @code{+} expected.
+
+The symbol @code{number-or-marker-p} says that the Lisp interpreter is
+trying to determine whether the information presented it (the value of
+the argument) is a number or a marker (a special object representing a
+buffer position). What it does is test to see whether the @code{+} is
+being given numbers to add. It also tests to see whether the
+argument is something called a marker, which is a specific feature of
+Emacs Lisp. (In Emacs, locations in a buffer are recorded as markers.
+When the mark is set with the @kbd{C-@@} or @address@hidden command,
+its position is kept as a marker. The mark can be considered a
+number---the number of characters the location is from the beginning
+of the buffer.) In Emacs Lisp, @code{+} can be used to add the
+numeric value of marker positions as numbers.
+
+The @samp{p} of @code{number-or-marker-p} is the embodiment of a
+practice started in the early days of Lisp programming. The @samp{p}
+stands for `predicate'. In the jargon used by the early Lisp
+researchers, a predicate refers to a function to determine whether some
+property is true or false. So the @samp{p} tells us that
address@hidden is the name of a function that determines
+whether it is true or false that the argument supplied is a number or
+a marker. Other Lisp symbols that end in @samp{p} include @code{zerop},
+a function that tests whether its argument has the value of zero, and
address@hidden, a function that tests whether its argument is a list.
+
+Finally, the last part of the error message is the symbol @code{hello}.
+This is the value of the argument that was passed to @code{+}. If the
+addition had been passed the correct type of object, the value passed
+would have been a number, such as 37, rather than a symbol like
address@hidden But then you would not have got the error message.
+
address@hidden
address@hidden 1250
+In GNU Emacs version 20 and before, the echo area displays an error
+message that says:
+
address@hidden
+Wrong type argument:@: number-or-marker-p, hello
address@hidden smallexample
+
+This says, in different words, the same as the top line of the
address@hidden buffer.
address@hidden ignore
+
address@hidden message, , Wrong Type of Argument, Arguments
address@hidden node-name, next, previous, up
address@hidden The @code{message} Function
address@hidden message
+
+Like @code{+}, the @code{message} function takes a variable number of
+arguments. It is used to send messages to the user and is so useful
+that we will describe it here.
+
address@hidden 1250
+A message is printed in the echo area. For example, you can print a
+message in your echo area by evaluating the following list:
+
address@hidden
+(message "This message appears in the echo area!")
address@hidden smallexample
+
+The whole string between double quotation marks is a single argument
+and is printed @i{in toto}. (Note that in this example, the message
+itself will appear in the echo area within double quotes; that is
+because you see the value returned by the @code{message} function. In
+most uses of @code{message} in programs that you write, the text will
+be printed in the echo area as a side-effect, without the quotes.
address@hidden in detail, , @code{multiply-by-seven} in
+detail}, for an example of this.)
+
+However, if there is a @samp{%s} in the quoted string of characters, the
address@hidden function does not print the @samp{%s} as such, but looks
+to the argument that follows the string. It evaluates the second
+argument and prints the value at the location in the string where the
address@hidden is.
+
address@hidden 1250
+You can see this by positioning the cursor after the following
+expression and typing @kbd{C-x C-e}:
+
address@hidden
+(message "The name of this buffer is: %s." (buffer-name))
address@hidden smallexample
+
address@hidden
+In Info, @code{"The name of this buffer is: *info*."} will appear in the
+echo area. The function @code{buffer-name} returns the name of the
+buffer as a string, which the @code{message} function inserts in place
+of @code{%s}.
+
+To print a value as an integer, use @samp{%d} in the same way as
address@hidden For example, to print a message in the echo area that
+states the value of the @code{fill-column}, evaluate the following:
+
address@hidden
+(message "The value of fill-column is %d." fill-column)
address@hidden smallexample
+
address@hidden
+On my system, when I evaluate this list, @code{"The value of
+fill-column is 72."} appears in my echo address@hidden, you
+can use @code{%s} to print a number. It is non-specific. @code{%d}
+prints only the part of a number left of a decimal point, and not
+anything that is not a number.}.
+
+If there is more than one @samp{%s} in the quoted string, the value of
+the first argument following the quoted string is printed at the
+location of the first @samp{%s} and the value of the second argument is
+printed at the location of the second @samp{%s}, and so on.
+
address@hidden 1250
+For example, if you evaluate the following,
+
address@hidden
address@hidden
+(message "There are %d %s in the office!"
+ (- fill-column 14) "pink elephants")
address@hidden group
address@hidden smallexample
+
address@hidden
+a rather whimsical message will appear in your echo area. On my system
+it says, @code{"There are 58 pink elephants in the office!"}.
+
+The expression @code{(- fill-column 14)} is evaluated and the resulting
+number is inserted in place of the @samp{%d}; and the string in double
+quotes, @code{"pink elephants"}, is treated as a single argument and
+inserted in place of the @samp{%s}. (That is to say, a string between
+double quotes evaluates to itself, like a number.)
+
+Finally, here is a somewhat complex example that not only illustrates
+the computation of a number, but also shows how you can use an
+expression within an expression to generate the text that is substituted
+for @samp{%s}:
+
address@hidden
address@hidden
+(message "He saw %d %s"
+ (- fill-column 32)
+ (concat "red "
+ (substring
+ "The quick brown foxes jumped." 16 21)
+ " leaping."))
address@hidden group
address@hidden smallexample
+
+In this example, @code{message} has three arguments: the string,
address@hidden"He saw %d %s"}, the expression, @code{(- fill-column 32)}, and
+the expression beginning with the function @code{concat}. The value
+resulting from the evaluation of @code{(- fill-column 32)} is inserted
+in place of the @samp{%d}; and the value returned by the expression
+beginning with @code{concat} is inserted in place of the @samp{%s}.
+
+When your fill column is 70 and you evaluate the expression, the
+message @code{"He saw 38 red foxes leaping."} appears in your echo
+area.
+
address@hidden set & setq, Summary, Arguments, List Processing
address@hidden node-name, next, previous, up
address@hidden Setting the Value of a Variable
address@hidden Variable, setting value
address@hidden Setting value of variable
+
address@hidden @samp{bind} defined
+There are several ways by which a variable can be given a value. One of
+the ways is to use either the function @code{set} or the function
address@hidden Another way is to use @code{let} (@pxref{let}). (The
+jargon for this process is to @dfn{bind} a variable to a value.)
+
+The following sections not only describe how @code{set} and @code{setq}
+work but also illustrate how arguments are passed.
+
address@hidden
+* Using set:: Setting values.
+* Using setq:: Setting a quoted value.
+* Counting:: Using @code{setq} to count.
address@hidden menu
+
address@hidden Using set, Using setq, set & setq, set & setq
address@hidden node-name, next, previous, up
address@hidden Using @code{set}
address@hidden set
+
+To set the value of the symbol @code{flowers} to the list @code{'(rose
+violet daisy buttercup)}, evaluate the following expression by
+positioning the cursor after the expression and typing @kbd{C-x C-e}.
+
address@hidden
+(set 'flowers '(rose violet daisy buttercup))
address@hidden smallexample
+
address@hidden
+The list @code{(rose violet daisy buttercup)} will appear in the echo
+area. This is what is @emph{returned} by the @code{set} function. As a
+side effect, the symbol @code{flowers} is bound to the list; that is,
+the symbol @code{flowers}, which can be viewed as a variable, is given
+the list as its value. (This process, by the way, illustrates how a
+side effect to the Lisp interpreter, setting the value, can be the
+primary effect that we humans are interested in. This is because every
+Lisp function must return a value if it does not get an error, but it
+will only have a side effect if it is designed to have one.)
+
+After evaluating the @code{set} expression, you can evaluate the symbol
address@hidden and it will return the value you just set. Here is the
+symbol. Place your cursor after it and type @kbd{C-x C-e}.
+
address@hidden
+flowers
address@hidden smallexample
+
address@hidden
+When you evaluate @code{flowers}, the list
address@hidden(rose violet daisy buttercup)} appears in the echo area.
+
+Incidentally, if you evaluate @code{'flowers}, the variable with a quote
+in front of it, what you will see in the echo area is the symbol itself,
address@hidden Here is the quoted symbol, so you can try this:
+
address@hidden
+'flowers
address@hidden smallexample
+
+Note also, that when you use @code{set}, you need to quote both
+arguments to @code{set}, unless you want them evaluated. Since we do
+not want either argument evaluated, neither the variable
address@hidden nor the list @code{(rose violet daisy buttercup)}, both
+are quoted. (When you use @code{set} without quoting its first
+argument, the first argument is evaluated before anything else is
+done. If you did this and @code{flowers} did not have a value
+already, you would get an error message that the @samp{Symbol's value
+as variable is void}; on the other hand, if @code{flowers} did return
+a value after it was evaluated, the @code{set} would attempt to set
+the value that was returned. There are situations where this is the
+right thing for the function to do; but such situations are rare.)
+
address@hidden Using setq, Counting, Using set, set & setq
address@hidden node-name, next, previous, up
address@hidden Using @code{setq}
address@hidden setq
+
+As a practical matter, you almost always quote the first argument to
address@hidden The combination of @code{set} and a quoted first argument
+is so common that it has its own name: the special form @code{setq}.
+This special form is just like @code{set} except that the first argument
+is quoted automatically, so you don't need to type the quote mark
+yourself. Also, as an added convenience, @code{setq} permits you to set
+several different variables to different values, all in one expression.
+
+To set the value of the variable @code{carnivores} to the list
address@hidden'(lion tiger leopard)} using @code{setq}, the following expression
+is used:
+
address@hidden
+(setq carnivores '(lion tiger leopard))
address@hidden smallexample
+
address@hidden
+This is exactly the same as using @code{set} except the first argument
+is automatically quoted by @code{setq}. (The @samp{q} in @code{setq}
+means @code{quote}.)
+
address@hidden 1250
+With @code{set}, the expression would look like this:
+
address@hidden
+(set 'carnivores '(lion tiger leopard))
address@hidden smallexample
+
+Also, @code{setq} can be used to assign different values to
+different variables. The first argument is bound to the value
+of the second argument, the third argument is bound to the value of the
+fourth argument, and so on. For example, you could use the following to
+assign a list of trees to the symbol @code{trees} and a list of herbivores
+to the symbol @code{herbivores}:
+
address@hidden
address@hidden
+(setq trees '(pine fir oak maple)
+ herbivores '(gazelle antelope zebra))
address@hidden group
address@hidden smallexample
+
address@hidden
+(The expression could just as well have been on one line, but it might
+not have fit on a page; and humans find it easier to read nicely
+formatted lists.)
+
+Although I have been using the term `assign', there is another way of
+thinking about the workings of @code{set} and @code{setq}; and that is to
+say that @code{set} and @code{setq} make the symbol @emph{point} to the
+list. This latter way of thinking is very common and in forthcoming
+chapters we shall come upon at least one symbol that has `pointer' as
+part of its name. The name is chosen because the symbol has a value,
+specifically a list, attached to it; or, expressed another way,
+the symbol is set to ``point'' to the list.
+
address@hidden Counting, , Using setq, set & setq
address@hidden node-name, next, previous, up
address@hidden Counting
address@hidden Counting
+
+Here is an example that shows how to use @code{setq} in a counter. You
+might use this to count how many times a part of your program repeats
+itself. First set a variable to zero; then add one to the number each
+time the program repeats itself. To do this, you need a variable that
+serves as a counter, and two expressions: an initial @code{setq}
+expression that sets the counter variable to zero; and a second
address@hidden expression that increments the counter each time it is
+evaluated.
+
address@hidden
address@hidden
+(setq counter 0) ; @r{Let's call this the initializer.}
+
+(setq counter (+ counter 1)) ; @r{This is the incrementer.}
+
+counter ; @r{This is the counter.}
address@hidden group
address@hidden smallexample
+
address@hidden
+(The text following the @samp{;} are comments. @xref{Change a
+defun, , Change a Function Definition}.)
+
+If you evaluate the first of these expressions, the initializer,
address@hidden(setq counter 0)}, and then evaluate the third expression,
address@hidden, the number @code{0} will appear in the echo area. If
+you then evaluate the second expression, the incrementer, @code{(setq
+counter (+ counter 1))}, the counter will get the value 1. So if you
+again evaluate @code{counter}, the number @code{1} will appear in the
+echo area. Each time you evaluate the second expression, the value of
+the counter will be incremented.
+
+When you evaluate the incrementer, @code{(setq counter (+ counter 1))},
+the Lisp interpreter first evaluates the innermost list; this is the
+addition. In order to evaluate this list, it must evaluate the variable
address@hidden and the number @code{1}. When it evaluates the variable
address@hidden, it receives its current value. It passes this value and
+the number @code{1} to the @code{+} which adds them together. The sum
+is then returned as the value of the inner list and passed to the
address@hidden which sets the variable @code{counter} to this new value.
+Thus, the value of the variable, @code{counter}, is changed.
+
address@hidden Summary, Error Message Exercises, set & setq, List Processing
address@hidden node-name, next, previous, up
address@hidden Summary
+
+Learning Lisp is like climbing a hill in which the first part is the
+steepest. You have now climbed the most difficult part; what remains
+becomes easier as you progress onwards.
+
address@hidden 1000
+In summary,
+
address@hidden @bullet
+
address@hidden
+Lisp programs are made up of expressions, which are lists or single atoms.
+
address@hidden
+Lists are made up of zero or more atoms or inner lists, separated by
whitespace and
+surrounded by parentheses. A list can be empty.
+
address@hidden
+Atoms are multi-character symbols, like @code{forward-paragraph}, single
+character symbols like @code{+}, strings of characters between double
+quotation marks, or numbers.
+
address@hidden
+A number evaluates to itself.
+
address@hidden
+A string between double quotes also evaluates to itself.
+
address@hidden
+When you evaluate a symbol by itself, its value is returned.
+
address@hidden
+When you evaluate a list, the Lisp interpreter looks at the first symbol
+in the list and then at the function definition bound to that symbol.
+Then the instructions in the function definition are carried out.
+
address@hidden
+A single quotation mark,
address@hidden
+'
address@hidden ifinfo
address@hidden
address@hidden'}
address@hidden ifnotinfo
+, tells the Lisp interpreter that it should
+return the following expression as written, and not evaluate it as it
+would if the quote were not there.
+
address@hidden
+Arguments are the information passed to a function. The arguments to a
+function are computed by evaluating the rest of the elements of the list
+of which the function is the first element.
+
address@hidden
+A function always returns a value when it is evaluated (unless it gets
+an error); in addition, it may also carry out some action called a
+``side effect''. In many cases, a function's primary purpose is to
+create a side effect.
address@hidden itemize
+
address@hidden Error Message Exercises, , Summary, List Processing
address@hidden node-name, next, previous, up
address@hidden Exercises
+
+A few simple exercises:
+
address@hidden @bullet
address@hidden
+Generate an error message by evaluating an appropriate symbol that is
+not within parentheses.
+
address@hidden
+Generate an error message by evaluating an appropriate symbol that is
+between parentheses.
+
address@hidden
+Create a counter that increments by two rather than one.
+
address@hidden
+Write an expression that prints a message in the echo area when
+evaluated.
address@hidden itemize
+
address@hidden Practicing Evaluation, Writing Defuns, List Processing, Top
address@hidden node-name, next, previous, up
address@hidden Practicing Evaluation
address@hidden Practicing evaluation
address@hidden Evaluation practice
+
+Before learning how to write a function definition in Emacs Lisp, it is
+useful to spend a little time evaluating various expressions that have
+already been written. These expressions will be lists with the
+functions as their first (and often only) element. Since some of the
+functions associated with buffers are both simple and interesting, we
+will start with those. In this section, we will evaluate a few of
+these. In another section, we will study the code of several other
+buffer-related functions, to see how they were written.
+
address@hidden
+* How to Evaluate:: Typing editing commands or @kbd{C-x C-e}
+ causes evaluation.
+* Buffer Names:: Buffers and files are different.
+* Getting Buffers:: Getting a buffer itself, not merely its name.
+* Switching Buffers:: How to change to another buffer.
+* Buffer Size & Locations:: Where point is located and the size of
+ the buffer.
+* Evaluation Exercise::
address@hidden menu
+
address@hidden How to Evaluate, Buffer Names, Practicing Evaluation, Practicing
Evaluation
address@hidden
address@hidden How to Evaluate
address@hidden ifnottex
+
address@hidden you give an editing command} to Emacs Lisp, such as the
+command to move the cursor or to scroll the screen, @i{you are evaluating
+an expression,} the first element of which is a function. @i{This is
+how Emacs works.}
+
address@hidden @samp{interactive function} defined
address@hidden @samp{command} defined
+When you type keys, you cause the Lisp interpreter to evaluate an
+expression and that is how you get your results. Even typing plain text
+involves evaluating an Emacs Lisp function, in this case, one that uses
address@hidden, which simply inserts the character you
+typed. The functions you evaluate by typing keystrokes are called
address@hidden functions, or @dfn{commands}; how you make a function
+interactive will be illustrated in the chapter on how to write function
+definitions. @xref{Interactive, , Making a Function Interactive}.
+
+In addition to typing keyboard commands, we have seen a second way to
+evaluate an expression: by positioning the cursor after a list and
+typing @kbd{C-x C-e}. This is what we will do in the rest of this
+section. There are other ways to evaluate an expression as well; these
+will be described as we come to them.
+
+Besides being used for practicing evaluation, the functions shown in the
+next few sections are important in their own right. A study of these
+functions makes clear the distinction between buffers and files, how to
+switch to a buffer, and how to determine a location within it.
+
address@hidden Buffer Names, Getting Buffers, How to Evaluate, Practicing
Evaluation
address@hidden node-name, next, previous, up
address@hidden Buffer Names
address@hidden buffer-name
address@hidden buffer-file-name
+
+The two functions, @code{buffer-name} and @code{buffer-file-name}, show
+the difference between a file and a buffer. When you evaluate the
+following expression, @code{(buffer-name)}, the name of the buffer
+appears in the echo area. When you evaluate @code{(buffer-file-name)},
+the name of the file to which the buffer refers appears in the echo
+area. Usually, the name returned by @code{(buffer-name)} is the same as
+the name of the file to which it refers, and the name returned by
address@hidden(buffer-file-name)} is the full path-name of the file.
+
+A file and a buffer are two different entities. A file is information
+recorded permanently in the computer (unless you delete it). A buffer,
+on the other hand, is information inside of Emacs that will vanish at
+the end of the editing session (or when you kill the buffer). Usually,
+a buffer contains information that you have copied from a file; we say
+the buffer is @dfn{visiting} that file. This copy is what you work on
+and modify. Changes to the buffer do not change the file, until you
+save the buffer. When you save the buffer, the buffer is copied to the file
+and is thus saved permanently.
+
address@hidden 1250
+If you are reading this in Info inside of GNU Emacs, you can evaluate
+each of the following expressions by positioning the cursor after it and
+typing @kbd{C-x C-e}.
+
address@hidden
address@hidden
+(buffer-name)
+
+(buffer-file-name)
address@hidden group
address@hidden example
+
address@hidden
+When I do this in Info, the value returned by evaluating
address@hidden(buffer-name)} is @file{"*info*"}, and the value returned by
+evaluating @code{(buffer-file-name)} is @file{nil}.
+
+On the other hand, while I am writing this Introduction, the value
+returned by evaluating @code{(buffer-name)} is
address@hidden"introduction.texinfo"}, and the value returned by evaluating
address@hidden(buffer-file-name)} is
address@hidden"/gnu/work/intro/introduction.texinfo"}.
+
address@hidden @code{nil}, history of word
+The former is the name of the buffer and the latter is the name of the
+file. In Info, the buffer name is @file{"*info*"}. Info does not
+point to any file, so the result of evaluating
address@hidden(buffer-file-name)} is @file{nil}. The symbol @code{nil} is
+from the Latin word for `nothing'; in this case, it means that the
+buffer is not associated with any file. (In Lisp, @code{nil} is also
+used to mean `false' and is a synonym for the empty list, @code{()}.)
+
+When I am writing, the name of my buffer is
address@hidden"introduction.texinfo"}. The name of the file to which it
+points is @file{"/gnu/work/intro/introduction.texinfo"}.
+
+(In the expressions, the parentheses tell the Lisp interpreter to
+treat @address@hidden and @address@hidden as
+functions; without the parentheses, the interpreter would attempt to
+evaluate the symbols as variables. @xref{Variables}.)
+
+In spite of the distinction between files and buffers, you will often
+find that people refer to a file when they mean a buffer and vice-verse.
+Indeed, most people say, ``I am editing a file,'' rather than saying,
+``I am editing a buffer which I will soon save to a file.'' It is
+almost always clear from context what people mean. When dealing with
+computer programs, however, it is important to keep the distinction in mind,
+since the computer is not as smart as a person.
+
address@hidden Buffer, history of word
+The word `buffer', by the way, comes from the meaning of the word as a
+cushion that deadens the force of a collision. In early computers, a
+buffer cushioned the interaction between files and the computer's
+central processing unit. The drums or tapes that held a file and the
+central processing unit were pieces of equipment that were very
+different from each other, working at their own speeds, in spurts. The
+buffer made it possible for them to work together effectively.
+Eventually, the buffer grew from being an intermediary, a temporary
+holding place, to being the place where work is done. This
+transformation is rather like that of a small seaport that grew into a
+great city: once it was merely the place where cargo was warehoused
+temporarily before being loaded onto ships; then it became a business
+and cultural center in its own right.
+
+Not all buffers are associated with files. For example, a
address@hidden buffer does not visit any file. Similarly, a
address@hidden buffer is not associated with any file.
+
+In the old days, when you lacked a @file{~/.emacs} file and started an
+Emacs session by typing the command @code{emacs} alone, without naming
+any files, Emacs started with the @file{*scratch*} buffer visible.
+Nowadays, you will see a splash screen. You can follow one of the
+commands suggested on the splash screen, visit a file, or press the
+spacebar to reach the @file{*scratch*} buffer.
+
+If you switch to the @file{*scratch*} buffer, type
address@hidden(buffer-name)}, position the cursor after it, and then type
address@hidden C-e} to evaluate the expression. The name @code{"*scratch*"}
+will be returned and will appear in the echo area. @code{"*scratch*"}
+is the name of the buffer. When you type @code{(buffer-file-name)} in
+the @file{*scratch*} buffer and evaluate that, @code{nil} will appear
+in the echo area, just as it does when you evaluate
address@hidden(buffer-file-name)} in Info.
+
+Incidentally, if you are in the @file{*scratch*} buffer and want the
+value returned by an expression to appear in the @file{*scratch*}
+buffer itself rather than in the echo area, type @kbd{C-u C-x C-e}
+instead of @kbd{C-x C-e}. This causes the value returned to appear
+after the expression. The buffer will look like this:
+
address@hidden
+(buffer-name)"*scratch*"
address@hidden smallexample
+
address@hidden
+You cannot do this in Info since Info is read-only and it will not allow
+you to change the contents of the buffer. But you can do this in any
+buffer you can edit; and when you write code or documentation (such as
+this book), this feature is very useful.
+
address@hidden Getting Buffers, Switching Buffers, Buffer Names, Practicing
Evaluation
address@hidden node-name, next, previous, up
address@hidden Getting Buffers
address@hidden current-buffer
address@hidden other-buffer
address@hidden Getting a buffer
+
+The @code{buffer-name} function returns the @emph{name} of the buffer;
+to get the buffer @emph{itself}, a different function is needed: the
address@hidden function. If you use this function in code, what
+you get is the buffer itself.
+
+A name and the object or entity to which the name refers are different
+from each other. You are not your name. You are a person to whom
+others refer by name. If you ask to speak to George and someone hands you
+a card with the letters @samp{G}, @samp{e}, @samp{o}, @samp{r},
address@hidden, and @samp{e} written on it, you might be amused, but you would
+not be satisfied. You do not want to speak to the name, but to the
+person to whom the name refers. A buffer is similar: the name of the
+scratch buffer is @file{*scratch*}, but the name is not the buffer. To
+get a buffer itself, you need to use a function such as
address@hidden
+
+However, there is a slight complication: if you evaluate
address@hidden in an expression on its own, as we will do here,
+what you see is a printed representation of the name of the buffer
+without the contents of the buffer. Emacs works this way for two
+reasons: the buffer may be thousands of lines long---too long to be
+conveniently displayed; and, another buffer may have the same contents
+but a different name, and it is important to distinguish between them.
+
address@hidden 800
+Here is an expression containing the function:
+
address@hidden
+(current-buffer)
address@hidden smallexample
+
address@hidden
+If you evaluate this expression in Info in Emacs in the usual way,
address@hidden<buffer *info*>} will appear in the echo area. The special
+format indicates that the buffer itself is being returned, rather than
+just its name.
+
+Incidentally, while you can type a number or symbol into a program, you
+cannot do that with the printed representation of a buffer: the only way
+to get a buffer itself is with a function such as @code{current-buffer}.
+
+A related function is @code{other-buffer}. This returns the most
+recently selected buffer other than the one you are in currently, not
+a printed representation of its name. If you have recently switched
+back and forth from the @file{*scratch*} buffer, @code{other-buffer}
+will return that buffer.
+
address@hidden 800
+You can see this by evaluating the expression:
+
address@hidden
+(other-buffer)
address@hidden smallexample
+
address@hidden
+You should see @file{#<buffer *scratch*>} appear in the echo area, or
+the name of whatever other buffer you switched back from most
address@hidden, by default, if the buffer from which you
+just switched is visible to you in another window, @code{other-buffer}
+will choose the most recent buffer that you cannot see; this is a
+subtlety that I often forget.}.
+
address@hidden Switching Buffers, Buffer Size & Locations, Getting Buffers,
Practicing Evaluation
address@hidden node-name, next, previous, up
address@hidden Switching Buffers
address@hidden switch-to-buffer
address@hidden set-buffer
address@hidden Switching to a buffer
+
+The @code{other-buffer} function actually provides a buffer when it is
+used as an argument to a function that requires one. We can see this
+by using @code{other-buffer} and @code{switch-to-buffer} to switch to a
+different buffer.
+
+But first, a brief introduction to the @code{switch-to-buffer}
+function. When you switched back and forth from Info to the
address@hidden buffer to evaluate @code{(buffer-name)}, you most
+likely typed @kbd{C-x b} and then typed @address@hidden
+rather, to save typing, you probably only typed @kbd{RET} if the
+default buffer was @file{*scratch*}, or if it was different, then you
+typed just part of the name, such as @code{*sc}, pressed your
address@hidden key to cause it to expand to the full name, and then typed
+your @kbd{RET} key.} when prompted in the minibuffer for the name of
+the buffer to which you wanted to switch. The keystrokes, @kbd{C-x
+b}, cause the Lisp interpreter to evaluate the interactive function
address@hidden As we said before, this is how Emacs works:
+different keystrokes call or run different functions. For example,
address@hidden calls @code{forward-char}, @kbd{M-e} calls
address@hidden, and so on.
+
+By writing @code{switch-to-buffer} in an expression, and giving it a
+buffer to switch to, we can switch buffers just the way @kbd{C-x b}
+does.
+
address@hidden 1000
+Here is the Lisp expression:
+
address@hidden
+(switch-to-buffer (other-buffer))
address@hidden smallexample
+
address@hidden
+The symbol @code{switch-to-buffer} is the first element of the list,
+so the Lisp interpreter will treat it as a function and carry out the
+instructions that are attached to it. But before doing that, the
+interpreter will note that @code{other-buffer} is inside parentheses
+and work on that symbol first. @code{other-buffer} is the first (and
+in this case, the only) element of this list, so the Lisp interpreter
+calls or runs the function. It returns another buffer. Next, the
+interpreter runs @code{switch-to-buffer}, passing to it, as an
+argument, the other buffer, which is what Emacs will switch to. If
+you are reading this in Info, try this now. Evaluate the expression.
+(To get back, type @kbd{C-x b @key{RET}}.)@footnote{Remember, this
+expression will move you to your most recent other buffer that you
+cannot see. If you really want to go to your most recently selected
+buffer, even if you can still see it, you need to evaluate the
+following more complex expression:
+
address@hidden
+(switch-to-buffer (other-buffer (current-buffer) t))
address@hidden smallexample
+
address@hidden noindent
+In this case, the first argument to @code{other-buffer} tells it which
+buffer to skip---the current one---and the second argument tells
address@hidden it is OK to switch to a visible buffer.
+In regular use, @code{switch-to-buffer} takes you to an invisible
+window since you would most likely use @kbd{C-x o} (@code{other-window})
+to go to another visible buffer.}
+
+In the programming examples in later sections of this document, you will
+see the function @code{set-buffer} more often than
address@hidden This is because of a difference between
+computer programs and humans: humans have eyes and expect to see the
+buffer on which they are working on their computer terminals. This is
+so obvious, it almost goes without saying. However, programs do not
+have eyes. When a computer program works on a buffer, that buffer does
+not need to be visible on the screen.
+
address@hidden is designed for humans and does two different
+things: it switches the buffer to which Emacs' attention is directed; and
+it switches the buffer displayed in the window to the new buffer.
address@hidden, on the other hand, does only one thing: it switches
+the attention of the computer program to a different buffer. The buffer
+on the screen remains unchanged (of course, normally nothing happens
+there until the command finishes running).
+
address@hidden @samp{call} defined
+Also, we have just introduced another jargon term, the word @dfn{call}.
+When you evaluate a list in which the first symbol is a function, you
+are calling that function. The use of the term comes from the notion of
+the function as an entity that can do something for you if you `call'
+it---just as a plumber is an entity who can fix a leak if you call him
+or her.
+
address@hidden Buffer Size & Locations, Evaluation Exercise, Switching Buffers,
Practicing Evaluation
address@hidden node-name, next, previous, up
address@hidden Buffer Size and the Location of Point
address@hidden Size of buffer
address@hidden Buffer size
address@hidden Point location
address@hidden Location of point
+
+Finally, let's look at several rather simple functions,
address@hidden, @code{point}, @code{point-min}, and
address@hidden These give information about the size of a buffer and
+the location of point within it.
+
+The function @code{buffer-size} tells you the size of the current
+buffer; that is, the function returns a count of the number of
+characters in the buffer.
+
address@hidden
+(buffer-size)
address@hidden smallexample
+
address@hidden
+You can evaluate this in the usual way, by positioning the
+cursor after the expression and typing @kbd{C-x C-e}.
+
address@hidden @samp{point} defined
+In Emacs, the current position of the cursor is called @dfn{point}.
+The expression @code{(point)} returns a number that tells you where the
+cursor is located as a count of the number of characters from the
+beginning of the buffer up to point.
+
address@hidden 1250
+You can see the character count for point in this buffer by evaluating
+the following expression in the usual way:
+
address@hidden
+(point)
address@hidden smallexample
+
address@hidden
+As I write this, the value of @code{point} is 65724. The @code{point}
+function is frequently used in some of the examples later in this
+book.
+
address@hidden 1250
+The value of point depends, of course, on its location within the
+buffer. If you evaluate point in this spot, the number will be larger:
+
address@hidden
+(point)
address@hidden smallexample
+
address@hidden
+For me, the value of point in this location is 66043, which means that
+there are 319 characters (including spaces) between the two
+expressions. (Doubtless, you will see different numbers, since I will
+have edited this since I first evaluated point.)
+
address@hidden @samp{narrowing} defined
+The function @code{point-min} is somewhat similar to @code{point}, but
+it returns the value of the minimum permissible value of point in the
+current buffer. This is the number 1 unless @dfn{narrowing} is in
+effect. (Narrowing is a mechanism whereby you can restrict yourself,
+or a program, to operations on just a part of a buffer.
address@hidden & Widening, , Narrowing and Widening}.) Likewise, the
+function @code{point-max} returns the value of the maximum permissible
+value of point in the current buffer.
+
address@hidden Evaluation Exercise, , Buffer Size & Locations, Practicing
Evaluation
address@hidden Exercise
+
+Find a file with which you are working and move towards its middle.
+Find its buffer name, file name, length, and your position in the file.
+
address@hidden Writing Defuns, Buffer Walk Through, Practicing Evaluation, Top
address@hidden node-name, next, previous, up
address@hidden How To Write Function Definitions
address@hidden Definition writing
address@hidden Function definition writing
address@hidden Writing a function definition
+
+When the Lisp interpreter evaluates a list, it looks to see whether the
+first symbol on the list has a function definition attached to it; or,
+put another way, whether the symbol points to a function definition. If
+it does, the computer carries out the instructions in the definition. A
+symbol that has a function definition is called, simply, a function
+(although, properly speaking, the definition is the function and the
+symbol refers to it.)
+
address@hidden
+* Primitive Functions::
+* defun:: The @code{defun} special form.
+* Install:: Install a function definition.
+* Interactive:: Making a function interactive.
+* Interactive Options:: Different options for @code{interactive}.
+* Permanent Installation:: Installing code permanently.
+* let:: Creating and initializing local variables.
+* if:: What if?
+* else:: If--then--else expressions.
+* Truth & Falsehood:: What Lisp considers false and true.
+* save-excursion:: Keeping track of point, mark, and buffer.
+* Review::
+* defun Exercises::
address@hidden menu
+
address@hidden Primitive Functions, defun, Writing Defuns, Writing Defuns
address@hidden
address@hidden An Aside about Primitive Functions
address@hidden ifnottex
address@hidden Primitive functions
address@hidden Functions, primitive
+
address@hidden C language primitives
address@hidden Primitives written in C
+All functions are defined in terms of other functions, except for a few
address@hidden functions that are written in the C programming
+language. When you write functions' definitions, you will write them in
+Emacs Lisp and use other functions as your building blocks. Some of the
+functions you will use will themselves be written in Emacs Lisp (perhaps
+by you) and some will be primitives written in C. The primitive
+functions are used exactly like those written in Emacs Lisp and behave
+like them. They are written in C so we can easily run GNU Emacs on any
+computer that has sufficient power and can run C.
+
+Let me re-emphasize this: when you write code in Emacs Lisp, you do not
+distinguish between the use of functions written in C and the use of
+functions written in Emacs Lisp. The difference is irrelevant. I
+mention the distinction only because it is interesting to know. Indeed,
+unless you investigate, you won't know whether an already-written
+function is written in Emacs Lisp or C.
+
address@hidden defun, Install, Primitive Functions, Writing Defuns
address@hidden node-name, next, previous, up
address@hidden The @code{defun} Special Form
address@hidden defun
address@hidden Special form of @code{defun}
+
address@hidden @samp{function definition} defined
+In Lisp, a symbol such as @code{mark-whole-buffer} has code attached to
+it that tells the computer what to do when the function is called.
+This code is called the @dfn{function definition} and is created by
+evaluating a Lisp expression that starts with the symbol @code{defun}
+(which is an abbreviation for @emph{define function}). Because
address@hidden does not evaluate its arguments in the usual way, it is
+called a @dfn{special form}.
+
+In subsequent sections, we will look at function definitions from the
+Emacs source code, such as @code{mark-whole-buffer}. In this section,
+we will describe a simple function definition so you can see how it
+looks. This function definition uses arithmetic because it makes for a
+simple example. Some people dislike examples using arithmetic; however,
+if you are such a person, do not despair. Hardly any of the code we
+will study in the remainder of this introduction involves arithmetic or
+mathematics. The examples mostly involve text in one way or another.
+
+A function definition has up to five parts following the word
address@hidden:
+
address@hidden
address@hidden
+The name of the symbol to which the function definition should be
+attached.
+
address@hidden
+A list of the arguments that will be passed to the function. If no
+arguments will be passed to the function, this is an empty list,
address@hidden()}.
+
address@hidden
+Documentation describing the function. (Technically optional, but
+strongly recommended.)
+
address@hidden
+Optionally, an expression to make the function interactive so you can
+use it by typing @kbd{M-x} and then the name of the function; or by
+typing an appropriate key or keychord.
+
address@hidden @samp{body} defined
address@hidden
+The code that instructs the computer what to do: the @dfn{body} of the
+function definition.
address@hidden enumerate
+
+It is helpful to think of the five parts of a function definition as
+being organized in a template, with slots for each part:
+
address@hidden
address@hidden
+(defun @var{function-name} (@address@hidden)
+ "@address@hidden"
+ (interactive @var{argument-passing-info}) ; @r{optional}
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+As an example, here is the code for a function that multiplies its
+argument by 7. (This example is not interactive. @xref{Interactive,
+, Making a Function Interactive}, for that information.)
+
address@hidden
address@hidden
+(defun multiply-by-seven (number)
+ "Multiply NUMBER by seven."
+ (* 7 number))
address@hidden group
address@hidden smallexample
+
+This definition begins with a parenthesis and the symbol @code{defun},
+followed by the name of the function.
+
address@hidden @samp{argument list} defined
+The name of the function is followed by a list that contains the
+arguments that will be passed to the function. This list is called
+the @dfn{argument list}. In this example, the list has only one
+element, the symbol, @code{number}. When the function is used, the
+symbol will be bound to the value that is used as the argument to the
+function.
+
+Instead of choosing the word @code{number} for the name of the argument,
+I could have picked any other name. For example, I could have chosen
+the word @code{multiplicand}. I picked the word `number' because it
+tells what kind of value is intended for this slot; but I could just as
+well have chosen the word `multiplicand' to indicate the role that the
+value placed in this slot will play in the workings of the function. I
+could have called it @code{foogle}, but that would have been a bad
+choice because it would not tell humans what it means. The choice of
+name is up to the programmer and should be chosen to make the meaning of
+the function clear.
+
+Indeed, you can choose any name you wish for a symbol in an argument
+list, even the name of a symbol used in some other function: the name
+you use in an argument list is private to that particular definition.
+In that definition, the name refers to a different entity than any use
+of the same name outside the function definition. Suppose you have a
+nick-name `Shorty' in your family; when your family members refer to
+`Shorty', they mean you. But outside your family, in a movie, for
+example, the name `Shorty' refers to someone else. Because a name in an
+argument list is private to the function definition, you can change the
+value of such a symbol inside the body of a function without changing
+its value outside the function. The effect is similar to that produced
+by a @code{let} expression. (@xref{let, , @code{let}}.)
+
address@hidden
+Note also that we discuss the word `number' in two different ways: as a
+symbol that appears in the code, and as the name of something that will
+be replaced by a something else during the evaluation of the function.
+In the first case, @code{number} is a symbol, not a number; it happens
+that within the function, it is a variable who value is the number in
+question, but our primary interest in it is as a symbol. On the other
+hand, when we are talking about the function, our interest is that we
+will substitute a number for the word @var{number}. To keep this
+distinction clear, we use different typography for the two
+circumstances. When we talk about this function, or about how it works,
+we refer to this number by writing @var{number}. In the function
+itself, we refer to it by writing @code{number}.
address@hidden ignore
+
+The argument list is followed by the documentation string that
+describes the function. This is what you see when you type
address@hidden@kbd{C-h f}} and the name of a function. Incidentally, when you
+write a documentation string like this, you should make the first line
+a complete sentence since some commands, such as @code{apropos}, print
+only the first line of a multi-line documentation string. Also, you
+should not indent the second line of a documentation string, if you
+have one, because that looks odd when you use @kbd{C-h f}
+(@code{describe-function}). The documentation string is optional, but
+it is so useful, it should be included in almost every function you
+write.
+
address@hidden * @r{(multiplication)}
+The third line of the example consists of the body of the function
+definition. (Most functions' definitions, of course, are longer than
+this.) In this function, the body is the list, @code{(* 7 number)}, which
+says to multiply the value of @var{number} by 7. (In Emacs Lisp,
address@hidden is the function for multiplication, just as @code{+} is the
+function for addition.)
+
+When you use the @code{multiply-by-seven} function, the argument
address@hidden evaluates to the actual number you want used. Here is an
+example that shows how @code{multiply-by-seven} is used; but don't try
+to evaluate this yet!
+
address@hidden
+(multiply-by-seven 3)
address@hidden smallexample
+
address@hidden
+The symbol @code{number}, specified in the function definition in the
+next section, is given or ``bound to'' the value 3 in the actual use of
+the function. Note that although @code{number} was inside parentheses
+in the function definition, the argument passed to the
address@hidden function is not in parentheses. The
+parentheses are written in the function definition so the computer can
+figure out where the argument list ends and the rest of the function
+definition begins.
+
+If you evaluate this example, you are likely to get an error message.
+(Go ahead, try it!) This is because we have written the function
+definition, but not yet told the computer about the definition---we have
+not yet installed (or `loaded') the function definition in Emacs.
+Installing a function is the process that tells the Lisp interpreter the
+definition of the function. Installation is described in the next
+section.
+
address@hidden Install, Interactive, defun, Writing Defuns
address@hidden node-name, next, previous, up
address@hidden Install a Function Definition
address@hidden Install a Function Definition
address@hidden Definition installation
address@hidden Function definition installation
+
+If you are reading this inside of Info in Emacs, you can try out the
address@hidden function by first evaluating the function
+definition and then evaluating @code{(multiply-by-seven 3)}. A copy of
+the function definition follows. Place the cursor after the last
+parenthesis of the function definition and type @kbd{C-x C-e}. When you
+do this, @code{multiply-by-seven} will appear in the echo area. (What
+this means is that when a function definition is evaluated, the value it
+returns is the name of the defined function.) At the same time, this
+action installs the function definition.
+
address@hidden
address@hidden
+(defun multiply-by-seven (number)
+ "Multiply NUMBER by seven."
+ (* 7 number))
address@hidden group
address@hidden smallexample
+
address@hidden
+By evaluating this @code{defun}, you have just installed
address@hidden in Emacs. The function is now just as much a
+part of Emacs as @code{forward-word} or any other editing function you
+use. (@code{multiply-by-seven} will stay installed until you quit
+Emacs. To reload code automatically whenever you start Emacs, see
address@hidden Installation, , Installing Code Permanently}.)
+
address@hidden
+* Effect of installation::
+* Change a defun:: How to change a function definition.
address@hidden menu
+
address@hidden Effect of installation, Change a defun, Install, Install
address@hidden
address@hidden The effect of installation
address@hidden ifnottex
+
+You can see the effect of installing @code{multiply-by-seven} by
+evaluating the following sample. Place the cursor after the following
+expression and type @kbd{C-x C-e}. The number 21 will appear in the
+echo area.
+
address@hidden
+(multiply-by-seven 3)
address@hidden smallexample
+
+If you wish, you can read the documentation for the function by typing
address@hidden f} (@code{describe-function}) and then the name of the
+function, @code{multiply-by-seven}. When you do this, a
address@hidden window will appear on your screen that says:
+
address@hidden
address@hidden
+multiply-by-seven is a Lisp function.
+(multiply-by-seven NUMBER)
+
+Multiply NUMBER by seven.
address@hidden group
address@hidden smallexample
+
address@hidden
+(To return to a single window on your screen, type @kbd{C-x 1}.)
+
address@hidden Change a defun, , Effect of installation, Install
address@hidden node-name, next, previous, up
address@hidden Change a Function Definition
address@hidden Changing a function definition
address@hidden Function definition, how to change
address@hidden Definition, how to change
+
+If you want to change the code in @code{multiply-by-seven}, just rewrite
+it. To install the new version in place of the old one, evaluate the
+function definition again. This is how you modify code in Emacs. It is
+very simple.
+
+As an example, you can change the @code{multiply-by-seven} function to
+add the number to itself seven times instead of multiplying the number
+by seven. It produces the same answer, but by a different path. At
+the same time, we will add a comment to the code; a comment is text
+that the Lisp interpreter ignores, but that a human reader may find
+useful or enlightening. The comment is that this is the ``second
+version''.
+
address@hidden
address@hidden
+(defun multiply-by-seven (number) ; @r{Second version.}
+ "Multiply NUMBER by seven."
+ (+ number number number number number number number))
address@hidden group
address@hidden smallexample
+
address@hidden Comments in Lisp code
+The comment follows a semicolon, @samp{;}. In Lisp, everything on a
+line that follows a semicolon is a comment. The end of the line is the
+end of the comment. To stretch a comment over two or more lines, begin
+each line with a semicolon.
+
address@hidden a .emacs File, , Beginning a @file{.emacs}
+File}, and @ref{Comments, , Comments, elisp, The GNU Emacs Lisp
+Reference Manual}, for more about comments.
+
+You can install this version of the @code{multiply-by-seven} function by
+evaluating it in the same way you evaluated the first function: place
+the cursor after the last parenthesis and type @kbd{C-x C-e}.
+
+In summary, this is how you write code in Emacs Lisp: you write a
+function; install it; test it; and then make fixes or enhancements and
+install it again.
+
address@hidden Interactive, Interactive Options, Install, Writing Defuns
address@hidden node-name, next, previous, up
address@hidden Make a Function Interactive
address@hidden Interactive functions
address@hidden interactive
+
+You make a function interactive by placing a list that begins with
+the special form @code{interactive} immediately after the
+documentation. A user can invoke an interactive function by typing
address@hidden and then the name of the function; or by typing the keys to
+which it is bound, for example, by typing @kbd{C-n} for
address@hidden or @kbd{C-x h} for @code{mark-whole-buffer}.
+
+Interestingly, when you call an interactive function interactively,
+the value returned is not automatically displayed in the echo area.
+This is because you often call an interactive function for its side
+effects, such as moving forward by a word or line, and not for the
+value returned. If the returned value were displayed in the echo area
+each time you typed a key, it would be very distracting.
+
address@hidden
+* Interactive multiply-by-seven:: An overview.
+* multiply-by-seven in detail:: The interactive version.
address@hidden menu
+
address@hidden Interactive multiply-by-seven, multiply-by-seven in detail,
Interactive, Interactive
address@hidden
address@hidden An Interactive @code{multiply-by-seven}, An Overview
address@hidden ifnottex
+
+Both the use of the special form @code{interactive} and one way to
+display a value in the echo area can be illustrated by creating an
+interactive version of @code{multiply-by-seven}.
+
address@hidden 1250
+Here is the code:
+
address@hidden
address@hidden
+(defun multiply-by-seven (number) ; @r{Interactive version.}
+ "Multiply NUMBER by seven."
+ (interactive "p")
+ (message "The result is %d" (* 7 number)))
address@hidden group
address@hidden smallexample
+
address@hidden
+You can install this code by placing your cursor after it and typing
address@hidden C-e}. The name of the function will appear in your echo area.
+Then, you can use this code by typing @kbd{C-u} and a number and then
+typing @kbd{M-x multiply-by-seven} and pressing @key{RET}. The phrase
address@hidden result is @dots{}} followed by the product will appear in the
+echo area.
+
+Speaking more generally, you invoke a function like this in either of two
+ways:
+
address@hidden
address@hidden
+By typing a prefix argument that contains the number to be passed, and
+then typing @kbd{M-x} and the name of the function, as with
address@hidden 3 M-x forward-sentence}; or,
+
address@hidden
+By typing whatever key or keychord the function is bound to, as with
address@hidden 3 M-e}.
address@hidden enumerate
+
address@hidden
+Both the examples just mentioned work identically to move point forward
+three sentences. (Since @code{multiply-by-seven} is not bound to a key,
+it could not be used as an example of key binding.)
+
+(@xref{Keybindings, , Some Keybindings}, to learn how to bind a command
+to a key.)
+
+A prefix argument is passed to an interactive function by typing the
address@hidden key followed by a number, for example, @kbd{M-3 M-e}, or by
+typing @kbd{C-u} and then a number, for example, @kbd{C-u 3 M-e} (if you
+type @kbd{C-u} without a number, it defaults to 4).
+
address@hidden multiply-by-seven in detail, , Interactive multiply-by-seven,
Interactive
address@hidden node-name, next, previous, up
address@hidden An Interactive @code{multiply-by-seven}
+
+Let's look at the use of the special form @code{interactive} and then at
+the function @code{message} in the interactive version of
address@hidden You will recall that the function definition
+looks like this:
+
address@hidden
address@hidden
+(defun multiply-by-seven (number) ; @r{Interactive version.}
+ "Multiply NUMBER by seven."
+ (interactive "p")
+ (message "The result is %d" (* 7 number)))
address@hidden group
address@hidden smallexample
+
+In this function, the expression, @code{(interactive "p")}, is a list of
+two elements. The @code{"p"} tells Emacs to pass the prefix argument to
+the function and use its value for the argument of the function.
+
address@hidden 1000
+The argument will be a number. This means that the symbol
address@hidden will be bound to a number in the line:
+
address@hidden
+(message "The result is %d" (* 7 number))
address@hidden smallexample
+
address@hidden 1250
address@hidden
+For example, if your prefix argument is 5, the Lisp interpreter will
+evaluate the line as if it were:
+
address@hidden
+(message "The result is %d" (* 7 5))
address@hidden smallexample
+
address@hidden
+(If you are reading this in GNU Emacs, you can evaluate this expression
+yourself.) First, the interpreter will evaluate the inner list, which
+is @code{(* 7 5)}. This returns a value of 35. Next, it
+will evaluate the outer list, passing the values of the second and
+subsequent elements of the list to the function @code{message}.
+
+As we have seen, @code{message} is an Emacs Lisp function especially
+designed for sending a one line message to a user. (@xref{message, ,
+The @code{message} function}.) In summary, the @code{message}
+function prints its first argument in the echo area as is, except for
+occurrences of @samp{%d} or @samp{%s} (and various other %-sequences
+which we have not mentioned). When it sees a control sequence, the
+function looks to the second or subsequent arguments and prints the
+value of the argument in the location in the string where the control
+sequence is located.
+
+In the interactive @code{multiply-by-seven} function, the control string
+is @samp{%d}, which requires a number, and the value returned by
+evaluating @code{(* 7 5)} is the number 35. Consequently, the number 35
+is printed in place of the @samp{%d} and the message is @samp{The result
+is 35}.
+
+(Note that when you call the function @code{multiply-by-seven}, the
+message is printed without quotes, but when you call @code{message}, the
+text is printed in double quotes. This is because the value returned by
address@hidden is what appears in the echo area when you evaluate an
+expression whose first element is @code{message}; but when embedded in a
+function, @code{message} prints the text as a side effect without
+quotes.)
+
address@hidden Interactive Options, Permanent Installation, Interactive,
Writing Defuns
address@hidden node-name, next, previous, up
address@hidden Different Options for @code{interactive}
address@hidden Options for @code{interactive}
address@hidden Interactive options
+
+In the example, @code{multiply-by-seven} used @code{"p"} as the
+argument to @code{interactive}. This argument told Emacs to interpret
+your typing either @kbd{C-u} followed by a number or @key{META}
+followed by a number as a command to pass that number to the function
+as its argument. Emacs has more than twenty characters predefined for
+use with @code{interactive}. In almost every case, one of these
+options will enable you to pass the right information interactively to
+a function. (@xref{Interactive Codes, , Code Characters for
address@hidden, elisp, The GNU Emacs Lisp Reference Manual}.)
+
address@hidden 1250
+Consider the function @code{zap-to-char}. Its interactive expression
+is
+
address@hidden
+(interactive "p\ncZap to char: ")
address@hidden smallexample
+
+The first part of the argument to @code{interactive} is @samp{p}, with
+which you are already familiar. This argument tells Emacs to
+interpret a `prefix', as a number to be passed to the function. You
+can specify a prefix either by typing @kbd{C-u} followed by a number
+or by typing @key{META} followed by a number. The prefix is the
+number of specified characters. Thus, if your prefix is three and the
+specified character is @samp{x}, then you will delete all the text up
+to and including the third next @samp{x}. If you do not set a prefix,
+then you delete all the text up to and including the specified
+character, but no more.
+
+The @samp{c} tells the function the name of the character to which to delete.
+
+More formally, a function with two or more arguments can have
+information passed to each argument by adding parts to the string that
+follows @code{interactive}. When you do this, the information is
+passed to each argument in the same order it is specified in the
address@hidden list. In the string, each part is separated from
+the next part by a @samp{\n}, which is a newline. For example, you
+can follow @samp{p} with a @samp{\n} and an @samp{cZap to char:@: }.
+This causes Emacs to pass the value of the prefix argument (if there
+is one) and the character.
+
+In this case, the function definition looks like the following, where
address@hidden and @code{char} are the symbols to which @code{interactive}
+binds the prefix argument and the specified character:
+
address@hidden
address@hidden
+(defun @var{name-of-function} (arg char)
+ "@address@hidden"
+ (interactive "p\ncZap to char: ")
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
address@hidden
+(The space after the colon in the prompt makes it look better when you
+are prompted. @xref{copy-to-buffer, , The Definition of
address@hidden, for an example.)
+
+When a function does not take arguments, @code{interactive} does not
+require any. Such a function contains the simple expression
address@hidden(interactive)}. The @code{mark-whole-buffer} function is like
+this.
+
+Alternatively, if the special letter-codes are not right for your
+application, you can pass your own arguments to @code{interactive} as
+a list.
+
address@hidden, , The Definition of @code{append-to-buffer}},
+for an example. @xref{Using Interactive, , Using @code{Interactive},
+elisp, The GNU Emacs Lisp Reference Manual}, for a more complete
+explanation about this technique.
+
address@hidden Permanent Installation, let, Interactive Options, Writing Defuns
address@hidden node-name, next, previous, up
address@hidden Install Code Permanently
address@hidden Install code permanently
address@hidden Permanent code installation
address@hidden Code installation
+
+When you install a function definition by evaluating it, it will stay
+installed until you quit Emacs. The next time you start a new session
+of Emacs, the function will not be installed unless you evaluate the
+function definition again.
+
+At some point, you may want to have code installed automatically
+whenever you start a new session of Emacs. There are several ways of
+doing this:
+
address@hidden @bullet
address@hidden
+If you have code that is just for yourself, you can put the code for the
+function definition in your @file{.emacs} initialization file. When you
+start Emacs, your @file{.emacs} file is automatically evaluated and all
+the function definitions within it are installed.
address@hidden Initialization, , Your @file{.emacs} File}.
+
address@hidden
+Alternatively, you can put the function definitions that you want
+installed in one or more files of their own and use the @code{load}
+function to cause Emacs to evaluate and thereby install each of the
+functions in the files.
address@hidden Files, , Loading Files}.
+
address@hidden
+Thirdly, if you have code that your whole site will use, it is usual
+to put it in a file called @file{site-init.el} that is loaded when
+Emacs is built. This makes the code available to everyone who uses
+your machine. (See the @file{INSTALL} file that is part of the Emacs
+distribution.)
address@hidden itemize
+
+Finally, if you have code that everyone who uses Emacs may want, you
+can post it on a computer network or send a copy to the Free Software
+Foundation. (When you do this, please license the code and its
+documentation under a license that permits other people to run, copy,
+study, modify, and redistribute the code and which protects you from
+having your work taken from you.) If you send a copy of your code to
+the Free Software Foundation, and properly protect yourself and
+others, it may be included in the next release of Emacs. In large
+part, this is how Emacs has grown over the past years, by donations.
+
address@hidden let, if, Permanent Installation, Writing Defuns
address@hidden node-name, next, previous, up
address@hidden @code{let}
address@hidden let
+
+The @code{let} expression is a special form in Lisp that you will need
+to use in most function definitions.
+
address@hidden is used to attach or bind a symbol to a value in such a way
+that the Lisp interpreter will not confuse the variable with a
+variable of the same name that is not part of the function.
+
+To understand why the @code{let} special form is necessary, consider
+the situation in which you own a home that you generally refer to as
+`the house', as in the sentence, ``The house needs painting.'' If you
+are visiting a friend and your host refers to `the house', he is
+likely to be referring to @emph{his} house, not yours, that is, to a
+different house.
+
+If your friend is referring to his house and you think he is referring
+to your house, you may be in for some confusion. The same thing could
+happen in Lisp if a variable that is used inside of one function has
+the same name as a variable that is used inside of another function,
+and the two are not intended to refer to the same value. The
address@hidden special form prevents this kind of confusion.
+
address@hidden
+* Prevent confusion::
+* Parts of let Expression::
+* Sample let Expression::
+* Uninitialized let Variables::
address@hidden menu
+
address@hidden Prevent confusion, Parts of let Expression, let, let
address@hidden
address@hidden @code{let} Prevents Confusion
address@hidden ifnottex
+
address@hidden @samp{local variable} defined
address@hidden @samp{variable, local}, defined
+The @code{let} special form prevents confusion. @code{let} creates a
+name for a @dfn{local variable} that overshadows any use of the same
+name outside the @code{let} expression. This is like understanding
+that whenever your host refers to `the house', he means his house, not
+yours. (Symbols used in argument lists work the same way.
address@hidden, , The @code{defun} Special Form}.)
+
+Local variables created by a @code{let} expression retain their value
address@hidden within the @code{let} expression itself (and within
+expressions called within the @code{let} expression); the local
+variables have no effect outside the @code{let} expression.
+
+Another way to think about @code{let} is that it is like a @code{setq}
+that is temporary and local. The values set by @code{let} are
+automatically undone when the @code{let} is finished. The setting
+only affects expressions that are inside the bounds of the @code{let}
+expression. In computer science jargon, we would say ``the binding of
+a symbol is visible only in functions called in the @code{let} form;
+in Emacs Lisp, scoping is dynamic, not lexical.''
+
address@hidden can create more than one variable at once. Also,
address@hidden gives each variable it creates an initial value, either a
+value specified by you, or @code{nil}. (In the jargon, this is called
+`binding the variable to the value'.) After @code{let} has created
+and bound the variables, it executes the code in the body of the
address@hidden, and returns the value of the last expression in the body,
+as the value of the whole @code{let} expression. (`Execute' is a jargon
+term that means to evaluate a list; it comes from the use of the word
+meaning `to give practical effect to' (@cite{Oxford English
+Dictionary}). Since you evaluate an expression to perform an action,
+`execute' has evolved as a synonym to `evaluate'.)
+
address@hidden Parts of let Expression, Sample let Expression, Prevent
confusion, let
address@hidden node-name, next, previous, up
address@hidden The Parts of a @code{let} Expression
address@hidden @code{let} expression, parts of
address@hidden Parts of @code{let} expression
+
address@hidden @samp{varlist} defined
+A @code{let} expression is a list of three parts. The first part is
+the symbol @code{let}. The second part is a list, called a
address@hidden, each element of which is either a symbol by itself or a
+two-element list, the first element of which is a symbol. The third
+part of the @code{let} expression is the body of the @code{let}. The
+body usually consists of one or more lists.
+
address@hidden 800
+A template for a @code{let} expression looks like this:
+
address@hidden
+(let @var{varlist} @address@hidden)
address@hidden smallexample
+
address@hidden
+The symbols in the varlist are the variables that are given initial
+values by the @code{let} special form. Symbols by themselves are given
+the initial value of @code{nil}; and each symbol that is the first
+element of a two-element list is bound to the value that is returned
+when the Lisp interpreter evaluates the second element.
+
+Thus, a varlist might look like this: @code{(thread (needles 3))}. In
+this case, in a @code{let} expression, Emacs binds the symbol
address@hidden to an initial value of @code{nil}, and binds the symbol
address@hidden to an initial value of 3.
+
+When you write a @code{let} expression, what you do is put the
+appropriate expressions in the slots of the @code{let} expression
+template.
+
+If the varlist is composed of two-element lists, as is often the case,
+the template for the @code{let} expression looks like this:
+
address@hidden
address@hidden
+(let ((@var{variable} @var{value})
+ (@var{variable} @var{value})
+ @dots{})
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
address@hidden Sample let Expression, Uninitialized let Variables, Parts of let
Expression, let
address@hidden node-name, next, previous, up
address@hidden Sample @code{let} Expression
address@hidden Sample @code{let} expression
address@hidden @code{let} expression sample
+
+The following expression creates and gives initial values
+to the two variables @code{zebra} and @code{tiger}. The body of the
address@hidden expression is a list which calls the @code{message} function.
+
address@hidden
address@hidden
+(let ((zebra 'stripes)
+ (tiger 'fierce))
+ (message "One kind of animal has %s and another is %s."
+ zebra tiger))
address@hidden group
address@hidden smallexample
+
+Here, the varlist is @code{((zebra 'stripes) (tiger 'fierce))}.
+
+The two variables are @code{zebra} and @code{tiger}. Each variable is
+the first element of a two-element list and each value is the second
+element of its two-element list. In the varlist, Emacs binds the
+variable @code{zebra} to the value @address@hidden
+to Jared Diamond in @cite{Guns, Germs, and Steel}, address@hidden zebras
+become impossibly dangerous as they grow older'' but the claim here is
+that they do not become fierce like a tiger. (1997, W. W. Norton and
+Co., ISBN 0-393-03894-2, page 171)}, and binds the
+variable @code{tiger} to the value @code{fierce}. In this example,
+both values are symbols preceded by a quote. The values could just as
+well have been another list or a string. The body of the @code{let}
+follows after the list holding the variables. In this example, the
+body is a list that uses the @code{message} function to print a string
+in the echo area.
+
address@hidden 1500
+You may evaluate the example in the usual fashion, by placing the
+cursor after the last parenthesis and typing @kbd{C-x C-e}. When you do
+this, the following will appear in the echo area:
+
address@hidden
+"One kind of animal has stripes and another is fierce."
address@hidden smallexample
+
+As we have seen before, the @code{message} function prints its first
+argument, except for @samp{%s}. In this example, the value of the variable
address@hidden is printed at the location of the first @samp{%s} and the
+value of the variable @code{tiger} is printed at the location of the
+second @samp{%s}.
+
address@hidden Uninitialized let Variables, , Sample let Expression, let
address@hidden node-name, next, previous, up
address@hidden Uninitialized Variables in a @code{let} Statement
address@hidden Uninitialized @code{let} variables
address@hidden @code{let} variables uninitialized
+
+If you do not bind the variables in a @code{let} statement to specific
+initial values, they will automatically be bound to an initial value of
address@hidden, as in the following expression:
+
address@hidden
address@hidden
+(let ((birch 3)
+ pine
+ fir
+ (oak 'some))
+ (message
+ "Here are %d variables with %s, %s, and %s value."
+ birch pine fir oak))
address@hidden group
address@hidden smallexample
+
address@hidden
+Here, the varlist is @code{((birch 3) pine fir (oak 'some))}.
+
address@hidden 1250
+If you evaluate this expression in the usual way, the following will
+appear in your echo area:
+
address@hidden
+"Here are 3 variables with nil, nil, and some value."
address@hidden smallexample
+
address@hidden
+In this example, Emacs binds the symbol @code{birch} to the number 3,
+binds the symbols @code{pine} and @code{fir} to @code{nil}, and binds
+the symbol @code{oak} to the value @code{some}.
+
+Note that in the first part of the @code{let}, the variables @code{pine}
+and @code{fir} stand alone as atoms that are not surrounded by
+parentheses; this is because they are being bound to @code{nil}, the
+empty list. But @code{oak} is bound to @code{some} and so is a part of
+the list @code{(oak 'some)}. Similarly, @code{birch} is bound to the
+number 3 and so is in a list with that number. (Since a number
+evaluates to itself, the number does not need to be quoted. Also, the
+number is printed in the message using a @samp{%d} rather than a
address@hidden) The four variables as a group are put into a list to
+delimit them from the body of the @code{let}.
+
address@hidden if, else, let, Writing Defuns
address@hidden node-name, next, previous, up
address@hidden The @code{if} Special Form
address@hidden if
address@hidden Conditional with @code{if}
+
+A third special form, in addition to @code{defun} and @code{let}, is the
+conditional @code{if}. This form is used to instruct the computer to
+make decisions. You can write function definitions without using
address@hidden, but it is used often enough, and is important enough, to be
+included here. It is used, for example, in the code for the
+function @code{beginning-of-buffer}.
+
+The basic idea behind an @code{if}, is that address@hidden a test is true,
address@hidden an expression is evaluated.'' If the test is not true, the
+expression is not evaluated. For example, you might make a decision
+such as, ``if it is warm and sunny, then go to the beach!''
+
address@hidden
+* if in more detail::
+* type-of-animal in detail:: An example of an @code{if} expression.
address@hidden menu
+
address@hidden if in more detail, type-of-animal in detail, if, if
address@hidden
address@hidden @code{if} in more detail
address@hidden ifnottex
+
address@hidden @samp{if-part} defined
address@hidden @samp{then-part} defined
+An @code{if} expression written in Lisp does not use the word `then';
+the test and the action are the second and third elements of the list
+whose first element is @code{if}. Nonetheless, the test part of an
address@hidden expression is often called the @dfn{if-part} and the second
+argument is often called the @dfn{then-part}.
+
+Also, when an @code{if} expression is written, the true-or-false-test
+is usually written on the same line as the symbol @code{if}, but the
+action to carry out if the test is true, the ``then-part'', is written
+on the second and subsequent lines. This makes the @code{if}
+expression easier to read.
+
address@hidden
address@hidden
+(if @var{true-or-false-test}
+ @var{action-to-carry-out-if-test-is-true})
address@hidden group
address@hidden smallexample
+
address@hidden
+The true-or-false-test will be an expression that
+is evaluated by the Lisp interpreter.
+
+Here is an example that you can evaluate in the usual manner. The test
+is whether the number 5 is greater than the number 4. Since it is, the
+message @samp{5 is greater than 4!} will be printed.
+
address@hidden
address@hidden
+(if (> 5 4) ; @r{if-part}
+ (message "5 is greater than 4!")) ; @r{then-part}
address@hidden group
address@hidden smallexample
+
address@hidden
+(The function @code{>} tests whether its first argument is greater than
+its second argument and returns true if it is.)
address@hidden > (greater than)
+
+Of course, in actual use, the test in an @code{if} expression will not
+be fixed for all time as it is by the expression @code{(> 5 4)}.
+Instead, at least one of the variables used in the test will be bound to
+a value that is not known ahead of time. (If the value were known ahead
+of time, we would not need to run the test!)
+
+For example, the value may be bound to an argument of a function
+definition. In the following function definition, the character of the
+animal is a value that is passed to the function. If the value bound to
address@hidden is @code{fierce}, then the message, @samp{It's a
+tiger!} will be printed; otherwise, @code{nil} will be returned.
+
address@hidden
address@hidden
+(defun type-of-animal (characteristic)
+ "Print message in echo area depending on CHARACTERISTIC.
+If the CHARACTERISTIC is the symbol `fierce',
+then warn of a tiger."
+ (if (equal characteristic 'fierce)
+ (message "It's a tiger!")))
address@hidden group
address@hidden smallexample
+
address@hidden 1500
address@hidden
+If you are reading this inside of GNU Emacs, you can evaluate the
+function definition in the usual way to install it in Emacs, and then you
+can evaluate the following two expressions to see the results:
+
address@hidden
address@hidden
+(type-of-animal 'fierce)
+
+(type-of-animal 'zebra)
+
address@hidden group
address@hidden smallexample
+
address@hidden Following sentences rewritten to prevent overfull hbox.
address@hidden
+When you evaluate @code{(type-of-animal 'fierce)}, you will see the
+following message printed in the echo area: @code{"It's a tiger!"}; and
+when you evaluate @code{(type-of-animal 'zebra)} you will see @code{nil}
+printed in the echo area.
+
address@hidden type-of-animal in detail, , if in more detail, if
address@hidden node-name, next, previous, up
address@hidden The @code{type-of-animal} Function in Detail
+
+Let's look at the @code{type-of-animal} function in detail.
+
+The function definition for @code{type-of-animal} was written by filling
+the slots of two templates, one for a function definition as a whole, and
+a second for an @code{if} expression.
+
address@hidden 1250
+The template for every function that is not interactive is:
+
address@hidden
address@hidden
+(defun @var{name-of-function} (@var{argument-list})
+ "@address@hidden"
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
address@hidden 800
+The parts of the function that match this template look like this:
+
address@hidden
address@hidden
+(defun type-of-animal (characteristic)
+ "Print message in echo area depending on CHARACTERISTIC.
+If the CHARACTERISTIC is the symbol `fierce',
+then warn of a tiger."
+ @var{body: the} @code{if} @var{expression})
address@hidden group
address@hidden smallexample
+
+The name of function is @code{type-of-animal}; it is passed the value
+of one argument. The argument list is followed by a multi-line
+documentation string. The documentation string is included in the
+example because it is a good habit to write documentation string for
+every function definition. The body of the function definition
+consists of the @code{if} expression.
+
address@hidden 800
+The template for an @code{if} expression looks like this:
+
address@hidden
address@hidden
+(if @var{true-or-false-test}
+ @var{action-to-carry-out-if-the-test-returns-true})
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+In the @code{type-of-animal} function, the code for the @code{if}
+looks like this:
+
address@hidden
address@hidden
+(if (equal characteristic 'fierce)
+ (message "It's a tiger!")))
address@hidden group
address@hidden smallexample
+
address@hidden 800
+Here, the true-or-false-test is the expression:
+
address@hidden
+(equal characteristic 'fierce)
address@hidden smallexample
+
address@hidden
+In Lisp, @code{equal} is a function that determines whether its first
+argument is equal to its second argument. The second argument is the
+quoted symbol @code{'fierce} and the first argument is the value of the
+symbol @code{characteristic}---in other words, the argument passed to
+this function.
+
+In the first exercise of @code{type-of-animal}, the argument
address@hidden is passed to @code{type-of-animal}. Since @code{fierce}
+is equal to @code{fierce}, the expression, @code{(equal characteristic
+'fierce)}, returns a value of true. When this happens, the @code{if}
+evaluates the second argument or then-part of the @code{if}:
address@hidden(message "It's tiger!")}.
+
+On the other hand, in the second exercise of @code{type-of-animal}, the
+argument @code{zebra} is passed to @code{type-of-animal}. @code{zebra}
+is not equal to @code{fierce}, so the then-part is not evaluated and
address@hidden is returned by the @code{if} expression.
+
address@hidden else, Truth & Falsehood, if, Writing Defuns
address@hidden node-name, next, previous, up
address@hidden If--then--else Expressions
address@hidden Else
+
+An @code{if} expression may have an optional third argument, called
+the @dfn{else-part}, for the case when the true-or-false-test returns
+false. When this happens, the second argument or then-part of the
+overall @code{if} expression is @emph{not} evaluated, but the third or
+else-part @emph{is} evaluated. You might think of this as the cloudy
+day alternative for the decision ``if it is warm and sunny, then go to
+the beach, else read a book!''.
+
+The word ``else'' is not written in the Lisp code; the else-part of an
address@hidden expression comes after the then-part. In the written Lisp, the
+else-part is usually written to start on a line of its own and is
+indented less than the then-part:
+
address@hidden
address@hidden
+(if @var{true-or-false-test}
+ @var{action-to-carry-out-if-the-test-returns-true}
+ @var{action-to-carry-out-if-the-test-returns-false})
address@hidden group
address@hidden smallexample
+
+For example, the following @code{if} expression prints the message @samp{4
+is not greater than 5!} when you evaluate it in the usual way:
+
address@hidden
address@hidden
+(if (> 4 5) ; @r{if-part}
+ (message "4 falsely greater than 5!") ; @r{then-part}
+ (message "4 is not greater than 5!")) ; @r{else-part}
address@hidden group
address@hidden smallexample
+
address@hidden
+Note that the different levels of indentation make it easy to
+distinguish the then-part from the else-part. (GNU Emacs has several
+commands that automatically indent @code{if} expressions correctly.
address@hidden Lists, , GNU Emacs Helps You Type Lists}.)
+
+We can extend the @code{type-of-animal} function to include an
+else-part by simply incorporating an additional part to the @code{if}
+expression.
+
address@hidden 1500
+You can see the consequences of doing this if you evaluate the following
+version of the @code{type-of-animal} function definition to install it
+and then evaluate the two subsequent expressions to pass different
+arguments to the function.
+
address@hidden
address@hidden
+(defun type-of-animal (characteristic) ; @r{Second version.}
+ "Print message in echo area depending on CHARACTERISTIC.
+If the CHARACTERISTIC is the symbol `fierce',
+then warn of a tiger;
+else say it's not fierce."
+ (if (equal characteristic 'fierce)
+ (message "It's a tiger!")
+ (message "It's not fierce!")))
address@hidden group
address@hidden smallexample
address@hidden 1
+
address@hidden
address@hidden
+(type-of-animal 'fierce)
+
+(type-of-animal 'zebra)
+
address@hidden group
address@hidden smallexample
+
address@hidden Following sentence rewritten to prevent overfull hbox.
address@hidden
+When you evaluate @code{(type-of-animal 'fierce)}, you will see the
+following message printed in the echo area: @code{"It's a tiger!"}; but
+when you evaluate @code{(type-of-animal 'zebra)}, you will see
address@hidden"It's not fierce!"}.
+
+(Of course, if the @var{characteristic} were @code{ferocious}, the
+message @code{"It's not fierce!"} would be printed; and it would be
+misleading! When you write code, you need to take into account the
+possibility that some such argument will be tested by the @code{if}
+and write your program accordingly.)
+
address@hidden Truth & Falsehood, save-excursion, else, Writing Defuns
address@hidden node-name, next, previous, up
address@hidden Truth and Falsehood in Emacs Lisp
address@hidden Truth and falsehood in Emacs Lisp
address@hidden Falsehood and truth in Emacs Lisp
address@hidden nil
+
+There is an important aspect to the truth test in an @code{if}
+expression. So far, we have spoken of `true' and `false' as values of
+predicates as if they were new kinds of Emacs Lisp objects. In fact,
+`false' is just our old friend @code{nil}. Anything else---anything
+at all---is `true'.
+
+The expression that tests for truth is interpreted as @dfn{true}
+if the result of evaluating it is a value that is not @code{nil}. In
+other words, the result of the test is considered true if the value
+returned is a number such as 47, a string such as @code{"hello"}, or a
+symbol (other than @code{nil}) such as @code{flowers}, or a list (so
+long as it is not empty), or even a buffer!
+
address@hidden
+* nil explained:: @code{nil} has two meanings.
address@hidden menu
+
address@hidden nil explained, , Truth & Falsehood, Truth & Falsehood
address@hidden
address@hidden An explanation of @code{nil}
address@hidden ifnottex
+
+Before illustrating a test for truth, we need an explanation of @code{nil}.
+
+In Emacs Lisp, the symbol @code{nil} has two meanings. First, it means the
+empty list. Second, it means false and is the value returned when a
+true-or-false-test tests false. @code{nil} can be written as an empty
+list, @code{()}, or as @code{nil}. As far as the Lisp interpreter is
+concerned, @code{()} and @code{nil} are the same. Humans, however, tend
+to use @code{nil} for false and @code{()} for the empty list.
+
+In Emacs Lisp, any value that is not @code{nil}---is not the empty
+list---is considered true. This means that if an evaluation returns
+something that is not an empty list, an @code{if} expression will test
+true. For example, if a number is put in the slot for the test, it
+will be evaluated and will return itself, since that is what numbers
+do when evaluated. In this conditional, the @code{if} expression will
+test true. The expression tests false only when @code{nil}, an empty
+list, is returned by evaluating the expression.
+
+You can see this by evaluating the two expressions in the following examples.
+
+In the first example, the number 4 is evaluated as the test in the
address@hidden expression and returns itself; consequently, the then-part
+of the expression is evaluated and returned: @samp{true} appears in
+the echo area. In the second example, the @code{nil} indicates false;
+consequently, the else-part of the expression is evaluated and
+returned: @samp{false} appears in the echo area.
+
address@hidden
address@hidden
+(if 4
+ 'true
+ 'false)
address@hidden group
+
address@hidden
+(if nil
+ 'true
+ 'false)
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+Incidentally, if some other useful value is not available for a test that
+returns true, then the Lisp interpreter will return the symbol @code{t}
+for true. For example, the expression @code{(> 5 4)} returns @code{t}
+when evaluated, as you can see by evaluating it in the usual way:
+
address@hidden
+(> 5 4)
address@hidden smallexample
+
address@hidden 1250
address@hidden
+On the other hand, this function returns @code{nil} if the test is false.
+
address@hidden
+(> 4 5)
address@hidden smallexample
+
address@hidden save-excursion, Review, Truth & Falsehood, Writing Defuns
address@hidden node-name, next, previous, up
address@hidden @code{save-excursion}
address@hidden save-excursion
address@hidden Region, what it is
address@hidden Preserving point, mark, and buffer
address@hidden Point, mark, buffer preservation
address@hidden point
address@hidden mark
+
+The @code{save-excursion} function is the fourth and final special form
+that we will discuss in this chapter.
+
+In Emacs Lisp programs used for editing, the @code{save-excursion}
+function is very common. It saves the location of point and mark,
+executes the body of the function, and then restores point and mark to
+their previous positions if their locations were changed. Its primary
+purpose is to keep the user from being surprised and disturbed by
+unexpected movement of point or mark.
+
address@hidden
+* Point and mark:: A review of various locations.
+* Template for save-excursion::
address@hidden menu
+
address@hidden Point and mark, Template for save-excursion, save-excursion,
save-excursion
address@hidden
address@hidden Point and Mark
address@hidden ifnottex
+
+Before discussing @code{save-excursion}, however, it may be useful
+first to review what point and mark are in GNU Emacs. @dfn{Point} is
+the current location of the cursor. Wherever the cursor
+is, that is point. More precisely, on terminals where the cursor
+appears to be on top of a character, point is immediately before the
+character. In Emacs Lisp, point is an integer. The first character in
+a buffer is number one, the second is number two, and so on. The
+function @code{point} returns the current position of the cursor as a
+number. Each buffer has its own value for point.
+
+The @dfn{mark} is another position in the buffer; its value can be set
+with a command such as @address@hidden (@code{set-mark-command}). If
+a mark has been set, you can use the command @kbd{C-x C-x}
+(@code{exchange-point-and-mark}) to cause the cursor to jump to the mark
+and set the mark to be the previous position of point. In addition, if
+you set another mark, the position of the previous mark is saved in the
+mark ring. Many mark positions can be saved this way. You can jump the
+cursor to a saved mark by typing @kbd{C-u address@hidden one or more
+times.
+
+The part of the buffer between point and mark is called @dfn{the
+region}. Numerous commands work on the region, including
address@hidden, @code{count-lines-region}, @code{kill-region}, and
address@hidden
+
+The @code{save-excursion} special form saves the locations of point and
+mark and restores those positions after the code within the body of the
+special form is evaluated by the Lisp interpreter. Thus, if point were
+in the beginning of a piece of text and some code moved point to the end
+of the buffer, the @code{save-excursion} would put point back to where
+it was before, after the expressions in the body of the function were
+evaluated.
+
+In Emacs, a function frequently moves point as part of its internal
+workings even though a user would not expect this. For example,
address@hidden moves point. To prevent the user from being
+bothered by jumps that are both unexpected and (from the user's point of
+view) unnecessary, @code{save-excursion} is often used to keep point and
+mark in the location expected by the user. The use of
address@hidden is good housekeeping.
+
+To make sure the house stays clean, @code{save-excursion} restores the
+values of point and mark even if something goes wrong in the code inside
+of it (or, to be more precise and to use the proper jargon, ``in case of
+abnormal exit''). This feature is very helpful.
+
+In addition to recording the values of point and mark,
address@hidden keeps track of the current buffer, and restores
+it, too. This means you can write code that will change the buffer and
+have @code{save-excursion} switch you back to the original buffer.
+This is how @code{save-excursion} is used in @code{append-to-buffer}.
+(@xref{append-to-buffer, , The Definition of @code{append-to-buffer}}.)
+
address@hidden Template for save-excursion, , Point and mark, save-excursion
address@hidden node-name, next, previous, up
address@hidden Template for a @code{save-excursion} Expression
+
address@hidden 800
+The template for code using @code{save-excursion} is simple:
+
address@hidden
address@hidden
+(save-excursion
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
address@hidden
+The body of the function is one or more expressions that will be
+evaluated in sequence by the Lisp interpreter. If there is more than
+one expression in the body, the value of the last one will be returned
+as the value of the @code{save-excursion} function. The other
+expressions in the body are evaluated only for their side effects; and
address@hidden itself is used only for its side effect (which
+is restoring the positions of point and mark).
+
address@hidden 1250
+In more detail, the template for a @code{save-excursion} expression
+looks like this:
+
address@hidden
address@hidden
+(save-excursion
+ @var{first-expression-in-body}
+ @var{second-expression-in-body}
+ @var{third-expression-in-body}
+ @dots{}
+ @var{last-expression-in-body})
address@hidden group
address@hidden smallexample
+
address@hidden
+An expression, of course, may be a symbol on its own or a list.
+
+In Emacs Lisp code, a @code{save-excursion} expression often occurs
+within the body of a @code{let} expression. It looks like this:
+
address@hidden
address@hidden
+(let @var{varlist}
+ (save-excursion
+ @address@hidden))
address@hidden group
address@hidden smallexample
+
address@hidden Review, defun Exercises, save-excursion, Writing Defuns
address@hidden node-name, next, previous, up
address@hidden Review
+
+In the last few chapters we have introduced a fair number of functions
+and special forms. Here they are described in brief, along with a few
+similar functions that have not been mentioned yet.
+
address@hidden @code
address@hidden eval-last-sexp
+Evaluate the last symbolic expression before the current location of
+point. The value is printed in the echo area unless the function is
+invoked with an argument; in that case, the output is printed in the
+current buffer. This command is normally bound to @kbd{C-x C-e}.
+
address@hidden defun
+Define function. This special form has up to five parts: the name,
+a template for the arguments that will be passed to the function,
+documentation, an optional interactive declaration, and the body of the
+definition.
+
address@hidden 1250
+For example, in an early version of Emacs, the function definition was
+as follows. (It is slightly more complex now that it seeks the first
+non-whitespace character rather than the first visible character.)
+
address@hidden
address@hidden
+(defun back-to-indentation ()
+ "Move point to first visible character on line."
+ (interactive)
+ (beginning-of-line 1)
+ (skip-chars-forward " \t"))
address@hidden group
address@hidden smallexample
+
address@hidden
+In GNU Emacs 22,
+
+(defun backward-to-indentation (&optional arg)
+ "Move backward ARG lines and position at first nonblank character."
+ (interactive "p")
+ (forward-line (- (or arg 1)))
+ (skip-chars-forward " \t"))
+
+(defun back-to-indentation ()
+ "Move point to the first non-whitespace character on this line."
+ (interactive)
+ (beginning-of-line 1)
+ (skip-syntax-forward " " (line-end-position))
+ ;; Move back over chars that have whitespace syntax but have the p flag.
+ (backward-prefix-chars))
address@hidden ignore
+
address@hidden interactive
+Declare to the interpreter that the function can be used
+interactively. This special form may be followed by a string with one
+or more parts that pass the information to the arguments of the
+function, in sequence. These parts may also tell the interpreter to
+prompt for information. Parts of the string are separated by
+newlines, @samp{\n}.
+
address@hidden 1000
+Common code characters are:
+
address@hidden @code
address@hidden b
+The name of an existing buffer.
+
address@hidden f
+The name of an existing file.
+
address@hidden p
+The numeric prefix argument. (Note that this `p' is lower case.)
+
address@hidden r
+Point and the mark, as two numeric arguments, smallest first. This
+is the only code letter that specifies two successive arguments
+rather than one.
address@hidden table
+
address@hidden Codes, , Code Characters for @samp{interactive},
+elisp, The GNU Emacs Lisp Reference Manual}, for a complete list of
+code characters.
+
address@hidden let
+Declare that a list of variables is for use within the body of the
address@hidden and give them an initial value, either @code{nil} or a
+specified value; then evaluate the rest of the expressions in the body
+of the @code{let} and return the value of the last one. Inside the
+body of the @code{let}, the Lisp interpreter does not see the values of
+the variables of the same names that are bound outside of the
address@hidden
+
address@hidden 1250
+For example,
+
address@hidden
address@hidden
+(let ((foo (buffer-name))
+ (bar (buffer-size)))
+ (message
+ "This buffer is %s and has %d characters."
+ foo bar))
address@hidden group
address@hidden smallexample
+
address@hidden save-excursion
+Record the values of point and mark and the current buffer before
+evaluating the body of this special form. Restore the values of point
+and mark and buffer afterward.
+
address@hidden 1250
+For example,
+
address@hidden
address@hidden
+(message "We are %d characters into this buffer."
+ (- (point)
+ (save-excursion
+ (goto-char (point-min)) (point))))
address@hidden group
address@hidden smallexample
+
address@hidden if
+Evaluate the first argument to the function; if it is true, evaluate
+the second argument; else evaluate the third argument, if there is one.
+
+The @code{if} special form is called a @dfn{conditional}. There are
+other conditionals in Emacs Lisp, but @code{if} is perhaps the most
+commonly used.
+
address@hidden 1250
+For example,
+
address@hidden
address@hidden
+(if (= 22 emacs-major-version)
+ (message "This is version 22 Emacs")
+ (message "This is not version 22 Emacs"))
address@hidden group
address@hidden smallexample
+
address@hidden 1250
address@hidden <
address@hidden >
address@hidden <=
address@hidden >=
+The @code{<} function tests whether its first argument is smaller than
+its second argument. A corresponding function, @code{>}, tests whether
+the first argument is greater than the second. Likewise, @code{<=}
+tests whether the first argument is less than or equal to the second and
address@hidden>=} tests whether the first argument is greater than or equal to
+the second. In all cases, both arguments must be numbers or markers
+(markers indicate positions in buffers).
+
address@hidden 800
address@hidden =
+The @code{=} function tests whether two arguments, both numbers or
+markers, are equal.
+
address@hidden 1250
address@hidden equal
address@hidden eq
+Test whether two objects are the same. @code{equal} uses one meaning
+of the word `same' and @code{eq} uses another: @code{equal} returns
+true if the two objects have a similar structure and contents, such as
+two copies of the same book. On the other hand, @code{eq}, returns
+true if both arguments are actually the same object.
address@hidden equal
address@hidden eq
+
address@hidden 1250
address@hidden string<
address@hidden string-lessp
address@hidden string=
address@hidden string-equal
+The @code{string-lessp} function tests whether its first argument is
+smaller than the second argument. A shorter, alternative name for the
+same function (a @code{defalias}) is @code{string<}.
+
+The arguments to @code{string-lessp} must be strings or symbols; the
+ordering is lexicographic, so case is significant. The print names of
+symbols are used instead of the symbols themselves.
+
address@hidden @samp{empty string} defined
+An empty string, @samp{""}, a string with no characters in it, is
+smaller than any string of characters.
+
address@hidden provides the corresponding test for equality. Its
+shorter, alternative name is @code{string=}. There are no string test
+functions that correspond to @var{>}, @code{>=}, or @code{<=}.
+
address@hidden message
+Print a message in the echo area. The first argument is a string that
+can contain @samp{%s}, @samp{%d}, or @samp{%c} to print the value of
+arguments that follow the string. The argument used by @samp{%s} must
+be a string or a symbol; the argument used by @samp{%d} must be a
+number. The argument used by @samp{%c} must be an @sc{ascii} code
+number; it will be printed as the character with that @sc{ascii} code.
+(Various other %-sequences have not been mentioned.)
+
address@hidden setq
address@hidden set
+The @code{setq} function sets the value of its first argument to the
+value of the second argument. The first argument is automatically
+quoted by @code{setq}. It does the same for succeeding pairs of
+arguments. Another function, @code{set}, takes only two arguments and
+evaluates both of them before setting the value returned by its first
+argument to the value returned by its second argument.
+
address@hidden buffer-name
+Without an argument, return the name of the buffer, as a string.
+
address@hidden buffer-file-name
+Without an argument, return the name of the file the buffer is
+visiting.
+
address@hidden current-buffer
+Return the buffer in which Emacs is active; it may not be
+the buffer that is visible on the screen.
+
address@hidden other-buffer
+Return the most recently selected buffer (other than the buffer passed
+to @code{other-buffer} as an argument and other than the current
+buffer).
+
address@hidden switch-to-buffer
+Select a buffer for Emacs to be active in and display it in the current
+window so users can look at it. Usually bound to @kbd{C-x b}.
+
address@hidden set-buffer
+Switch Emacs' attention to a buffer on which programs will run. Don't
+alter what the window is showing.
+
address@hidden buffer-size
+Return the number of characters in the current buffer.
+
address@hidden point
+Return the value of the current position of the cursor, as an
+integer counting the number of characters from the beginning of the
+buffer.
+
address@hidden point-min
+Return the minimum permissible value of point in
+the current buffer. This is 1, unless narrowing is in effect.
+
address@hidden point-max
+Return the value of the maximum permissible value of point in the
+current buffer. This is the end of the buffer, unless narrowing is in
+effect.
address@hidden table
+
address@hidden 1500
address@hidden defun Exercises, , Review, Writing Defuns
address@hidden Exercises
+
address@hidden @bullet
address@hidden
+Write a non-interactive function that doubles the value of its
+argument, a number. Make that function interactive.
+
address@hidden
+Write a function that tests whether the current value of
address@hidden is greater than the argument passed to the function,
+and if so, prints an appropriate message.
address@hidden itemize
+
address@hidden Buffer Walk Through, More Complex, Writing Defuns, Top
address@hidden node-name, next, previous, up
address@hidden A Few Buffer--Related Functions
+
+In this chapter we study in detail several of the functions used in GNU
+Emacs. This is called a ``walk-through''. These functions are used as
+examples of Lisp code, but are not imaginary examples; with the
+exception of the first, simplified function definition, these functions
+show the actual code used in GNU Emacs. You can learn a great deal from
+these definitions. The functions described here are all related to
+buffers. Later, we will study other functions.
+
address@hidden
+* Finding More:: How to find more information.
+* simplified-beginning-of-buffer:: Shows @code{goto-char},
+ @code{point-min}, and @code{push-mark}.
+* mark-whole-buffer:: Almost the same as @code{beginning-of-buffer}.
+* append-to-buffer:: Uses @code{save-excursion} and
+ @code{insert-buffer-substring}.
+* Buffer Related Review:: Review.
+* Buffer Exercises::
address@hidden menu
+
address@hidden Finding More, simplified-beginning-of-buffer, Buffer Walk
Through, Buffer Walk Through
address@hidden Finding More Information
+
address@hidden describe-function, @r{introduced}
address@hidden Find function documentation
+In this walk-through, I will describe each new function as we come to
+it, sometimes in detail and sometimes briefly. If you are interested,
+you can get the full documentation of any Emacs Lisp function at any
+time by typing @kbd{C-h f} and then the name of the function (and then
address@hidden). Similarly, you can get the full documentation for a
+variable by typing @kbd{C-h v} and then the name of the variable (and
+then @key{RET}).
+
address@hidden Find source of function
address@hidden In version 22, tells location both of C and of Emacs Lisp
+Also, @code{describe-function} will tell you the location of the
+function definition.
+
+Put point into the name of the file that contains the function and
+press the @key{RET} key. In this case, @key{RET} means
address@hidden rather than `return' or `enter'. Emacs will take
+you directly to the function definition.
+
address@hidden
+Not In version 22
+
+If you move point over the file name and press
+the @key{RET} key, which in this case means @code{help-follow} rather
+than `return' or `enter', Emacs will take you directly to the function
+definition.
address@hidden ignore
+
+More generally, if you want to see a function in its original source
+file, you can use the @code{find-tags} function to jump to it.
address@hidden works with a wide variety of languages, not just
+Lisp, and C, and it works with non-programming text as well. For
+example, @code{find-tags} will jump to the various nodes in the
+Texinfo source file of this document.
+The @code{find-tags} function depends on `tags tables' that record
+the locations of the functions, variables, and other items to which
address@hidden jumps.
+
+To use the @code{find-tags} command, type @kbd{M-.} (i.e., press the
+period key while holding down the @key{META} key, or else type the
address@hidden key and then type the period key), and then, at the prompt,
+type in the name of the function whose source code you want to see,
+such as @code{mark-whole-buffer}, and then type @key{RET}. Emacs will
+switch buffers and display the source code for the function on your
+screen. To switch back to your current buffer, type @kbd{C-x b
address@hidden (On some keyboards, the @key{META} key is labelled
address@hidden)
+
address@hidden !!! 22.1.1 tags table location in this paragraph
address@hidden TAGS table, specifying
address@hidden find-tags
+Depending on how the initial default values of your copy of Emacs are
+set, you may also need to specify the location of your `tags table',
+which is a file called @file{TAGS}. For example, if you are
+interested in Emacs sources, the tags table you will most likely want,
+if it has already been created for you, will be in a subdirectory of
+the @file{/usr/local/share/emacs/} directory; thus you would use the
address@hidden visit-tags-table} command and specify a pathname such as
address@hidden/usr/local/share/emacs/22.1.1/lisp/TAGS}. If the tags table
+has not already been created, you will have to create it yourself. It
+will in a file such as @file{/usr/local/src/emacs/src/TAGS}.
+
address@hidden 1250
+To create a @file{TAGS} file in a specific directory, switch to that
+directory in Emacs using @kbd{M-x cd} command, or list the directory
+with @kbd{C-x d} (@code{dired}). Then run the compile command, with
address@hidden@code{etags *.el}} as the command to execute:
+
address@hidden
+M-x compile RET etags *.el RET
address@hidden smallexample
+
+For more information, see @ref{etags, , Create Your Own @file{TAGS} File}.
+
+After you become more familiar with Emacs Lisp, you will find that you will
+frequently use @code{find-tags} to navigate your way around source code;
+and you will create your own @file{TAGS} tables.
+
address@hidden Library, as term for `file'
+Incidentally, the files that contain Lisp code are conventionally
+called @dfn{libraries}. The metaphor is derived from that of a
+specialized library, such as a law library or an engineering library,
+rather than a general library. Each library, or file, contains
+functions that relate to a particular topic or activity, such as
address@hidden for handling abbreviations and other typing
+shortcuts, and @file{help.el} for on-line help. (Sometimes several
+libraries provide code for a single activity, as the various
address@hidden@dots{}} files provide code for reading electronic mail.)
+In @cite{The GNU Emacs Manual}, you will see sentences such as ``The
address@hidden p} command lets you search the standard Emacs Lisp libraries
+by topic keywords.''
+
address@hidden simplified-beginning-of-buffer, mark-whole-buffer, Finding More,
Buffer Walk Through
address@hidden node-name, next, previous, up
address@hidden A Simplified @code{beginning-of-buffer} Definition
address@hidden simplified-beginning-of-buffer
+
+The @code{beginning-of-buffer} command is a good function to start with
+since you are likely to be familiar with it and it is easy to
+understand. Used as an interactive command, @code{beginning-of-buffer}
+moves the cursor to the beginning of the buffer, leaving the mark at the
+previous position. It is generally bound to @kbd{M-<}.
+
+In this section, we will discuss a shortened version of the function
+that shows how it is most frequently used. This shortened function
+works as written, but it does not contain the code for a complex option.
+In another section, we will describe the entire function.
+(@xref{beginning-of-buffer, , Complete Definition of
address@hidden)
+
+Before looking at the code, let's consider what the function
+definition has to contain: it must include an expression that makes
+the function interactive so it can be called by typing @kbd{M-x
+beginning-of-buffer} or by typing a keychord such as @kbd{M-<}; it
+must include code to leave a mark at the original position in the
+buffer; and it must include code to move the cursor to the beginning
+of the buffer.
+
address@hidden 1250
+Here is the complete text of the shortened version of the function:
+
address@hidden
address@hidden
+(defun simplified-beginning-of-buffer ()
+ "Move point to the beginning of the buffer;
+leave mark at previous position."
+ (interactive)
+ (push-mark)
+ (goto-char (point-min)))
address@hidden group
address@hidden smallexample
+
+Like all function definitions, this definition has five parts following
+the special form @code{defun}:
+
address@hidden
address@hidden
+The name: in this example, @code{simplified-beginning-of-buffer}.
+
address@hidden
+A list of the arguments: in this example, an empty list, @code{()},
+
address@hidden
+The documentation string.
+
address@hidden
+The interactive expression.
+
address@hidden
+The body.
address@hidden enumerate
+
address@hidden
+In this function definition, the argument list is empty; this means that
+this function does not require any arguments. (When we look at the
+definition for the complete function, we will see that it may be passed
+an optional argument.)
+
+The interactive expression tells Emacs that the function is intended to
+be used interactively. In this example, @code{interactive} does not have
+an argument because @code{simplified-beginning-of-buffer} does not
+require one.
+
address@hidden 800
+The body of the function consists of the two lines:
+
address@hidden
address@hidden
+(push-mark)
+(goto-char (point-min))
address@hidden group
address@hidden smallexample
+
+The first of these lines is the expression, @code{(push-mark)}. When
+this expression is evaluated by the Lisp interpreter, it sets a mark at
+the current position of the cursor, wherever that may be. The position
+of this mark is saved in the mark ring.
+
+The next line is @code{(goto-char (point-min))}. This expression
+jumps the cursor to the minimum point in the buffer, that is, to the
+beginning of the buffer (or to the beginning of the accessible portion
+of the buffer if it is narrowed. @xref{Narrowing & Widening, ,
+Narrowing and Widening}.)
+
+The @code{push-mark} command sets a mark at the place where the cursor
+was located before it was moved to the beginning of the buffer by the
address@hidden(goto-char (point-min))} expression. Consequently, you can, if
+you wish, go back to where you were originally by typing @kbd{C-x C-x}.
+
+That is all there is to the function definition!
+
address@hidden describe-function
+When you are reading code such as this and come upon an unfamiliar
+function, such as @code{goto-char}, you can find out what it does by
+using the @code{describe-function} command. To use this command, type
address@hidden f} and then type in the name of the function and press
address@hidden The @code{describe-function} command will print the
+function's documentation string in a @file{*Help*} window. For
+example, the documentation for @code{goto-char} is:
+
address@hidden
address@hidden
+Set point to POSITION, a number or marker.
+Beginning of buffer is position (point-min), end is (point-max).
address@hidden group
address@hidden smallexample
+
address@hidden
+The function's one argument is the desired position.
+
address@hidden
+(The prompt for @code{describe-function} will offer you the symbol
+under or preceding the cursor, so you can save typing by positioning
+the cursor right over or after the function and then typing @kbd{C-h f
address@hidden)
+
+The @code{end-of-buffer} function definition is written in the same way as
+the @code{beginning-of-buffer} definition except that the body of the
+function contains the expression @code{(goto-char (point-max))} in place
+of @code{(goto-char (point-min))}.
+
address@hidden mark-whole-buffer, append-to-buffer,
simplified-beginning-of-buffer, Buffer Walk Through
address@hidden node-name, next, previous, up
address@hidden The Definition of @code{mark-whole-buffer}
address@hidden mark-whole-buffer
+
+The @code{mark-whole-buffer} function is no harder to understand than the
address@hidden function. In this case, however,
+we will look at the complete function, not a shortened version.
+
+The @code{mark-whole-buffer} function is not as commonly used as the
address@hidden function, but is useful nonetheless: it
+marks a whole buffer as a region by putting point at the beginning and
+a mark at the end of the buffer. It is generally bound to @kbd{C-x
+h}.
+
address@hidden
+* mark-whole-buffer overview::
+* Body of mark-whole-buffer:: Only three lines of code.
address@hidden menu
+
address@hidden mark-whole-buffer overview, Body of mark-whole-buffer,
mark-whole-buffer, mark-whole-buffer
address@hidden
address@hidden An overview of @code{mark-whole-buffer}
address@hidden ifnottex
+
address@hidden 1250
+In GNU Emacs 22, the code for the complete function looks like this:
+
address@hidden
address@hidden
+(defun mark-whole-buffer ()
+ "Put point at beginning and mark at end of buffer.
+You probably should not use this function in Lisp programs;
+it is usually a mistake for a Lisp function to use any subroutine
+that uses or sets the mark."
+ (interactive)
+ (push-mark (point))
+ (push-mark (point-max) nil t)
+ (goto-char (point-min)))
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+Like all other functions, the @code{mark-whole-buffer} function fits
+into the template for a function definition. The template looks like
+this:
+
address@hidden
address@hidden
+(defun @var{name-of-function} (@var{argument-list})
+ "@address@hidden"
+ (@address@hidden)
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+Here is how the function works: the name of the function is
address@hidden; it is followed by an empty argument list,
address@hidden()}, which means that the function does not require arguments.
+The documentation comes next.
+
+The next line is an @code{(interactive)} expression that tells Emacs
+that the function will be used interactively. These details are similar
+to the @code{simplified-beginning-of-buffer} function described in the
+previous section.
+
address@hidden 1250
address@hidden Body of mark-whole-buffer, , mark-whole-buffer overview,
mark-whole-buffer
address@hidden node-name, next, previous, up
address@hidden Body of @code{mark-whole-buffer}
+
+The body of the @code{mark-whole-buffer} function consists of three
+lines of code:
+
address@hidden GNU Emacs 22
address@hidden
address@hidden
+(push-mark (point))
+(push-mark (point-max) nil t)
+(goto-char (point-min))
address@hidden group
address@hidden smallexample
+
+The first of these lines is the expression, @code{(push-mark (point))}.
+
+This line does exactly the same job as the first line of the body of
+the @code{simplified-beginning-of-buffer} function, which is written
address@hidden(push-mark)}. In both cases, the Lisp interpreter sets a mark
+at the current position of the cursor.
+
+I don't know why the expression in @code{mark-whole-buffer} is written
address@hidden(push-mark (point))} and the expression in
address@hidden is written @code{(push-mark)}. Perhaps
+whoever wrote the code did not know that the arguments for
address@hidden are optional and that if @code{push-mark} is not
+passed an argument, the function automatically sets mark at the
+location of point by default. Or perhaps the expression was written
+so as to parallel the structure of the next line. In any case, the
+line causes Emacs to determine the position of point and set a mark
+there.
+
+In earlier versions of GNU Emacs, the next line of
address@hidden was @code{(push-mark (point-max))}. This
+expression sets a mark at the point in the buffer that has the highest
+number. This will be the end of the buffer (or, if the buffer is
+narrowed, the end of the accessible portion of the buffer.
address@hidden & Widening, , Narrowing and Widening}, for more about
+narrowing.) After this mark has been set, the previous mark, the one
+set at point, is no longer set, but Emacs remembers its position, just
+as all other recent marks are always remembered. This means that you
+can, if you wish, go back to that position by typing @kbd{C-u
address@hidden twice.
+
address@hidden 1250
+In GNU Emacs 22, the @code{(point-max)} is slightly more complicated.
+The line reads
+
address@hidden
+(push-mark (point-max) nil t)
address@hidden smallexample
+
address@hidden
+The expression works nearly the same as before. It sets a mark at the
+highest numbered place in the buffer that it can. However, in this
+version, @code{push-mark} has two additional arguments. The second
+argument to @code{push-mark} is @code{nil}. This tells the function
+it @emph{should} display a message that says `Mark set' when it pushes
+the mark. The third argument is @code{t}. This tells
address@hidden to activate the mark when Transient Mark mode is
+turned on. Transient Mark mode highlights the currently active
+region. It is often turned off.
+
+Finally, the last line of the function is @code{(goto-char
+(point-min)))}. This is written exactly the same way as it is written
+in @code{beginning-of-buffer}. The expression moves the cursor to
+the minimum point in the buffer, that is, to the beginning of the buffer
+(or to the beginning of the accessible portion of the buffer). As a
+result of this, point is placed at the beginning of the buffer and mark
+is set at the end of the buffer. The whole buffer is, therefore, the
+region.
+
address@hidden append-to-buffer, Buffer Related Review, mark-whole-buffer,
Buffer Walk Through
address@hidden node-name, next, previous, up
address@hidden The Definition of @code{append-to-buffer}
address@hidden append-to-buffer
+
+The @code{append-to-buffer} command is more complex than the
address@hidden command. What it does is copy the region
+(that is, the part of the buffer between point and mark) from the
+current buffer to a specified buffer.
+
address@hidden
+* append-to-buffer overview::
+* append interactive:: A two part interactive expression.
+* append-to-buffer body:: Incorporates a @code{let} expression.
+* append save-excursion:: How the @code{save-excursion} works.
address@hidden menu
+
address@hidden append-to-buffer overview, append interactive, append-to-buffer,
append-to-buffer
address@hidden
address@hidden An Overview of @code{append-to-buffer}
address@hidden ifnottex
+
address@hidden insert-buffer-substring
+The @code{append-to-buffer} command uses the
address@hidden function to copy the region.
address@hidden is described by its name: it takes a
+string of characters from part of a buffer, a ``substring'', and
+inserts them into another buffer.
+
+Most of @code{append-to-buffer} is
+concerned with setting up the conditions for
address@hidden to work: the code must specify both the
+buffer to which the text will go, the window it comes from and goes
+to, and the region that will be copied.
+
address@hidden 1250
+Here is the complete text of the function:
+
address@hidden
address@hidden
+(defun append-to-buffer (buffer start end)
+ "Append to specified buffer the text of the region.
+It is inserted into that buffer before its point.
address@hidden group
+
address@hidden
+When calling from a program, give three arguments:
+BUFFER (or buffer name), START and END.
+START and END specify the portion of the current buffer to be copied."
+ (interactive
+ (list (read-buffer "Append to buffer: " (other-buffer
+ (current-buffer) t))
+ (region-beginning) (region-end)))
address@hidden group
address@hidden
+ (let ((oldbuf (current-buffer)))
+ (save-excursion
+ (let* ((append-to (get-buffer-create buffer))
+ (windows (get-buffer-window-list append-to t t))
+ point)
+ (set-buffer append-to)
+ (setq point (point))
+ (barf-if-buffer-read-only)
+ (insert-buffer-substring oldbuf start end)
+ (dolist (window windows)
+ (when (= (window-point window) point)
+ (set-window-point window (point))))))))
address@hidden group
address@hidden smallexample
+
+The function can be understood by looking at it as a series of
+filled-in templates.
+
+The outermost template is for the function definition. In this
+function, it looks like this (with several slots filled in):
+
address@hidden
address@hidden
+(defun append-to-buffer (buffer start end)
+ "@address@hidden"
+ (interactive @dots{})
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+The first line of the function includes its name and three arguments.
+The arguments are the @code{buffer} to which the text will be copied, and
+the @code{start} and @code{end} of the region in the current buffer that
+will be copied.
+
+The next part of the function is the documentation, which is clear and
+complete. As is conventional, the three arguments are written in
+upper case so you will notice them easily. Even better, they are
+described in the same order as in the argument list.
+
+Note that the documentation distinguishes between a buffer and its
+name. (The function can handle either.)
+
address@hidden append interactive, append-to-buffer body, append-to-buffer
overview, append-to-buffer
address@hidden node-name, next, previous, up
address@hidden The @code{append-to-buffer} Interactive Expression
+
+Since the @code{append-to-buffer} function will be used interactively,
+the function must have an @code{interactive} expression. (For a
+review of @code{interactive}, see @ref{Interactive, , Making a
+Function Interactive}.) The expression reads as follows:
+
address@hidden
address@hidden
+(interactive
+ (list (read-buffer
+ "Append to buffer: "
+ (other-buffer (current-buffer) t))
+ (region-beginning)
+ (region-end)))
address@hidden group
address@hidden smallexample
+
address@hidden
+This expression is not one with letters standing for parts, as
+described earlier. Instead, it starts a list with these parts:
+
+The first part of the list is an expression to read the name of a
+buffer and return it as a string. That is @code{read-buffer}. The
+function requires a prompt as its first argument, @samp{"Append to
+buffer: "}. Its second argument tells the command what value to
+provide if you don't specify anything.
+
+In this case that second argument is an expression containing the
+function @code{other-buffer}, an exception, and a @samp{t}, standing
+for true.
+
+The first argument to @code{other-buffer}, the exception, is yet
+another function, @code{current-buffer}. That is not going to be
+returned. The second argument is the symbol for true, @code{t}. that
+tells @code{other-buffer} that it may show visible buffers (except in
+this case, it will not show the current buffer, which makes sense).
+
address@hidden 1250
+The expression looks like this:
+
address@hidden
+(other-buffer (current-buffer) t)
address@hidden smallexample
+
+The second and third arguments to the @code{list} expression are
address@hidden(region-beginning)} and @code{(region-end)}. These two
+functions specify the beginning and end of the text to be appended.
+
address@hidden 1250
+Originally, the command used the letters @samp{B} and @samp{r}.
+The whole @code{interactive} expression looked like this:
+
address@hidden
+(interactive "BAppend to buffer:@: \nr")
address@hidden smallexample
+
address@hidden
+But when that was done, the default value of the buffer switched to
+was invisible. That was not wanted.
+
+(The prompt was separated from the second argument with a newline,
address@hidden It was followed by an @samp{r} that told Emacs to bind the
+two arguments that follow the symbol @code{buffer} in the function's
+argument list (that is, @code{start} and @code{end}) to the values of
+point and mark. That argument worked fine.)
+
address@hidden append-to-buffer body, append save-excursion, append
interactive, append-to-buffer
address@hidden node-name, next, previous, up
address@hidden The Body of @code{append-to-buffer}
+
address@hidden
+in GNU Emacs 22 in /usr/local/src/emacs/lisp/simple.el
+
+(defun append-to-buffer (buffer start end)
+ "Append to specified buffer the text of the region.
+It is inserted into that buffer before its point.
+
+When calling from a program, give three arguments:
+BUFFER (or buffer name), START and END.
+START and END specify the portion of the current buffer to be copied."
+ (interactive
+ (list (read-buffer "Append to buffer: " (other-buffer (current-buffer) t))
+ (region-beginning) (region-end)))
+ (let ((oldbuf (current-buffer)))
+ (save-excursion
+ (let* ((append-to (get-buffer-create buffer))
+ (windows (get-buffer-window-list append-to t t))
+ point)
+ (set-buffer append-to)
+ (setq point (point))
+ (barf-if-buffer-read-only)
+ (insert-buffer-substring oldbuf start end)
+ (dolist (window windows)
+ (when (= (window-point window) point)
+ (set-window-point window (point))))))))
address@hidden ignore
+
+The body of the @code{append-to-buffer} function begins with @code{let}.
+
+As we have seen before (@pxref{let, , @code{let}}), the purpose of a
address@hidden expression is to create and give initial values to one or
+more variables that will only be used within the body of the
address@hidden This means that such a variable will not be confused with
+any variable of the same name outside the @code{let} expression.
+
+We can see how the @code{let} expression fits into the function as a
+whole by showing a template for @code{append-to-buffer} with the
address@hidden expression in outline:
+
address@hidden
address@hidden
+(defun append-to-buffer (buffer start end)
+ "@address@hidden"
+ (interactive @dots{})
+ (let ((@var{variable} @var{value}))
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+The @code{let} expression has three elements:
+
address@hidden
address@hidden
+The symbol @code{let};
+
address@hidden
+A varlist containing, in this case, a single two-element list,
address@hidden(@var{variable} @var{value})};
+
address@hidden
+The body of the @code{let} expression.
address@hidden enumerate
+
address@hidden 800
+In the @code{append-to-buffer} function, the varlist looks like this:
+
address@hidden
+(oldbuf (current-buffer))
address@hidden smallexample
+
address@hidden
+In this part of the @code{let} expression, the one variable,
address@hidden, is bound to the value returned by the
address@hidden(current-buffer)} expression. The variable, @code{oldbuf}, is
+used to keep track of the buffer in which you are working and from
+which you will copy.
+
+The element or elements of a varlist are surrounded by a set of
+parentheses so the Lisp interpreter can distinguish the varlist from
+the body of the @code{let}. As a consequence, the two-element list
+within the varlist is surrounded by a circumscribing set of parentheses.
+The line looks like this:
+
address@hidden
address@hidden
+(let ((oldbuf (current-buffer)))
+ @dots{} )
address@hidden group
address@hidden smallexample
+
address@hidden
+The two parentheses before @code{oldbuf} might surprise you if you did
+not realize that the first parenthesis before @code{oldbuf} marks the
+boundary of the varlist and the second parenthesis marks the beginning
+of the two-element list, @code{(oldbuf (current-buffer))}.
+
address@hidden append save-excursion, , append-to-buffer body, append-to-buffer
address@hidden node-name, next, previous, up
address@hidden @code{save-excursion} in @code{append-to-buffer}
+
+The body of the @code{let} expression in @code{append-to-buffer}
+consists of a @code{save-excursion} expression.
+
+The @code{save-excursion} function saves the locations of point and
+mark, and restores them to those positions after the expressions in the
+body of the @code{save-excursion} complete execution. In addition,
address@hidden keeps track of the original buffer, and
+restores it. This is how @code{save-excursion} is used in
address@hidden
+
address@hidden 1500
address@hidden Indentation for formatting
address@hidden Formatting convention
+Incidentally, it is worth noting here that a Lisp function is normally
+formatted so that everything that is enclosed in a multi-line spread is
+indented more to the right than the first symbol. In this function
+definition, the @code{let} is indented more than the @code{defun}, and
+the @code{save-excursion} is indented more than the @code{let}, like
+this:
+
address@hidden
address@hidden
+(defun @dots{}
+ @dots{}
+ @dots{}
+ (address@hidden
+ (save-excursion
+ @dots{}
address@hidden group
address@hidden smallexample
+
address@hidden 1500
address@hidden
+This formatting convention makes it easy to see that the lines in
+the body of the @code{save-excursion} are enclosed by the parentheses
+associated with @code{save-excursion}, just as the
address@hidden itself is enclosed by the parentheses associated
+with the @code{let}:
+
address@hidden
address@hidden
+(let ((oldbuf (current-buffer)))
+ (save-excursion
+ @dots{}
+ (set-buffer @dots{})
+ (insert-buffer-substring oldbuf start end)
+ @dots{}))
address@hidden group
address@hidden smallexample
+
address@hidden 1200
+The use of the @code{save-excursion} function can be viewed as a process
+of filling in the slots of a template:
+
address@hidden
address@hidden
+(save-excursion
+ @var{first-expression-in-body}
+ @var{second-expression-in-body}
+ @dots{}
+ @var{last-expression-in-body})
address@hidden group
address@hidden smallexample
+
address@hidden 1200
address@hidden
+In this function, the body of the @code{save-excursion} contains only
+one expression, the @code{let*} expression. You know about a
address@hidden function. The @code{let*} function is different. It has a
address@hidden in its name. It enables Emacs to set each variable in its
+varlist in sequence, one after another.
+
+Its critical feature is that variables later in the varlist can make
+use of the values to which Emacs set variables earlier in the varlist.
address@hidden let, , The @code{let*} expression}.
+
+We will skip functions like @code{let*} and focus on two: the
address@hidden function and the @code{insert-buffer-substring}
+function.
+
address@hidden 1250
+In the old days, the @code{set-buffer} expression was simply
+
address@hidden
+(set-buffer (get-buffer-create buffer))
address@hidden smallexample
+
address@hidden 1250
address@hidden
+but now it is
+
address@hidden
+(set-buffer append-to)
address@hidden smallexample
+
address@hidden
address@hidden is bound to @code{(get-buffer-create buffer)} earlier
+on in the @code{let*} expression. That extra binding would not be
+necessary except for that @code{append-to} is used later in the
+varlist as an argument to @code{get-buffer-window-list}.
+
address@hidden
+in GNU Emacs 22
+
+ (let ((oldbuf (current-buffer)))
+ (save-excursion
+ (let* ((append-to (get-buffer-create buffer))
+ (windows (get-buffer-window-list append-to t t))
+ point)
+ (set-buffer append-to)
+ (setq point (point))
+ (barf-if-buffer-read-only)
+ (insert-buffer-substring oldbuf start end)
+ (dolist (window windows)
+ (when (= (window-point window) point)
+ (set-window-point window (point))))))))
address@hidden ignore
+
+The @code{append-to-buffer} function definition inserts text from the
+buffer in which you are currently to a named buffer. It happens that
address@hidden copies text from another buffer to the
+current buffer, just the reverse---that is why the
address@hidden definition starts out with a @code{let} that
+binds the local symbol @code{oldbuf} to the value returned by
address@hidden
+
address@hidden 1250
+The @code{insert-buffer-substring} expression looks like this:
+
address@hidden
+(insert-buffer-substring oldbuf start end)
address@hidden smallexample
+
address@hidden
+The @code{insert-buffer-substring} function copies a string
address@hidden the buffer specified as its first argument and inserts the
+string into the present buffer. In this case, the argument to
address@hidden is the value of the variable created
+and bound by the @code{let}, namely the value of @code{oldbuf}, which
+was the current buffer when you gave the @code{append-to-buffer}
+command.
+
+After @code{insert-buffer-substring} has done its work,
address@hidden will restore the action to the original buffer
+and @code{append-to-buffer} will have done its job.
+
address@hidden 800
+Written in skeletal form, the workings of the body look like this:
+
address@hidden
address@hidden
+(let (@address@hidden@address@hidden)
+ (save-excursion ; @r{Keep track of buffer.}
+ @var{change-buffer}
+ @address@hidden@var{-into-buffer})
+
+ @var{change-back-to-original-buffer-when-finished}
address@hidden@address@hidden
address@hidden group
address@hidden smallexample
+
+In summary, @code{append-to-buffer} works as follows: it saves the
+value of the current buffer in the variable called @code{oldbuf}. It
+gets the new buffer (creating one if need be) and switches Emacs'
+attention to it. Using the value of @code{oldbuf}, it inserts the
+region of text from the old buffer into the new buffer; and then using
address@hidden, it brings you back to your original buffer.
+
+In looking at @code{append-to-buffer}, you have explored a fairly
+complex function. It shows how to use @code{let} and
address@hidden, and how to change to and come back from another
+buffer. Many function definitions use @code{let},
address@hidden, and @code{set-buffer} this way.
+
address@hidden Buffer Related Review, Buffer Exercises, append-to-buffer,
Buffer Walk Through
address@hidden node-name, next, previous, up
address@hidden Review
+
+Here is a brief summary of the various functions discussed in this chapter.
+
address@hidden @code
address@hidden describe-function
address@hidden describe-variable
+Print the documentation for a function or variable.
+Conventionally bound to @kbd{C-h f} and @kbd{C-h v}.
+
address@hidden find-tag
+Find the file containing the source for a function or variable and
+switch buffers to it, positioning point at the beginning of the item.
+Conventionally bound to @kbd{M-.} (that's a period following the
address@hidden key).
+
address@hidden save-excursion
+Save the location of point and mark and restore their values after the
+arguments to @code{save-excursion} have been evaluated. Also, remember
+the current buffer and return to it.
+
address@hidden push-mark
+Set mark at a location and record the value of the previous mark on the
+mark ring. The mark is a location in the buffer that will keep its
+relative position even if text is added to or removed from the buffer.
+
address@hidden goto-char
+Set point to the location specified by the value of the argument, which
+can be a number, a marker, or an expression that returns the number of
+a position, such as @code{(point-min)}.
+
address@hidden insert-buffer-substring
+Copy a region of text from a buffer that is passed to the function as
+an argument and insert the region into the current buffer.
+
address@hidden mark-whole-buffer
+Mark the whole buffer as a region. Normally bound to @kbd{C-x h}.
+
address@hidden set-buffer
+Switch the attention of Emacs to another buffer, but do not change the
+window being displayed. Used when the program rather than a human is
+to work on a different buffer.
+
address@hidden get-buffer-create
address@hidden get-buffer
+Find a named buffer or create one if a buffer of that name does not
+exist. The @code{get-buffer} function returns @code{nil} if the named
+buffer does not exist.
address@hidden table
+
address@hidden 1500
address@hidden Buffer Exercises, , Buffer Related Review, Buffer Walk Through
address@hidden Exercises
+
address@hidden @bullet
address@hidden
+Write your own @code{simplified-end-of-buffer} function definition;
+then test it to see whether it works.
+
address@hidden
+Use @code{if} and @code{get-buffer} to write a function that prints a
+message telling you whether a buffer exists.
+
address@hidden
+Using @code{find-tag}, find the source for the @code{copy-to-buffer}
+function.
address@hidden itemize
+
address@hidden More Complex, Narrowing & Widening, Buffer Walk Through, Top
address@hidden node-name, next, previous, up
address@hidden A Few More Complex Functions
+
+In this chapter, we build on what we have learned in previous chapters
+by looking at more complex functions. The @code{copy-to-buffer}
+function illustrates use of two @code{save-excursion} expressions in
+one definition, while the @code{insert-buffer} function illustrates
+use of an asterisk in an @code{interactive} expression, use of
address@hidden, and the important distinction between a name and the object
+to which the name refers.
+
address@hidden
+* copy-to-buffer:: With @code{set-buffer},
@code{get-buffer-create}.
+* insert-buffer:: Read-only, and with @code{or}.
+* beginning-of-buffer:: Shows @code{goto-char},
+ @code{point-min}, and @code{push-mark}.
+* Second Buffer Related Review::
+* optional Exercise::
address@hidden menu
+
address@hidden copy-to-buffer, insert-buffer, More Complex, More Complex
address@hidden node-name, next, previous, up
address@hidden The Definition of @code{copy-to-buffer}
address@hidden copy-to-buffer
+
+After understanding how @code{append-to-buffer} works, it is easy to
+understand @code{copy-to-buffer}. This function copies text into a
+buffer, but instead of adding to the second buffer, it replaces all the
+previous text in the second buffer.
+
address@hidden 800
+The body of @code{copy-to-buffer} looks like this,
+
address@hidden
address@hidden
address@hidden
+(interactive "BCopy to buffer: \nr")
+(let ((oldbuf (current-buffer)))
+ (with-current-buffer (get-buffer-create buffer)
+ (barf-if-buffer-read-only)
+ (erase-buffer)
+ (save-excursion
+ (insert-buffer-substring oldbuf start end)))))
address@hidden group
address@hidden smallexample
+
+The @code{copy-to-buffer} function has a simpler @code{interactive}
+expression than @code{append-to-buffer}.
+
address@hidden 800
+The definition then says
+
address@hidden
+(with-current-buffer (get-buffer-create buffer) @dots{}
address@hidden smallexample
+
+First, look at the earliest inner expression; that is evaluated first.
+That expression starts with @code{get-buffer-create buffer}. The
+function tells the computer to use the buffer with the name specified
+as the one to which you are copying, or if such a buffer does not
+exist, to create it. Then, the @code{with-current-buffer} function
+evaluates its body with that buffer temporarily current.
+
+(This demonstrates another way to shift the computer's attention but
+not the user's. The @code{append-to-buffer} function showed how to do
+the same with @code{save-excursion} and @code{set-buffer}.
address@hidden is a newer, and arguably easier,
+mechanism.)
+
+The @code{barf-if-buffer-read-only} function sends you an error
+message saying the buffer is read-only if you cannot modify it.
+
+The next line has the @code{erase-buffer} function as its sole
+contents. That function erases the buffer.
+
+Finally, the last two lines contain the @code{save-excursion}
+expression with @code{insert-buffer-substring} as its body.
+The @code{insert-buffer-substring} expression copies the text from
+the buffer you are in (and you have not seen the computer shift its
+attention, so you don't know that that buffer is now called
address@hidden).
+
+Incidentally, this is what is meant by `replacement'. To replace text,
+Emacs erases the previous text and then inserts new text.
+
address@hidden 1250
+In outline, the body of @code{copy-to-buffer} looks like this:
+
address@hidden
address@hidden
+(let (@address@hidden@address@hidden)
+ (@var{with-the-buffer-you-are-copying-to}
+ (@var{but-do-not-erase-or-copy-to-a-read-only-buffer})
+ (erase-buffer)
+ (save-excursion
+ @address@hidden@var{-into-buffer})))
address@hidden group
address@hidden smallexample
+
address@hidden insert-buffer, beginning-of-buffer, copy-to-buffer, More Complex
address@hidden node-name, next, previous, up
address@hidden The Definition of @code{insert-buffer}
address@hidden insert-buffer
+
address@hidden is yet another buffer-related function. This
+command copies another buffer @emph{into} the current buffer. It is the
+reverse of @code{append-to-buffer} or @code{copy-to-buffer}, since they
+copy a region of text @emph{from} the current buffer to another buffer.
+
+Here is a discussion based on the original code. The code was
+simplified in 2003 and is harder to understand.
+
+(@xref{New insert-buffer, , New Body for @code{insert-buffer}}, to see
+a discussion of the new body.)
+
+In addition, this code illustrates the use of @code{interactive} with a
+buffer that might be @dfn{read-only} and the important distinction
+between the name of an object and the object actually referred to.
+
address@hidden
+* insert-buffer code::
+* insert-buffer interactive:: When you can read, but not write.
+* insert-buffer body:: The body has an @code{or} and a @code{let}.
+* if & or:: Using an @code{if} instead of an @code{or}.
+* Insert or:: How the @code{or} expression works.
+* Insert let:: Two @code{save-excursion} expressions.
+* New insert-buffer::
address@hidden menu
+
address@hidden insert-buffer code, insert-buffer interactive, insert-buffer,
insert-buffer
address@hidden
address@hidden The Code for @code{insert-buffer}
address@hidden ifnottex
+
address@hidden 800
+Here is the earlier code:
+
address@hidden
address@hidden
+(defun insert-buffer (buffer)
+ "Insert after point the contents of BUFFER.
+Puts mark after the inserted text.
+BUFFER may be a buffer or a buffer name."
+ (interactive "*bInsert buffer:@: ")
address@hidden group
address@hidden
+ (or (bufferp buffer)
+ (setq buffer (get-buffer buffer)))
+ (let (start end newmark)
+ (save-excursion
+ (save-excursion
+ (set-buffer buffer)
+ (setq start (point-min) end (point-max)))
address@hidden group
address@hidden
+ (insert-buffer-substring buffer start end)
+ (setq newmark (point)))
+ (push-mark newmark)))
address@hidden group
address@hidden smallexample
+
address@hidden 1200
+As with other function definitions, you can use a template to see an
+outline of the function:
+
address@hidden
address@hidden
+(defun insert-buffer (buffer)
+ "@address@hidden"
+ (interactive "*bInsert buffer:@: ")
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
address@hidden insert-buffer interactive, insert-buffer body, insert-buffer
code, insert-buffer
address@hidden node-name, next, previous, up
address@hidden The Interactive Expression in @code{insert-buffer}
address@hidden interactive, @r{example use of}
+
+In @code{insert-buffer}, the argument to the @code{interactive}
+declaration has two parts, an asterisk, @samp{*}, and @samp{bInsert
+buffer:@: }.
+
address@hidden
+* Read-only buffer:: When a buffer cannot be modified.
+* b for interactive:: An existing buffer or else its name.
address@hidden menu
+
address@hidden Read-only buffer, b for interactive, insert-buffer interactive,
insert-buffer interactive
address@hidden node-name, next, previous, up
address@hidden A Read-only Buffer
address@hidden Read-only buffer
address@hidden Asterisk for read-only buffer
address@hidden * @r{for read-only buffer}
+
+The asterisk is for the situation when the current buffer is a
+read-only buffer---a buffer that cannot be modified. If
address@hidden is called when the current buffer is read-only, a
+message to this effect is printed in the echo area and the terminal
+may beep or blink at you; you will not be permitted to insert anything
+into current buffer. The asterisk does not need to be followed by a
+newline to separate it from the next argument.
+
address@hidden b for interactive, , Read-only buffer, insert-buffer interactive
address@hidden node-name, next, previous, up
address@hidden @samp{b} in an Interactive Expression
+
+The next argument in the interactive expression starts with a lower
+case @samp{b}. (This is different from the code for
address@hidden, which uses an upper-case @samp{B}.
address@hidden, , The Definition of @code{append-to-buffer}}.)
+The lower-case @samp{b} tells the Lisp interpreter that the argument
+for @code{insert-buffer} should be an existing buffer or else its
+name. (The upper-case @samp{B} option provides for the possibility
+that the buffer does not exist.) Emacs will prompt you for the name
+of the buffer, offering you a default buffer, with name completion
+enabled. If the buffer does not exist, you receive a message that
+says ``No match''; your terminal may beep at you as well.
+
+The new and simplified code generates a list for @code{interactive}.
+It uses the @code{barf-if-buffer-read-only} and @code{read-buffer}
+functions with which we are already familiar and the @code{progn}
+special form with which we are not. (It will be described later.)
+
address@hidden insert-buffer body, if & or, insert-buffer interactive,
insert-buffer
address@hidden node-name, next, previous, up
address@hidden The Body of the @code{insert-buffer} Function
+
+The body of the @code{insert-buffer} function has two major parts: an
address@hidden expression and a @code{let} expression. The purpose of the
address@hidden expression is to ensure that the argument @code{buffer} is
+bound to a buffer and not just the name of a buffer. The body of the
address@hidden expression contains the code which copies the other buffer
+into the current buffer.
+
address@hidden 1250
+In outline, the two expressions fit into the @code{insert-buffer}
+function like this:
+
address@hidden
address@hidden
+(defun insert-buffer (buffer)
+ "@address@hidden"
+ (interactive "*bInsert buffer:@: ")
+ (or @dots{}
+ @dots{}
address@hidden group
address@hidden
+ (let (@var{varlist})
+ @address@hidden@dots{} )
address@hidden group
address@hidden smallexample
+
+To understand how the @code{or} expression ensures that the argument
address@hidden is bound to a buffer and not to the name of a buffer, it
+is first necessary to understand the @code{or} function.
+
+Before doing this, let me rewrite this part of the function using
address@hidden so that you can see what is done in a manner that will be
familiar.
+
address@hidden if & or, Insert or, insert-buffer body, insert-buffer
address@hidden node-name, next, previous, up
address@hidden @code{insert-buffer} With an @code{if} Instead of an @code{or}
+
+The job to be done is to make sure the value of @code{buffer} is a
+buffer itself and not the name of a buffer. If the value is the name,
+then the buffer itself must be got.
+
+You can imagine yourself at a conference where an usher is wandering
+around holding a list with your name on it and looking for you: the
+usher is ``bound'' to your name, not to you; but when the usher finds
+you and takes your arm, the usher becomes ``bound'' to you.
+
address@hidden 800
+In Lisp, you might describe this situation like this:
+
address@hidden
address@hidden
+(if (not (holding-on-to-guest))
+ (find-and-take-arm-of-guest))
address@hidden group
address@hidden smallexample
+
+We want to do the same thing with a buffer---if we do not have the
+buffer itself, we want to get it.
+
address@hidden 1200
+Using a predicate called @code{bufferp} that tells us whether we have a
+buffer (rather than its name), we can write the code like this:
+
address@hidden
address@hidden
+(if (not (bufferp buffer)) ; @r{if-part}
+ (setq buffer (get-buffer buffer))) ; @r{then-part}
address@hidden group
address@hidden smallexample
+
address@hidden
+Here, the true-or-false-test of the @code{if} expression is
address@hidden@code{(not (bufferp buffer))}}; and the then-part is the
expression
address@hidden@code{(setq buffer (get-buffer buffer))}}.
+
+In the test, the function @code{bufferp} returns true if its argument is
+a buffer---but false if its argument is the name of the buffer. (The
+last character of the function name @code{bufferp} is the character
address@hidden; as we saw earlier, such use of @samp{p} is a convention that
+indicates that the function is a predicate, which is a term that means
+that the function will determine whether some property is true or false.
address@hidden Type of Argument, , Using the Wrong Type Object as an
+Argument}.)
+
address@hidden 1200
+The function @code{not} precedes the expression @code{(bufferp buffer)},
+so the true-or-false-test looks like this:
+
address@hidden
+(not (bufferp buffer))
address@hidden smallexample
+
address@hidden
address@hidden is a function that returns true if its argument is false
+and false if its argument is true. So if @code{(bufferp buffer)}
+returns true, the @code{not} expression returns false and vice-verse:
+what is ``not true'' is false and what is ``not false'' is true.
+
+Using this test, the @code{if} expression works as follows: when the
+value of the variable @code{buffer} is actually a buffer rather than
+its name, the true-or-false-test returns false and the @code{if}
+expression does not evaluate the then-part. This is fine, since we do
+not need to do anything to the variable @code{buffer} if it really is
+a buffer.
+
+On the other hand, when the value of @code{buffer} is not a buffer
+itself, but the name of a buffer, the true-or-false-test returns true
+and the then-part of the expression is evaluated. In this case, the
+then-part is @code{(setq buffer (get-buffer buffer))}. This
+expression uses the @code{get-buffer} function to return an actual
+buffer itself, given its name. The @code{setq} then sets the variable
address@hidden to the value of the buffer itself, replacing its previous
+value (which was the name of the buffer).
+
address@hidden Insert or, Insert let, if & or, insert-buffer
address@hidden node-name, next, previous, up
address@hidden The @code{or} in the Body
+
+The purpose of the @code{or} expression in the @code{insert-buffer}
+function is to ensure that the argument @code{buffer} is bound to a
+buffer and not just to the name of a buffer. The previous section shows
+how the job could have been done using an @code{if} expression.
+However, the @code{insert-buffer} function actually uses @code{or}.
+To understand this, it is necessary to understand how @code{or} works.
+
address@hidden or
+An @code{or} function can have any number of arguments. It evaluates
+each argument in turn and returns the value of the first of its
+arguments that is not @code{nil}. Also, and this is a crucial feature
+of @code{or}, it does not evaluate any subsequent arguments after
+returning the first address@hidden value.
+
address@hidden 800
+The @code{or} expression looks like this:
+
address@hidden
address@hidden
+(or (bufferp buffer)
+ (setq buffer (get-buffer buffer)))
address@hidden group
address@hidden smallexample
+
address@hidden
+The first argument to @code{or} is the expression @code{(bufferp buffer)}.
+This expression returns true (a address@hidden value) if the buffer is
+actually a buffer, and not just the name of a buffer. In the @code{or}
+expression, if this is the case, the @code{or} expression returns this
+true value and does not evaluate the next expression---and this is fine
+with us, since we do not want to do anything to the value of
address@hidden if it really is a buffer.
+
+On the other hand, if the value of @code{(bufferp buffer)} is @code{nil},
+which it will be if the value of @code{buffer} is the name of a buffer,
+the Lisp interpreter evaluates the next element of the @code{or}
+expression. This is the expression @code{(setq buffer (get-buffer
+buffer))}. This expression returns a address@hidden value, which
+is the value to which it sets the variable @code{buffer}---and this
+value is a buffer itself, not the name of a buffer.
+
+The result of all this is that the symbol @code{buffer} is always
+bound to a buffer itself rather than to the name of a buffer. All
+this is necessary because the @code{set-buffer} function in a
+following line only works with a buffer itself, not with the name to a
+buffer.
+
address@hidden 1250
+Incidentally, using @code{or}, the situation with the usher would be
+written like this:
+
address@hidden
+(or (holding-on-to-guest) (find-and-take-arm-of-guest))
address@hidden smallexample
+
address@hidden Insert let, New insert-buffer, Insert or, insert-buffer
address@hidden node-name, next, previous, up
address@hidden The @code{let} Expression in @code{insert-buffer}
+
+After ensuring that the variable @code{buffer} refers to a buffer itself
+and not just to the name of a buffer, the @code{insert-buffer function}
+continues with a @code{let} expression. This specifies three local
+variables, @code{start}, @code{end}, and @code{newmark} and binds them
+to the initial value @code{nil}. These variables are used inside the
+remainder of the @code{let} and temporarily hide any other occurrence of
+variables of the same name in Emacs until the end of the @code{let}.
+
address@hidden 1200
+The body of the @code{let} contains two @code{save-excursion}
+expressions. First, we will look at the inner @code{save-excursion}
+expression in detail. The expression looks like this:
+
address@hidden
address@hidden
+(save-excursion
+ (set-buffer buffer)
+ (setq start (point-min) end (point-max)))
address@hidden group
address@hidden smallexample
+
address@hidden
+The expression @code{(set-buffer buffer)} changes Emacs' attention
+from the current buffer to the one from which the text will copied.
+In that buffer, the variables @code{start} and @code{end} are set to
+the beginning and end of the buffer, using the commands
address@hidden and @code{point-max}. Note that we have here an
+illustration of how @code{setq} is able to set two variables in the
+same expression. The first argument of @code{setq} is set to the
+value of its second, and its third argument is set to the value of its
+fourth.
+
+After the body of the inner @code{save-excursion} is evaluated, the
address@hidden restores the original buffer, but @code{start} and
address@hidden remain set to the values of the beginning and end of the
+buffer from which the text will be copied.
+
address@hidden 1250
+The outer @code{save-excursion} expression looks like this:
+
address@hidden
address@hidden
+(save-excursion
+ (@address@hidden@var{-expression}
+ (@address@hidden@address@hidden)
+ (insert-buffer-substring buffer start end)
+ (setq newmark (point)))
address@hidden group
address@hidden smallexample
+
address@hidden
+The @code{insert-buffer-substring} function copies the text
address@hidden the current buffer @emph{from} the region indicated by
address@hidden and @code{end} in @code{buffer}. Since the whole of the
+second buffer lies between @code{start} and @code{end}, the whole of
+the second buffer is copied into the buffer you are editing. Next,
+the value of point, which will be at the end of the inserted text, is
+recorded in the variable @code{newmark}.
+
+After the body of the outer @code{save-excursion} is evaluated, point
+and mark are relocated to their original places.
+
+However, it is convenient to locate a mark at the end of the newly
+inserted text and locate point at its beginning. The @code{newmark}
+variable records the end of the inserted text. In the last line of
+the @code{let} expression, the @code{(push-mark newmark)} expression
+function sets a mark to this location. (The previous location of the
+mark is still accessible; it is recorded on the mark ring and you can
+go back to it with @kbd{C-u address@hidden) Meanwhile, point is
+located at the beginning of the inserted text, which is where it was
+before you called the insert function, the position of which was saved
+by the first @code{save-excursion}.
+
address@hidden 1250
+The whole @code{let} expression looks like this:
+
address@hidden
address@hidden
+(let (start end newmark)
+ (save-excursion
+ (save-excursion
+ (set-buffer buffer)
+ (setq start (point-min) end (point-max)))
+ (insert-buffer-substring buffer start end)
+ (setq newmark (point)))
+ (push-mark newmark))
address@hidden group
address@hidden smallexample
+
+Like the @code{append-to-buffer} function, the @code{insert-buffer}
+function uses @code{let}, @code{save-excursion}, and
address@hidden In addition, the function illustrates one way to
+use @code{or}. All these functions are building blocks that we will
+find and use again and again.
+
address@hidden New insert-buffer, , Insert let, insert-buffer
address@hidden node-name, next, previous, up
address@hidden New Body for @code{insert-buffer}
address@hidden insert-buffer, new version body
address@hidden new version body for insert-buffer
+
+The body in the GNU Emacs 22 version is more confusing than the original.
+
address@hidden 1250
+It consists of two expressions,
+
address@hidden
address@hidden
+ (push-mark
+ (save-excursion
+ (insert-buffer-substring (get-buffer buffer))
+ (point)))
+
+ nil
address@hidden group
address@hidden smallexample
+
address@hidden
+except, and this is what confuses novices, very important work is done
+inside the @code{push-mark} expression.
+
+The @code{get-buffer} function returns a buffer with the name
+provided. You will note that the function is @emph{not} called
address@hidden; it does not create a buffer if one does not
+already exist. The buffer returned by @code{get-buffer}, an existing
+buffer, is passed to @code{insert-buffer-substring}, which inserts the
+whole of the buffer (since you did not specify anything else).
+
+The location into which the buffer is inserted is recorded by
address@hidden Then the function returns @code{nil}, the value of
+its last command. Put another way, the @code{insert-buffer} function
+exists only to produce a side effect, inserting another buffer, not to
+return any value.
+
address@hidden beginning-of-buffer, Second Buffer Related Review,
insert-buffer, More Complex
address@hidden node-name, next, previous, up
address@hidden Complete Definition of @code{beginning-of-buffer}
address@hidden beginning-of-buffer
+
+The basic structure of the @code{beginning-of-buffer} function has
+already been discussed. (@xref{simplified-beginning-of-buffer, , A
+Simplified @code{beginning-of-buffer} Definition}.)
+This section describes the complex part of the definition.
+
+As previously described, when invoked without an argument,
address@hidden moves the cursor to the beginning of the
+buffer (in truth, the beginning of the accessible portion of the
+buffer), leaving the mark at the previous position. However, when the
+command is invoked with a number between one and ten, the function
+considers that number to be a fraction of the length of the buffer,
+measured in tenths, and Emacs moves the cursor that fraction of the
+way from the beginning of the buffer. Thus, you can either call this
+function with the key command @kbd{M-<}, which will move the cursor to
+the beginning of the buffer, or with a key command such as @kbd{C-u 7
+M-<} which will move the cursor to a point 70% of the way through the
+buffer. If a number bigger than ten is used for the argument, it
+moves to the end of the buffer.
+
+The @code{beginning-of-buffer} function can be called with or without an
+argument. The use of the argument is optional.
+
address@hidden
+* Optional Arguments::
+* beginning-of-buffer opt arg:: Example with optional argument.
+* beginning-of-buffer complete::
address@hidden menu
+
address@hidden Optional Arguments, beginning-of-buffer opt arg,
beginning-of-buffer, beginning-of-buffer
address@hidden Optional Arguments
+
+Unless told otherwise, Lisp expects that a function with an argument in
+its function definition will be called with a value for that argument.
+If that does not happen, you get an error and a message that says
address@hidden number of arguments}.
+
address@hidden Optional arguments
address@hidden Keyword
address@hidden optional
+However, optional arguments are a feature of Lisp: a particular
address@hidden is used to tell the Lisp interpreter that an argument is
+optional. The keyword is @code{&optional}. (The @samp{&} in front of
address@hidden is part of the keyword.) In a function definition, if
+an argument follows the keyword @code{&optional}, no value need be
+passed to that argument when the function is called.
+
address@hidden 1200
+The first line of the function definition of @code{beginning-of-buffer}
+therefore looks like this:
+
address@hidden
+(defun beginning-of-buffer (&optional arg)
address@hidden smallexample
+
address@hidden 1250
+In outline, the whole function looks like this:
+
address@hidden
address@hidden
+(defun beginning-of-buffer (&optional arg)
+ "@address@hidden"
+ (interactive "P")
+ (or (@var{is-the-argument-a-cons-cell} arg)
+ (and @var{are-both-transient-mark-mode-and-mark-active-true})
+ (push-mark))
+ (let (@var{determine-size-and-set-it})
+ (goto-char
+ (@var{if-there-is-an-argument}
+ @var{figure-out-where-to-go}
+ @var{else-go-to}
+ (point-min))))
+ @var{do-nicety}
address@hidden group
address@hidden smallexample
+
+The function is similar to the @code{simplified-beginning-of-buffer}
+function except that the @code{interactive} expression has @code{"P"}
+as an argument and the @code{goto-char} function is followed by an
+if-then-else expression that figures out where to put the cursor if
+there is an argument that is not a cons cell.
+
+(Since I do not explain a cons cell for many more chapters, please
+consider ignoring the function @code{consp}. @xref{List
+Implementation, , How Lists are Implemented}, and @ref{Cons Cell Type,
+, Cons Cell and List Types, elisp, The GNU Emacs Lisp Reference
+Manual}.)
+
+The @code{"P"} in the @code{interactive} expression tells Emacs to
+pass a prefix argument, if there is one, to the function in raw form.
+A prefix argument is made by typing the @key{META} key followed by a
+number, or by typing @kbd{C-u} and then a number. (If you don't type
+a number, @kbd{C-u} defaults to a cons cell with a 4. A lowercase
address@hidden"p"} in the @code{interactive} expression causes the function to
+convert a prefix arg to a number.)
+
+The true-or-false-test of the @code{if} expression looks complex, but
+it is not: it checks whether @code{arg} has a value that is not
address@hidden and whether it is a cons cell. (That is what @code{consp}
+does; it checks whether its argument is a cons cell.) If @code{arg}
+has a value that is not @code{nil} (and is not a cons cell), which
+will be the case if @code{beginning-of-buffer} is called with a
+numeric argument, then this true-or-false-test will return true and
+the then-part of the @code{if} expression will be evaluated. On the
+other hand, if @code{beginning-of-buffer} is not called with an
+argument, the value of @code{arg} will be @code{nil} and the else-part
+of the @code{if} expression will be evaluated. The else-part is
+simply @code{point-min}, and when this is the outcome, the whole
address@hidden expression is @code{(goto-char (point-min))}, which
+is how we saw the @code{beginning-of-buffer} function in its
+simplified form.
+
address@hidden beginning-of-buffer opt arg, beginning-of-buffer complete,
Optional Arguments, beginning-of-buffer
address@hidden @code{beginning-of-buffer} with an Argument
+
+When @code{beginning-of-buffer} is called with an argument, an
+expression is evaluated which calculates what value to pass to
address@hidden This expression is rather complicated at first sight.
+It includes an inner @code{if} expression and much arithmetic. It looks
+like this:
+
address@hidden
address@hidden
+(if (> (buffer-size) 10000)
+ ;; @r{Avoid overflow for large buffer sizes!}
+ (* (prefix-numeric-value arg)
+ (/ size 10))
+ (/
+ (+ 10
+ (*
+ size (prefix-numeric-value arg))) 10)))
address@hidden group
address@hidden smallexample
+
address@hidden
+* Disentangle beginning-of-buffer::
+* Large buffer case::
+* Small buffer case::
address@hidden menu
+
address@hidden Disentangle beginning-of-buffer, Large buffer case,
beginning-of-buffer opt arg, beginning-of-buffer opt arg
address@hidden
address@hidden Disentangle @code{beginning-of-buffer}
address@hidden ifnottex
+
+Like other complex-looking expressions, the conditional expression
+within @code{beginning-of-buffer} can be disentangled by looking at it
+as parts of a template, in this case, the template for an if-then-else
+expression. In skeletal form, the expression looks like this:
+
address@hidden
address@hidden
+(if (@var{buffer-is-large}
+ @var{divide-buffer-size-by-10-and-multiply-by-arg}
+ @var{else-use-alternate-calculation}
address@hidden group
address@hidden smallexample
+
+The true-or-false-test of this inner @code{if} expression checks the
+size of the buffer. The reason for this is that the old version 18
+Emacs used numbers that are no bigger than eight million or so and in
+the computation that followed, the programmer feared that Emacs might
+try to use over-large numbers if the buffer were large. The term
+`overflow', mentioned in the comment, means numbers that are over
+large. More recent versions of Emacs use larger numbers, but this
+code has not been touched, if only because people now look at buffers
+that are far, far larger than ever before.
+
+There are two cases: if the buffer is large and if it is not.
+
address@hidden Large buffer case, Small buffer case, Disentangle
beginning-of-buffer, beginning-of-buffer opt arg
address@hidden node-name, next, previous, up
address@hidden What happens in a large buffer
+
+In @code{beginning-of-buffer}, the inner @code{if} expression tests
+whether the size of the buffer is greater than 10,000 characters. To do
+this, it uses the @code{>} function and the computation of @code{size}
+that comes from the let expression.
+
+In the old days, the function @code{buffer-size} was used. Not only
+was that function called several times, it gave the size of the whole
+buffer, not the accessible part. The computation makes much more
+sense when it handles just the accessible part. (@xref{Narrowing &
+Widening, , Narrowing and Widening}, for more information on focusing
+attention to an `accessible' part.)
+
address@hidden 800
+The line looks like this:
+
address@hidden
+(if (> size 10000)
address@hidden smallexample
+
address@hidden 1200
address@hidden
+When the buffer is large, the then-part of the @code{if} expression is
+evaluated. It reads like this (after formatting for easy reading):
+
address@hidden
address@hidden
+(*
+ (prefix-numeric-value arg)
+ (/ size 10))
address@hidden group
address@hidden smallexample
+
address@hidden
+This expression is a multiplication, with two arguments to the function
address@hidden
+
+The first argument is @code{(prefix-numeric-value arg)}. When
address@hidden"P"} is used as the argument for @code{interactive}, the value
+passed to the function as its argument is passed a ``raw prefix
+argument'', and not a number. (It is a number in a list.) To perform
+the arithmetic, a conversion is necessary, and
address@hidden does the job.
+
address@hidden / @r{(division)}
address@hidden Division
+The second argument is @code{(/ size 10)}. This expression divides
+the numeric value by ten --- the numeric value of the size of the
+accessible portion of the buffer. This produces a number that tells
+how many characters make up one tenth of the buffer size. (In Lisp,
address@hidden/} is used for division, just as @code{*} is used for
+multiplication.)
+
address@hidden 1200
+In the multiplication expression as a whole, this amount is multiplied
+by the value of the prefix argument---the multiplication looks like this:
+
address@hidden
address@hidden
+(* @var{numeric-value-of-prefix-arg}
+ @var{number-of-characters-in-one-tenth-of-the-accessible-buffer})
address@hidden group
address@hidden smallexample
+
address@hidden
+If, for example, the prefix argument is @samp{7}, the one-tenth value
+will be multiplied by 7 to give a position 70% of the way through.
+
address@hidden 1200
+The result of all this is that if the accessible portion of the buffer
+is large, the @code{goto-char} expression reads like this:
+
address@hidden
address@hidden
+(goto-char (* (prefix-numeric-value arg)
+ (/ size 10)))
address@hidden group
address@hidden smallexample
+
+This puts the cursor where we want it.
+
address@hidden Small buffer case, , Large buffer case, beginning-of-buffer opt
arg
address@hidden node-name, next, previous, up
address@hidden What happens in a small buffer
+
+If the buffer contains fewer than 10,000 characters, a slightly
+different computation is performed. You might think this is not
+necessary, since the first computation could do the job. However, in
+a small buffer, the first method may not put the cursor on exactly the
+desired line; the second method does a better job.
+
address@hidden 800
+The code looks like this:
+
address@hidden Keep this on one line.
address@hidden
+(/ (+ 10 (* size (prefix-numeric-value arg))) 10))
address@hidden smallexample
+
address@hidden 1200
address@hidden
+This is code in which you figure out what happens by discovering how the
+functions are embedded in parentheses. It is easier to read if you
+reformat it with each expression indented more deeply than its
+enclosing expression:
+
address@hidden
address@hidden
+ (/
+ (+ 10
+ (*
+ size
+ (prefix-numeric-value arg)))
+ 10))
address@hidden group
address@hidden smallexample
+
address@hidden 1200
address@hidden
+Looking at parentheses, we see that the innermost operation is
address@hidden(prefix-numeric-value arg)}, which converts the raw argument to
+a number. In the following expression, this number is multiplied by
+the size of the accessible portion of the buffer:
+
address@hidden
+(* size (prefix-numeric-value arg))
address@hidden smallexample
+
address@hidden
+This multiplication creates a number that may be larger than the size of
+the buffer---seven times larger if the argument is 7, for example. Ten
+is then added to this number and finally the large number is divided by
+ten to provide a value that is one character larger than the percentage
+position in the buffer.
+
+The number that results from all this is passed to @code{goto-char} and
+the cursor is moved to that point.
+
address@hidden 1500
address@hidden beginning-of-buffer complete, , beginning-of-buffer opt arg,
beginning-of-buffer
address@hidden node-name, next, previous, up
address@hidden The Complete @code{beginning-of-buffer}
+
address@hidden 1000
+Here is the complete text of the @code{beginning-of-buffer} function:
address@hidden 1
+
address@hidden In GNU Emacs 22
address@hidden
address@hidden
+(defun beginning-of-buffer (&optional arg)
+ "Move point to the beginning of the buffer;
+leave mark at previous position.
+With \\[universal-argument] prefix,
+do not set mark at previous position.
+With numeric arg N,
+put point N/10 of the way from the beginning.
+
+If the buffer is narrowed,
+this command uses the beginning and size
+of the accessible part of the buffer.
address@hidden group
+
address@hidden
+Don't use this command in Lisp programs!
+\(goto-char (point-min)) is faster
+and avoids clobbering the mark."
+ (interactive "P")
+ (or (consp arg)
+ (and transient-mark-mode mark-active)
+ (push-mark))
address@hidden group
address@hidden
+ (let ((size (- (point-max) (point-min))))
+ (goto-char (if (and arg (not (consp arg)))
+ (+ (point-min)
+ (if (> size 10000)
+ ;; Avoid overflow for large buffer sizes!
+ (* (prefix-numeric-value arg)
+ (/ size 10))
+ (/ (+ 10 (* size (prefix-numeric-value arg))) 10)))
+ (point-min))))
+ (if arg (forward-line 1)))
address@hidden group
address@hidden smallexample
+
address@hidden
+From before GNU Emacs 22
address@hidden
address@hidden
+(defun beginning-of-buffer (&optional arg)
+ "Move point to the beginning of the buffer;
+leave mark at previous position.
+With arg N, put point N/10 of the way
+from the true beginning.
address@hidden group
address@hidden
+Don't use this in Lisp programs!
+\(goto-char (point-min)) is faster
+and does not set the mark."
+ (interactive "P")
+ (push-mark)
address@hidden group
address@hidden
+ (goto-char
+ (if arg
+ (if (> (buffer-size) 10000)
+ ;; @r{Avoid overflow for large buffer sizes!}
+ (* (prefix-numeric-value arg)
+ (/ (buffer-size) 10))
address@hidden group
address@hidden
+ (/ (+ 10 (* (buffer-size)
+ (prefix-numeric-value arg)))
+ 10))
+ (point-min)))
+ (if arg (forward-line 1)))
address@hidden group
address@hidden smallexample
address@hidden ignore
+
address@hidden
+Except for two small points, the previous discussion shows how this
+function works. The first point deals with a detail in the
+documentation string, and the second point concerns the last line of
+the function.
+
address@hidden 800
+In the documentation string, there is reference to an expression:
+
address@hidden
+\\[universal-argument]
address@hidden smallexample
+
address@hidden
+A @samp{\\} is used before the first square bracket of this
+expression. This @samp{\\} tells the Lisp interpreter to substitute
+whatever key is currently bound to the @address@hidden In the case
+of @code{universal-argument}, that is usually @kbd{C-u}, but it might
+be different. (@xref{Documentation Tips, , Tips for Documentation
+Strings, elisp, The GNU Emacs Lisp Reference Manual}, for more
+information.)
+
address@hidden 1200
+Finally, the last line of the @code{beginning-of-buffer} command says
+to move point to the beginning of the next line if the command is
+invoked with an argument:
+
address@hidden
+(if arg (forward-line 1)))
address@hidden smallexample
+
address@hidden
+This puts the cursor at the beginning of the first line after the
+appropriate tenths position in the buffer. This is a flourish that
+means that the cursor is always located @emph{at least} the requested
+tenths of the way through the buffer, which is a nicety that is,
+perhaps, not necessary, but which, if it did not occur, would be sure
+to draw complaints.
+
+On the other hand, it also means that if you specify the command with
+a @kbd{C-u}, but without a number, that is to say, if the `raw prefix
+argument' is simply a cons cell, then the command puts you at the
+beginning of the second line @dots{} I don't know whether this is
+intended or whether no one has dealt with the code to avoid this
+happening.
+
address@hidden Second Buffer Related Review, optional Exercise,
beginning-of-buffer, More Complex
address@hidden node-name, next, previous, up
address@hidden Review
+
+Here is a brief summary of some of the topics covered in this chapter.
+
address@hidden @code
address@hidden or
+Evaluate each argument in sequence, and return the value of the first
+argument that is not @code{nil}; if none return a value that is not
address@hidden, return @code{nil}. In brief, return the first true value
+of the arguments; return a true value if one @emph{or} any of the
+others are true.
+
address@hidden and
+Evaluate each argument in sequence, and if any are @code{nil}, return
address@hidden; if none are @code{nil}, return the value of the last
+argument. In brief, return a true value only if all the arguments are
+true; return a true value if one @emph{and} each of the others is
+true.
+
address@hidden &optional
+A keyword used to indicate that an argument to a function definition
+is optional; this means that the function can be evaluated without the
+argument, if desired.
+
address@hidden prefix-numeric-value
+Convert the `raw prefix argument' produced by @code{(interactive
+"P")} to a numeric value.
+
address@hidden forward-line
+Move point forward to the beginning of the next line, or if the argument
+is greater than one, forward that many lines. If it can't move as far
+forward as it is supposed to, @code{forward-line} goes forward as far as
+it can and then returns a count of the number of additional lines it was
+supposed to move but couldn't.
+
address@hidden erase-buffer
+Delete the entire contents of the current buffer.
+
address@hidden bufferp
+Return @code{t} if its argument is a buffer; otherwise return @code{nil}.
address@hidden table
+
address@hidden optional Exercise, , Second Buffer Related Review, More Complex
address@hidden @code{optional} Argument Exercise
+
+Write an interactive function with an optional argument that tests
+whether its argument, a number, is greater than or equal to, or else,
+less than the value of @code{fill-column}, and tells you which, in a
+message. However, if you do not pass an argument to the function, use
+56 as a default value.
+
address@hidden Narrowing & Widening, car cdr & cons, More Complex, Top
address@hidden node-name, next, previous, up
address@hidden Narrowing and Widening
address@hidden Focusing attention (narrowing)
address@hidden Narrowing
address@hidden Widening
+
+Narrowing is a feature of Emacs that makes it possible for you to focus
+on a specific part of a buffer, and work without accidentally changing
+other parts. Narrowing is normally disabled since it can confuse
+novices.
+
address@hidden
+* Narrowing advantages:: The advantages of narrowing
+* save-restriction:: The @code{save-restriction} special form.
+* what-line:: The number of the line that point is on.
+* narrow Exercise::
address@hidden menu
+
address@hidden Narrowing advantages, save-restriction, Narrowing & Widening,
Narrowing & Widening
address@hidden
address@hidden The Advantages of Narrowing
address@hidden ifnottex
+
+With narrowing, the rest of a buffer is made invisible, as if it weren't
+there. This is an advantage if, for example, you want to replace a word
+in one part of a buffer but not in another: you narrow to the part you want
+and the replacement is carried out only in that section, not in the rest
+of the buffer. Searches will only work within a narrowed region, not
+outside of one, so if you are fixing a part of a document, you can keep
+yourself from accidentally finding parts you do not need to fix by
+narrowing just to the region you want.
+(The key binding for @code{narrow-to-region} is @kbd{C-x n n}.)
+
+However, narrowing does make the rest of the buffer invisible, which
+can scare people who inadvertently invoke narrowing and think they
+have deleted a part of their file. Moreover, the @code{undo} command
+(which is usually bound to @kbd{C-x u}) does not turn off narrowing
+(nor should it), so people can become quite desperate if they do not
+know that they can return the rest of a buffer to visibility with the
address@hidden command.
+(The key binding for @code{widen} is @kbd{C-x n w}.)
+
+Narrowing is just as useful to the Lisp interpreter as to a human.
+Often, an Emacs Lisp function is designed to work on just part of a
+buffer; or conversely, an Emacs Lisp function needs to work on all of a
+buffer that has been narrowed. The @code{what-line} function, for
+example, removes the narrowing from a buffer, if it has any narrowing
+and when it has finished its job, restores the narrowing to what it was.
+On the other hand, the @code{count-lines} function, which is called by
address@hidden, uses narrowing to restrict itself to just that portion
+of the buffer in which it is interested and then restores the previous
+situation.
+
address@hidden save-restriction, what-line, Narrowing advantages, Narrowing &
Widening
address@hidden node-name, next, previous, up
address@hidden The @code{save-restriction} Special Form
address@hidden save-restriction
+
+In Emacs Lisp, you can use the @code{save-restriction} special form to
+keep track of whatever narrowing is in effect, if any. When the Lisp
+interpreter meets with @code{save-restriction}, it executes the code
+in the body of the @code{save-restriction} expression, and then undoes
+any changes to narrowing that the code caused. If, for example, the
+buffer is narrowed and the code that follows @code{save-restriction}
+gets rid of the narrowing, @code{save-restriction} returns the buffer
+to its narrowed region afterwards. In the @code{what-line} command,
+any narrowing the buffer may have is undone by the @code{widen}
+command that immediately follows the @code{save-restriction} command.
+Any original narrowing is restored just before the completion of the
+function.
+
address@hidden 1250
+The template for a @code{save-restriction} expression is simple:
+
address@hidden
address@hidden
+(save-restriction
+ @address@hidden )
address@hidden group
address@hidden smallexample
+
address@hidden
+The body of the @code{save-restriction} is one or more expressions that
+will be evaluated in sequence by the Lisp interpreter.
+
+Finally, a point to note: when you use both @code{save-excursion} and
address@hidden, one right after the other, you should use
address@hidden outermost. If you write them in reverse order,
+you may fail to record narrowing in the buffer to which Emacs switches
+after calling @code{save-excursion}. Thus, when written together,
address@hidden and @code{save-restriction} should be written
+like this:
+
address@hidden
address@hidden
+(save-excursion
+ (save-restriction
+ @address@hidden))
address@hidden group
address@hidden smallexample
+
+In other circumstances, when not written together, the
address@hidden and @code{save-restriction} special forms must
+be written in the order appropriate to the function.
+
address@hidden 1250
+For example,
+
address@hidden
address@hidden
+ (save-restriction
+ (widen)
+ (save-excursion
+ @address@hidden))
address@hidden group
address@hidden smallexample
+
address@hidden
+Emacs 22
+/usr/local/src/emacs/lisp/simple.el
+
+(defun what-line ()
+ "Print the current buffer line number and narrowed line number of point."
+ (interactive)
+ (let ((start (point-min))
+ (n (line-number-at-pos)))
+ (if (= start 1)
+ (message "Line %d" n)
+ (save-excursion
+ (save-restriction
+ (widen)
+ (message "line %d (narrowed line %d)"
+ (+ n (line-number-at-pos start) -1) n))))))
+
+(defun line-number-at-pos (&optional pos)
+ "Return (narrowed) buffer line number at position POS.
+If POS is nil, use current buffer location.
+Counting starts at (point-min), so the value refers
+to the contents of the accessible portion of the buffer."
+ (let ((opoint (or pos (point))) start)
+ (save-excursion
+ (goto-char (point-min))
+ (setq start (point))
+ (goto-char opoint)
+ (forward-line 0)
+ (1+ (count-lines start (point))))))
+
+(defun count-lines (start end)
+ "Return number of lines between START and END.
+This is usually the number of newlines between them,
+but can be one more if START is not equal to END
+and the greater of them is not at the start of a line."
+ (save-excursion
+ (save-restriction
+ (narrow-to-region start end)
+ (goto-char (point-min))
+ (if (eq selective-display t)
+ (save-match-data
+ (let ((done 0))
+ (while (re-search-forward "[\n\C-m]" nil t 40)
+ (setq done (+ 40 done)))
+ (while (re-search-forward "[\n\C-m]" nil t 1)
+ (setq done (+ 1 done)))
+ (goto-char (point-max))
+ (if (and (/= start end)
+ (not (bolp)))
+ (1+ done)
+ done)))
+ (- (buffer-size) (forward-line (buffer-size)))))))
address@hidden ignore
+
address@hidden what-line, narrow Exercise, save-restriction, Narrowing &
Widening
address@hidden node-name, next, previous, up
address@hidden @code{what-line}
address@hidden what-line
address@hidden Widening, example of
+
+The @code{what-line} command tells you the number of the line in which
+the cursor is located. The function illustrates the use of the
address@hidden and @code{save-excursion} commands. Here is the
+original text of the function:
+
address@hidden
address@hidden
+(defun what-line ()
+ "Print the current line number (in the buffer) of point."
+ (interactive)
+ (save-restriction
+ (widen)
+ (save-excursion
+ (beginning-of-line)
+ (message "Line %d"
+ (1+ (count-lines 1 (point)))))))
address@hidden group
address@hidden smallexample
+
+(In recent versions of GNU Emacs, the @code{what-line} function has
+been expanded to tell you your line number in a narrowed buffer as
+well as your line number in a widened buffer. The recent version is
+more complex than the version shown here. If you feel adventurous,
+you might want to look at it after figuring out how this version
+works. You will probably need to use @kbd{C-h f}
+(@code{describe-function}). The newer version uses a conditional to
+determine whether the buffer has been narrowed.
+
+(Also, it uses @code{line-number-at-pos}, which among other simple
+expressions, such as @code{(goto-char (point-min))}, moves point to
+the beginning of the current line with @code{(forward-line 0)} rather
+than @code{beginning-of-line}.)
+
+The @code{what-line} function as shown here has a documentation line
+and is interactive, as you would expect. The next two lines use the
+functions @code{save-restriction} and @code{widen}.
+
+The @code{save-restriction} special form notes whatever narrowing is in
+effect, if any, in the current buffer and restores that narrowing after
+the code in the body of the @code{save-restriction} has been evaluated.
+
+The @code{save-restriction} special form is followed by @code{widen}.
+This function undoes any narrowing the current buffer may have had
+when @code{what-line} was called. (The narrowing that was there is
+the narrowing that @code{save-restriction} remembers.) This widening
+makes it possible for the line counting commands to count from the
+beginning of the buffer. Otherwise, they would have been limited to
+counting within the accessible region. Any original narrowing is
+restored just before the completion of the function by the
address@hidden special form.
+
+The call to @code{widen} is followed by @code{save-excursion}, which
+saves the location of the cursor (i.e., of point) and of the mark, and
+restores them after the code in the body of the @code{save-excursion}
+uses the @code{beginning-of-line} function to move point.
+
+(Note that the @code{(widen)} expression comes between the
address@hidden and @code{save-excursion} special forms. When
+you write the two @code{save- @dots{}} expressions in sequence, write
address@hidden outermost.)
+
address@hidden 1200
+The last two lines of the @code{what-line} function are functions to
+count the number of lines in the buffer and then print the number in the
+echo area.
+
address@hidden
address@hidden
+(message "Line %d"
+ (1+ (count-lines 1 (point)))))))
address@hidden group
address@hidden smallexample
+
+The @code{message} function prints a one-line message at the bottom of
+the Emacs screen. The first argument is inside of quotation marks and
+is printed as a string of characters. However, it may contain a
address@hidden expression to print a following argument. @samp{%d} prints
+the argument as a decimal, so the message will say something such as
address@hidden 243}.
+
address@hidden 1200
+The number that is printed in place of the @samp{%d} is computed by the
+last line of the function:
+
address@hidden
+(1+ (count-lines 1 (point)))
address@hidden smallexample
+
address@hidden
+GNU Emacs 22
+
+(defun count-lines (start end)
+ "Return number of lines between START and END.
+This is usually the number of newlines between them,
+but can be one more if START is not equal to END
+and the greater of them is not at the start of a line."
+ (save-excursion
+ (save-restriction
+ (narrow-to-region start end)
+ (goto-char (point-min))
+ (if (eq selective-display t)
+ (save-match-data
+ (let ((done 0))
+ (while (re-search-forward "[\n\C-m]" nil t 40)
+ (setq done (+ 40 done)))
+ (while (re-search-forward "[\n\C-m]" nil t 1)
+ (setq done (+ 1 done)))
+ (goto-char (point-max))
+ (if (and (/= start end)
+ (not (bolp)))
+ (1+ done)
+ done)))
+ (- (buffer-size) (forward-line (buffer-size)))))))
address@hidden ignore
+
address@hidden
+What this does is count the lines from the first position of the
+buffer, indicated by the @code{1}, up to @code{(point)}, and then add
+one to that number. (The @code{1+} function adds one to its
+argument.) We add one to it because line 2 has only one line before
+it, and @code{count-lines} counts only the lines @emph{before} the
+current line.
+
+After @code{count-lines} has done its job, and the message has been
+printed in the echo area, the @code{save-excursion} restores point and
+mark to their original positions; and @code{save-restriction} restores
+the original narrowing, if any.
+
address@hidden narrow Exercise, , what-line, Narrowing & Widening
address@hidden Exercise with Narrowing
+
+Write a function that will display the first 60 characters of the
+current buffer, even if you have narrowed the buffer to its latter
+half so that the first line is inaccessible. Restore point, mark, and
+narrowing. For this exercise, you need to use a whole potpourri of
+functions, including @code{save-restriction}, @code{widen},
address@hidden, @code{point-min}, @code{message}, and
address@hidden
+
address@hidden Properties, mention of @code{buffer-substring-no-properties}
+(@code{buffer-substring} is a previously unmentioned function you will
+have to investigate yourself; or perhaps you will have to use
address@hidden or
address@hidden @dots{}, yet other functions. Text
+properties are a feature otherwise not discussed here. @xref{Text
+Properties, , Text Properties, elisp, The GNU Emacs Lisp Reference
+Manual}.)
+
+Additionally, do you really need @code{goto-char} or @code{point-min}?
+Or can you write the function without them?
+
address@hidden car cdr & cons, Cutting & Storing Text, Narrowing & Widening, Top
address@hidden node-name, next, previous, up
address@hidden @code{car}, @code{cdr}, @code{cons}: Fundamental Functions
address@hidden car, @r{introduced}
address@hidden cdr, @r{introduced}
+
+In Lisp, @code{car}, @code{cdr}, and @code{cons} are fundamental
+functions. The @code{cons} function is used to construct lists, and
+the @code{car} and @code{cdr} functions are used to take them apart.
+
+In the walk through of the @code{copy-region-as-kill} function, we
+will see @code{cons} as well as two variants on @code{cdr},
+namely, @code{setcdr} and @code{nthcdr}. (@xref{copy-region-as-kill}.)
+
address@hidden
+* Strange Names:: An historical aside: why the strange names?
+* car & cdr:: Functions for extracting part of a list.
+* cons:: Constructing a list.
+* nthcdr:: Calling @code{cdr} repeatedly.
+* nth::
+* setcar:: Changing the first element of a list.
+* setcdr:: Changing the rest of a list.
+* cons Exercise::
address@hidden menu
+
address@hidden Strange Names, car & cdr, car cdr & cons, car cdr & cons
address@hidden
address@hidden Strange Names
address@hidden ifnottex
+
+The name of the @code{cons} function is not unreasonable: it is an
+abbreviation of the word `construct'. The origins of the names for
address@hidden and @code{cdr}, on the other hand, are esoteric: @code{car}
+is an acronym from the phrase `Contents of the Address part of the
+Register'; and @code{cdr} (pronounced `could-er') is an acronym from
+the phrase `Contents of the Decrement part of the Register'. These
+phrases refer to specific pieces of hardware on the very early
+computer on which the original Lisp was developed. Besides being
+obsolete, the phrases have been completely irrelevant for more than 25
+years to anyone thinking about Lisp. Nonetheless, although a few
+brave scholars have begun to use more reasonable names for these
+functions, the old terms are still in use. In particular, since the
+terms are used in the Emacs Lisp source code, we will use them in this
+introduction.
+
address@hidden car & cdr, cons, Strange Names, car cdr & cons
address@hidden node-name, next, previous, up
address@hidden @code{car} and @code{cdr}
+
+The @sc{car} of a list is, quite simply, the first item in the list.
+Thus the @sc{car} of the list @code{(rose violet daisy buttercup)} is
address@hidden
+
address@hidden 1200
+If you are reading this in Info in GNU Emacs, you can see this by
+evaluating the following:
+
address@hidden
+(car '(rose violet daisy buttercup))
address@hidden smallexample
+
address@hidden
+After evaluating the expression, @code{rose} will appear in the echo
+area.
+
+Clearly, a more reasonable name for the @code{car} function would be
address@hidden and this is often suggested.
+
address@hidden does not remove the first item from the list; it only reports
+what it is. After @code{car} has been applied to a list, the list is
+still the same as it was. In the jargon, @code{car} is
+`non-destructive'. This feature turns out to be important.
+
+The @sc{cdr} of a list is the rest of the list, that is, the
address@hidden function returns the part of the list that follows the
+first item. Thus, while the @sc{car} of the list @code{'(rose violet
+daisy buttercup)} is @code{rose}, the rest of the list, the value
+returned by the @code{cdr} function, is @code{(violet daisy
+buttercup)}.
+
address@hidden 800
+You can see this by evaluating the following in the usual way:
+
address@hidden
+(cdr '(rose violet daisy buttercup))
address@hidden smallexample
+
address@hidden
+When you evaluate this, @code{(violet daisy buttercup)} will appear in
+the echo area.
+
+Like @code{car}, @code{cdr} does not remove any elements from the
+list---it just returns a report of what the second and subsequent
+elements are.
+
+Incidentally, in the example, the list of flowers is quoted. If it were
+not, the Lisp interpreter would try to evaluate the list by calling
address@hidden as a function. In this example, we do not want to do that.
+
+Clearly, a more reasonable name for @code{cdr} would be @code{rest}.
+
+(There is a lesson here: when you name new functions, consider very
+carefully what you are doing, since you may be stuck with the names
+for far longer than you expect. The reason this document perpetuates
+these names is that the Emacs Lisp source code uses them, and if I did
+not use them, you would have a hard time reading the code; but do,
+please, try to avoid using these terms yourself. The people who come
+after you will be grateful to you.)
+
+When @code{car} and @code{cdr} are applied to a list made up of symbols,
+such as the list @code{(pine fir oak maple)}, the element of the list
+returned by the function @code{car} is the symbol @code{pine} without
+any parentheses around it. @code{pine} is the first element in the
+list. However, the @sc{cdr} of the list is a list itself, @code{(fir
+oak maple)}, as you can see by evaluating the following expressions in
+the usual way:
+
address@hidden
address@hidden
+(car '(pine fir oak maple))
+
+(cdr '(pine fir oak maple))
address@hidden group
address@hidden smallexample
+
+On the other hand, in a list of lists, the first element is itself a
+list. @code{car} returns this first element as a list. For example,
+the following list contains three sub-lists, a list of carnivores, a
+list of herbivores and a list of sea mammals:
+
address@hidden
address@hidden
+(car '((lion tiger cheetah)
+ (gazelle antelope zebra)
+ (whale dolphin seal)))
address@hidden group
address@hidden smallexample
+
address@hidden
+In this example, the first element or @sc{car} of the list is the list of
+carnivores, @code{(lion tiger cheetah)}, and the rest of the list is
address@hidden((gazelle antelope zebra) (whale dolphin seal))}.
+
address@hidden
address@hidden
+(cdr '((lion tiger cheetah)
+ (gazelle antelope zebra)
+ (whale dolphin seal)))
address@hidden group
address@hidden smallexample
+
+It is worth saying again that @code{car} and @code{cdr} are
+non-destructive---that is, they do not modify or change lists to which
+they are applied. This is very important for how they are used.
+
+Also, in the first chapter, in the discussion about atoms, I said that
+in Lisp, ``certain kinds of atom, such as an array, can be separated
+into parts; but the mechanism for doing this is different from the
+mechanism for splitting a list. As far as Lisp is concerned, the
+atoms of a list are unsplittable.'' (@xref{Lisp Atoms}.) The
address@hidden and @code{cdr} functions are used for splitting lists and
+are considered fundamental to Lisp. Since they cannot split or gain
+access to the parts of an array, an array is considered an atom.
+Conversely, the other fundamental function, @code{cons}, can put
+together or construct a list, but not an array. (Arrays are handled
+by array-specific functions. @xref{Arrays, , Arrays, elisp, The GNU
+Emacs Lisp Reference Manual}.)
+
address@hidden cons, nthcdr, car & cdr, car cdr & cons
address@hidden node-name, next, previous, up
address@hidden @code{cons}
address@hidden cons, @r{introduced}
+
+The @code{cons} function constructs lists; it is the inverse of
address@hidden and @code{cdr}. For example, @code{cons} can be used to make
+a four element list from the three element list, @code{(fir oak maple)}:
+
address@hidden
+(cons 'pine '(fir oak maple))
address@hidden smallexample
+
address@hidden 800
address@hidden
+After evaluating this list, you will see
+
address@hidden
+(pine fir oak maple)
address@hidden smallexample
+
address@hidden
+appear in the echo area. @code{cons} causes the creation of a new
+list in which the element is followed by the elements of the original
+list.
+
+We often say that address@hidden puts a new element at the beginning of
+a list; it attaches or pushes elements onto the list', but this
+phrasing can be misleading, since @code{cons} does not change an
+existing list, but creates a new one.
+
+Like @code{car} and @code{cdr}, @code{cons} is non-destructive.
+
address@hidden
+* Build a list::
+* length:: How to find the length of a list.
address@hidden menu
+
address@hidden Build a list, length, cons, cons
address@hidden
address@hidden Build a list
address@hidden ifnottex
+
address@hidden must have a list to attach address@hidden, you can
address@hidden an element to an atom to produce a dotted pair. Dotted
+pairs are not discussed here; see @ref{Dotted Pair Notation, , Dotted
+Pair Notation, elisp, The GNU Emacs Lisp Reference Manual}.} You
+cannot start from absolutely nothing. If you are building a list, you
+need to provide at least an empty list at the beginning. Here is a
+series of @code{cons} expressions that build up a list of flowers. If
+you are reading this in Info in GNU Emacs, you can evaluate each of
+the expressions in the usual way; the value is printed in this text
+after @address@hidden, which you may read as `evaluates to'.
+
address@hidden
address@hidden
+(cons 'buttercup ())
+ @result{} (buttercup)
address@hidden group
+
address@hidden
+(cons 'daisy '(buttercup))
+ @result{} (daisy buttercup)
address@hidden group
+
address@hidden
+(cons 'violet '(daisy buttercup))
+ @result{} (violet daisy buttercup)
address@hidden group
+
address@hidden
+(cons 'rose '(violet daisy buttercup))
+ @result{} (rose violet daisy buttercup)
address@hidden group
address@hidden smallexample
+
address@hidden
+In the first example, the empty list is shown as @code{()} and a list
+made up of @code{buttercup} followed by the empty list is constructed.
+As you can see, the empty list is not shown in the list that was
+constructed. All that you see is @code{(buttercup)}. The empty list is
+not counted as an element of a list because there is nothing in an empty
+list. Generally speaking, an empty list is invisible.
+
+The second example, @code{(cons 'daisy '(buttercup))} constructs a new,
+two element list by putting @code{daisy} in front of @code{buttercup};
+and the third example constructs a three element list by putting
address@hidden in front of @code{daisy} and @code{buttercup}.
+
address@hidden length, , Build a list, cons
address@hidden node-name, next, previous, up
address@hidden Find the Length of a List: @code{length}
address@hidden length
+
+You can find out how many elements there are in a list by using the Lisp
+function @code{length}, as in the following examples:
+
address@hidden
address@hidden
+(length '(buttercup))
+ @result{} 1
address@hidden group
+
address@hidden
+(length '(daisy buttercup))
+ @result{} 2
address@hidden group
+
address@hidden
+(length (cons 'violet '(daisy buttercup)))
+ @result{} 3
address@hidden group
address@hidden smallexample
+
address@hidden
+In the third example, the @code{cons} function is used to construct a
+three element list which is then passed to the @code{length} function as
+its argument.
+
address@hidden 1200
+We can also use @code{length} to count the number of elements in an
+empty list:
+
address@hidden
address@hidden
+(length ())
+ @result{} 0
address@hidden group
address@hidden smallexample
+
address@hidden
+As you would expect, the number of elements in an empty list is zero.
+
+An interesting experiment is to find out what happens if you try to find
+the length of no list at all; that is, if you try to call @code{length}
+without giving it an argument, not even an empty list:
+
address@hidden
+(length )
address@hidden smallexample
+
address@hidden 800
address@hidden
+What you see, if you evaluate this, is the error message
+
address@hidden
+Lisp error: (wrong-number-of-arguments length 0)
address@hidden smallexample
+
address@hidden
+This means that the function receives the wrong number of
+arguments, zero, when it expects some other number of arguments. In
+this case, one argument is expected, the argument being a list whose
+length the function is measuring. (Note that @emph{one} list is
address@hidden argument, even if the list has many elements inside it.)
+
+The part of the error message that says @samp{length} is the name of
+the function.
+
address@hidden
address@hidden is still a subroutine, but you need C-h f to discover that.
+
+In an earlier version:
+ This is written with a special notation, @samp{#<subr},
+ that indicates that the function @code{length} is one of the primitive
+ functions written in C rather than in Emacs Lisp. (@samp{subr} is an
+ abbreviation for `subroutine'.) @xref{What Is a Function, , What Is a
+ Function?, elisp , The GNU Emacs Lisp Reference Manual}, for more
+ about subroutines.
address@hidden ignore
+
address@hidden nthcdr, nth, cons, car cdr & cons
address@hidden node-name, next, previous, up
address@hidden @code{nthcdr}
address@hidden nthcdr
+
+The @code{nthcdr} function is associated with the @code{cdr} function.
+What it does is take the @sc{cdr} of a list repeatedly.
+
+If you take the @sc{cdr} of the list @code{(pine fir
+oak maple)}, you will be returned the list @code{(fir oak maple)}. If you
+repeat this on what was returned, you will be returned the list
address@hidden(oak maple)}. (Of course, repeated @sc{cdr}ing on the original
+list will just give you the original @sc{cdr} since the function does
+not change the list. You need to evaluate the @sc{cdr} of the
address@hidden and so on.) If you continue this, eventually you will be
+returned an empty list, which in this case, instead of being shown as
address@hidden()} is shown as @code{nil}.
+
address@hidden 1200
+For review, here is a series of repeated @sc{cdr}s, the text following
+the @address@hidden shows what is returned.
+
address@hidden
address@hidden
+(cdr '(pine fir oak maple))
+ @result{}(fir oak maple)
address@hidden group
+
address@hidden
+(cdr '(fir oak maple))
+ @result{} (oak maple)
address@hidden group
+
address@hidden
+(cdr '(oak maple))
+ @result{}(maple)
address@hidden group
+
address@hidden
+(cdr '(maple))
+ @result{} nil
address@hidden group
+
address@hidden
+(cdr 'nil)
+ @result{} nil
address@hidden group
+
address@hidden
+(cdr ())
+ @result{} nil
address@hidden group
address@hidden smallexample
+
address@hidden 1200
+You can also do several @sc{cdr}s without printing the values in
+between, like this:
+
address@hidden
address@hidden
+(cdr (cdr '(pine fir oak maple)))
+ @result{} (oak maple)
address@hidden group
address@hidden smallexample
+
address@hidden
+In this example, the Lisp interpreter evaluates the innermost list first.
+The innermost list is quoted, so it just passes the list as it is to the
+innermost @code{cdr}. This @code{cdr} passes a list made up of the
+second and subsequent elements of the list to the outermost @code{cdr},
+which produces a list composed of the third and subsequent elements of
+the original list. In this example, the @code{cdr} function is repeated
+and returns a list that consists of the original list without its
+first two elements.
+
+The @code{nthcdr} function does the same as repeating the call to
address@hidden In the following example, the argument 2 is passed to the
+function @code{nthcdr}, along with the list, and the value returned is
+the list without its first two items, which is exactly the same
+as repeating @code{cdr} twice on the list:
+
address@hidden
address@hidden
+(nthcdr 2 '(pine fir oak maple))
+ @result{} (oak maple)
address@hidden group
address@hidden smallexample
+
address@hidden 1200
+Using the original four element list, we can see what happens when
+various numeric arguments are passed to @code{nthcdr}, including 0, 1,
+and 5:
+
address@hidden
address@hidden
+;; @r{Leave the list as it was.}
+(nthcdr 0 '(pine fir oak maple))
+ @result{} (pine fir oak maple)
address@hidden group
+
address@hidden
+;; @r{Return a copy without the first element.}
+(nthcdr 1 '(pine fir oak maple))
+ @result{} (fir oak maple)
address@hidden group
+
address@hidden
+;; @r{Return a copy of the list without three elements.}
+(nthcdr 3 '(pine fir oak maple))
+ @result{} (maple)
address@hidden group
+
address@hidden
+;; @r{Return a copy lacking all four elements.}
+(nthcdr 4 '(pine fir oak maple))
+ @result{} nil
address@hidden group
+
address@hidden
+;; @r{Return a copy lacking all elements.}
+(nthcdr 5 '(pine fir oak maple))
+ @result{} nil
address@hidden group
address@hidden smallexample
+
address@hidden nth, setcar, nthcdr, car cdr & cons
address@hidden node-name, next, previous, up
address@hidden @code{nth}
address@hidden nth
+
+The @code{nthcdr} function takes the @sc{cdr} of a list repeatedly.
+The @code{nth} function takes the @sc{car} of the result returned by
address@hidden It returns the Nth element of the list.
+
address@hidden 1500
+Thus, if it were not defined in C for speed, the definition of
address@hidden would be:
+
address@hidden
address@hidden
+(defun nth (n list)
+ "Returns the Nth element of LIST.
+N counts from zero. If LIST is not that long, nil is returned."
+ (car (nthcdr n list)))
address@hidden group
address@hidden smallexample
+
address@hidden
+(Originally, @code{nth} was defined in Emacs Lisp in @file{subr.el},
+but its definition was redone in C in the 1980s.)
+
+The @code{nth} function returns a single element of a list.
+This can be very convenient.
+
+Note that the elements are numbered from zero, not one. That is to
+say, the first element of a list, its @sc{car} is the zeroth element.
+This is called `zero-based' counting and often bothers people who
+are accustomed to the first element in a list being number one, which
+is `one-based'.
+
address@hidden 1250
+For example:
+
address@hidden
address@hidden
+(nth 0 '("one" "two" "three"))
+ @result{} "one"
+
+(nth 1 '("one" "two" "three"))
+ @result{} "two"
address@hidden group
address@hidden smallexample
+
+It is worth mentioning that @code{nth}, like @code{nthcdr} and
address@hidden, does not change the original list---the function is
+non-destructive. This is in sharp contrast to the @code{setcar} and
address@hidden functions.
+
address@hidden setcar, setcdr, nth, car cdr & cons
address@hidden node-name, next, previous, up
address@hidden @code{setcar}
address@hidden setcar
+
+As you might guess from their names, the @code{setcar} and @code{setcdr}
+functions set the @sc{car} or the @sc{cdr} of a list to a new value.
+They actually change the original list, unlike @code{car} and @code{cdr}
+which leave the original list as it was. One way to find out how this
+works is to experiment. We will start with the @code{setcar} function.
+
address@hidden 1200
+First, we can make a list and then set the value of a variable to the
+list, using the @code{setq} function. Here is a list of animals:
+
address@hidden
+(setq animals '(antelope giraffe lion tiger))
address@hidden smallexample
+
address@hidden
+If you are reading this in Info inside of GNU Emacs, you can evaluate
+this expression in the usual fashion, by positioning the cursor after
+the expression and typing @kbd{C-x C-e}. (I'm doing this right here
+as I write this. This is one of the advantages of having the
+interpreter built into the computing environment. Incidentally, when
+there is nothing on the line after the final parentheses, such as a
+comment, point can be on the next line. Thus, if your cursor is in
+the first column of the next line, you do not need to move it.
+Indeed, Emacs permits any amount of white space after the final
+parenthesis.)
+
address@hidden 1200
+When we evaluate the variable @code{animals}, we see that it is bound to
+the list @code{(antelope giraffe lion tiger)}:
+
address@hidden
address@hidden
+animals
+ @result{} (antelope giraffe lion tiger)
address@hidden group
address@hidden smallexample
+
address@hidden
+Put another way, the variable @code{animals} points to the list
address@hidden(antelope giraffe lion tiger)}.
+
+Next, evaluate the function @code{setcar} while passing it two
+arguments, the variable @code{animals} and the quoted symbol
address@hidden; this is done by writing the three element list
address@hidden(setcar animals 'hippopotamus)} and then evaluating it in the
+usual fashion:
+
address@hidden
+(setcar animals 'hippopotamus)
address@hidden smallexample
+
address@hidden 1200
address@hidden
+After evaluating this expression, evaluate the variable @code{animals}
+again. You will see that the list of animals has changed:
+
address@hidden
address@hidden
+animals
+ @result{} (hippopotamus giraffe lion tiger)
address@hidden group
address@hidden smallexample
+
address@hidden
+The first element on the list, @code{antelope} is replaced by
address@hidden
+
+So we can see that @code{setcar} did not add a new element to the list
+as @code{cons} would have; it replaced @code{antelope} with
address@hidden; it @emph{changed} the list.
+
address@hidden setcdr, cons Exercise, setcar, car cdr & cons
address@hidden node-name, next, previous, up
address@hidden @code{setcdr}
address@hidden setcdr
+
+The @code{setcdr} function is similar to the @code{setcar} function,
+except that the function replaces the second and subsequent elements of
+a list rather than the first element.
+
+(To see how to change the last element of a list, look ahead to
address@hidden function, , The @code{kill-new} function}, which uses
+the @code{nthcdr} and @code{setcdr} functions.)
+
address@hidden 1200
+To see how this works, set the value of the variable to a list of
+domesticated animals by evaluating the following expression:
+
address@hidden
+(setq domesticated-animals '(horse cow sheep goat))
address@hidden smallexample
+
address@hidden 1200
address@hidden
+If you now evaluate the list, you will be returned the list
address@hidden(horse cow sheep goat)}:
+
address@hidden
address@hidden
+domesticated-animals
+ @result{} (horse cow sheep goat)
address@hidden group
address@hidden smallexample
+
address@hidden 1200
+Next, evaluate @code{setcdr} with two arguments, the name of the
+variable which has a list as its value, and the list to which the
address@hidden of the first list will be set;
+
address@hidden
+(setcdr domesticated-animals '(cat dog))
address@hidden smallexample
+
address@hidden
+If you evaluate this expression, the list @code{(cat dog)} will appear
+in the echo area. This is the value returned by the function. The
+result we are interested in is the ``side effect'', which we can see by
+evaluating the variable @code{domesticated-animals}:
+
address@hidden
address@hidden
+domesticated-animals
+ @result{} (horse cat dog)
address@hidden group
address@hidden smallexample
+
address@hidden
+Indeed, the list is changed from @code{(horse cow sheep goat)} to
address@hidden(horse cat dog)}. The @sc{cdr} of the list is changed from
address@hidden(cow sheep goat)} to @code{(cat dog)}.
+
address@hidden cons Exercise, , setcdr, car cdr & cons
address@hidden Exercise
+
+Construct a list of four birds by evaluating several expressions with
address@hidden Find out what happens when you @code{cons} a list onto
+itself. Replace the first element of the list of four birds with a
+fish. Replace the rest of that list with a list of other fish.
+
address@hidden Cutting & Storing Text, List Implementation, car cdr & cons, Top
address@hidden node-name, next, previous, up
address@hidden Cutting and Storing Text
address@hidden Cutting and storing text
address@hidden Storing and cutting text
address@hidden Killing text
address@hidden Clipping text
address@hidden Erasing text
address@hidden Deleting text
+
+Whenever you cut or clip text out of a buffer with a `kill' command in
+GNU Emacs, it is stored in a list and you can bring it back with a
+`yank' command.
+
+(The use of the word `kill' in Emacs for processes which specifically
address@hidden not} destroy the values of the entities is an unfortunate
+historical accident. A much more appropriate word would be `clip' since
+that is what the kill commands do; they clip text out of a buffer and
+put it into storage from which it can be brought back. I have often
+been tempted to replace globally all occurrences of `kill' in the Emacs
+sources with `clip' and all occurrences of `killed' with `clipped'.)
+
address@hidden
+* Storing Text:: Text is stored in a list.
+* zap-to-char:: Cutting out text up to a character.
+* kill-region:: Cutting text out of a region.
+* copy-region-as-kill:: A definition for copying text.
+* Digression into C:: Minor note on C programming language macros.
+* defvar:: How to give a variable an initial value.
+* cons & search-fwd Review::
+* search Exercises::
address@hidden menu
+
address@hidden Storing Text, zap-to-char, Cutting & Storing Text, Cutting &
Storing Text
address@hidden
address@hidden Storing Text in a List
address@hidden ifnottex
+
+When text is cut out of a buffer, it is stored on a list. Successive
+pieces of text are stored on the list successively, so the list might
+look like this:
+
address@hidden
+("a piece of text" "previous piece")
address@hidden smallexample
+
address@hidden 1200
address@hidden
+The function @code{cons} can be used to create a new list from a piece
+of text (an `atom', to use the jargon) and an existing list, like
+this:
+
address@hidden
address@hidden
+(cons "another piece"
+ '("a piece of text" "previous piece"))
address@hidden group
address@hidden smallexample
+
address@hidden 1200
address@hidden
+If you evaluate this expression, a list of three elements will appear in
+the echo area:
+
address@hidden
+("another piece" "a piece of text" "previous piece")
address@hidden smallexample
+
+With the @code{car} and @code{nthcdr} functions, you can retrieve
+whichever piece of text you want. For example, in the following code,
address@hidden 1 @dots{}} returns the list with the first item removed;
+and the @code{car} returns the first element of that remainder---the
+second element of the original list:
+
address@hidden
address@hidden
+(car (nthcdr 1 '("another piece"
+ "a piece of text"
+ "previous piece")))
+ @result{} "a piece of text"
address@hidden group
address@hidden smallexample
+
+The actual functions in Emacs are more complex than this, of course.
+The code for cutting and retrieving text has to be written so that
+Emacs can figure out which element in the list you want---the first,
+second, third, or whatever. In addition, when you get to the end of
+the list, Emacs should give you the first element of the list, rather
+than nothing at all.
+
+The list that holds the pieces of text is called the @dfn{kill ring}.
+This chapter leads up to a description of the kill ring and how it is
+used by first tracing how the @code{zap-to-char} function works. This
+function uses (or `calls') a function that invokes a function that
+manipulates the kill ring. Thus, before reaching the mountains, we
+climb the foothills.
+
+A subsequent chapter describes how text that is cut from the buffer is
+retrieved. @xref{Yanking, , Yanking Text Back}.
+
address@hidden zap-to-char, kill-region, Storing Text, Cutting & Storing Text
address@hidden node-name, next, previous, up
address@hidden @code{zap-to-char}
address@hidden zap-to-char
+
+The @code{zap-to-char} function changed little between GNU Emacs
+version 19 and GNU Emacs version 22. However, @code{zap-to-char}
+calls another function, @code{kill-region}, which enjoyed a major
+rewrite.
+
+The @code{kill-region} function in Emacs 19 is complex, but does not
+use code that is important at this time. We will skip it.
+
+The @code{kill-region} function in Emacs 22 is easier to read than the
+same function in Emacs 19 and introduces a very important concept,
+that of error handling. We will walk through the function.
+
+But first, let us look at the interactive @code{zap-to-char} function.
+
address@hidden
+* Complete zap-to-char:: The complete implementation.
+* zap-to-char interactive:: A three part interactive expression.
+* zap-to-char body:: A short overview.
+* search-forward:: How to search for a string.
+* progn:: The @code{progn} special form.
+* Summing up zap-to-char:: Using @code{point} and @code{search-forward}.
address@hidden menu
+
address@hidden Complete zap-to-char, zap-to-char interactive, zap-to-char,
zap-to-char
address@hidden
address@hidden The Complete @code{zap-to-char} Implementation
address@hidden ifnottex
+
+The @code{zap-to-char} function removes the text in the region between
+the location of the cursor (i.e., of point) up to and including the
+next occurrence of a specified character. The text that
address@hidden removes is put in the kill ring; and it can be
+retrieved from the kill ring by typing @kbd{C-y} (@code{yank}). If
+the command is given an argument, it removes text through that number
+of occurrences. Thus, if the cursor were at the beginning of this
+sentence and the character were @samp{s}, @samp{Thus} would be
+removed. If the argument were two, @samp{Thus, if the curs} would be
+removed, up to and including the @samp{s} in @samp{cursor}.
+
+If the specified character is not found, @code{zap-to-char} will say
+``Search failed'', tell you the character you typed, and not remove
+any text.
+
+In order to determine how much text to remove, @code{zap-to-char} uses
+a search function. Searches are used extensively in code that
+manipulates text, and we will focus attention on them as well as on the
+deletion command.
+
address@hidden
address@hidden GNU Emacs version 19
+(defun zap-to-char (arg char) ; version 19 implementation
+ "Kill up to and including ARG'th occurrence of CHAR.
+Goes backward if ARG is negative; error if CHAR not found."
+ (interactive "*p\ncZap to char: ")
+ (kill-region (point)
+ (progn
+ (search-forward
+ (char-to-string char) nil nil arg)
+ (point))))
address@hidden ignore
+
address@hidden 1250
+Here is the complete text of the version 22 implementation of the function:
+
address@hidden GNU Emacs 22
address@hidden
address@hidden
+(defun zap-to-char (arg char)
+ "Kill up to and including ARG'th occurrence of CHAR.
+Case is ignored if `case-fold-search' is non-nil in the current buffer.
+Goes backward if ARG is negative; error if CHAR not found."
+ (interactive "p\ncZap to char: ")
+ (if (char-table-p translation-table-for-input)
+ (setq char (or (aref translation-table-for-input char) char)))
+ (kill-region (point) (progn
+ (search-forward (char-to-string char) nil nil arg)
+ (point))))
address@hidden group
address@hidden smallexample
+
+The documentation is thorough. You do need to know the jargon meaning
+of the word `kill'.
+
address@hidden zap-to-char interactive, zap-to-char body, Complete zap-to-char,
zap-to-char
address@hidden node-name, next, previous, up
address@hidden The @code{interactive} Expression
+
address@hidden 800
+The interactive expression in the @code{zap-to-char} command looks like
+this:
+
address@hidden
+(interactive "p\ncZap to char: ")
address@hidden smallexample
+
+The part within quotation marks, @code{"p\ncZap to char:@: "}, specifies
+two different things. First, and most simply, is the @samp{p}.
+This part is separated from the next part by a newline, @samp{\n}.
+The @samp{p} means that the first argument to the function will be
+passed the value of a `processed prefix'. The prefix argument is
+passed by typing @kbd{C-u} and a number, or @kbd{M-} and a number. If
+the function is called interactively without a prefix, 1 is passed to
+this argument.
+
+The second part of @code{"p\ncZap to char:@: "} is
address@hidden to char:@: }. In this part, the lower case @samp{c}
+indicates that @code{interactive} expects a prompt and that the
+argument will be a character. The prompt follows the @samp{c} and is
+the string @samp{Zap to char:@: } (with a space after the colon to
+make it look good).
+
+What all this does is prepare the arguments to @code{zap-to-char} so they
+are of the right type, and give the user a prompt.
+
+In a read-only buffer, the @code{zap-to-char} function copies the text
+to the kill ring, but does not remove it. The echo area displays a
+message saying that the buffer is read-only. Also, the terminal may
+beep or blink at you.
+
address@hidden zap-to-char body, search-forward, zap-to-char interactive,
zap-to-char
address@hidden node-name, next, previous, up
address@hidden The Body of @code{zap-to-char}
+
+The body of the @code{zap-to-char} function contains the code that
+kills (that is, removes) the text in the region from the current
+position of the cursor up to and including the specified character.
+
+The first part of the code looks like this:
+
address@hidden
+(if (char-table-p translation-table-for-input)
+ (setq char (or (aref translation-table-for-input char) char)))
+(kill-region (point) (progn
+ (search-forward (char-to-string char) nil nil arg)
+ (point)))
address@hidden smallexample
+
address@hidden
address@hidden is an hitherto unseen function. It determines
+whether its argument is a character table. When it is, it sets the
+character passed to @code{zap-to-char} to one of them, if that
+character exists, or to the character itself. (This becomes important
+for certain characters in non-European languages. The @code{aref}
+function extracts an element from an array. It is an array-specific
+function that is not described in this document. @xref{Arrays, ,
+Arrays, elisp, The GNU Emacs Lisp Reference Manual}.)
+
address@hidden
address@hidden(point)} is the current position of the cursor.
+
+The next part of the code is an expression using @code{progn}. The body
+of the @code{progn} consists of calls to @code{search-forward} and
address@hidden
+
+It is easier to understand how @code{progn} works after learning about
address@hidden, so we will look at @code{search-forward} and
+then at @code{progn}.
+
address@hidden search-forward, progn, zap-to-char body, zap-to-char
address@hidden node-name, next, previous, up
address@hidden The @code{search-forward} Function
address@hidden search-forward
+
+The @code{search-forward} function is used to locate the
+zapped-for-character in @code{zap-to-char}. If the search is
+successful, @code{search-forward} leaves point immediately after the
+last character in the target string. (In @code{zap-to-char}, the
+target string is just one character long. @code{zap-to-char} uses the
+function @code{char-to-string} to ensure that the computer treats that
+character as a string.) If the search is backwards,
address@hidden leaves point just before the first character in
+the target. Also, @code{search-forward} returns @code{t} for true.
+(Moving point is therefore a `side effect'.)
+
address@hidden 1250
+In @code{zap-to-char}, the @code{search-forward} function looks like this:
+
address@hidden
+(search-forward (char-to-string char) nil nil arg)
address@hidden smallexample
+
+The @code{search-forward} function takes four arguments:
+
address@hidden
address@hidden
+The first argument is the target, what is searched for. This must be a
+string, such as @samp{"z"}.
+
+As it happens, the argument passed to @code{zap-to-char} is a single
+character. Because of the way computers are built, the Lisp
+interpreter may treat a single character as being different from a
+string of characters. Inside the computer, a single character has a
+different electronic format than a string of one character. (A single
+character can often be recorded in the computer using exactly one
+byte; but a string may be longer, and the computer needs to be ready
+for this.) Since the @code{search-forward} function searches for a
+string, the character that the @code{zap-to-char} function receives as
+its argument must be converted inside the computer from one format to
+the other; otherwise the @code{search-forward} function will fail.
+The @code{char-to-string} function is used to make this conversion.
+
address@hidden
+The second argument bounds the search; it is specified as a position in
+the buffer. In this case, the search can go to the end of the buffer,
+so no bound is set and the second argument is @code{nil}.
+
address@hidden
+The third argument tells the function what it should do if the search
+fails---it can signal an error (and print a message) or it can return
address@hidden A @code{nil} as the third argument causes the function to
+signal an error when the search fails.
+
address@hidden
+The fourth argument to @code{search-forward} is the repeat count---how
+many occurrences of the string to look for. This argument is optional
+and if the function is called without a repeat count, this argument is
+passed the value 1. If this argument is negative, the search goes
+backwards.
address@hidden enumerate
+
address@hidden 800
+In template form, a @code{search-forward} expression looks like this:
+
address@hidden
address@hidden
+(search-forward "@var{target-string}"
+ @var{limit-of-search}
+ @var{what-to-do-if-search-fails}
+ @var{repeat-count})
address@hidden group
address@hidden smallexample
+
+We will look at @code{progn} next.
+
address@hidden progn, Summing up zap-to-char, search-forward, zap-to-char
address@hidden node-name, next, previous, up
address@hidden The @code{progn} Special Form
address@hidden progn
+
address@hidden is a special form that causes each of its arguments to be
+evaluated in sequence and then returns the value of the last one. The
+preceding expressions are evaluated only for the side effects they
+perform. The values produced by them are discarded.
+
address@hidden 800
+The template for a @code{progn} expression is very simple:
+
address@hidden
address@hidden
+(progn
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+In @code{zap-to-char}, the @code{progn} expression has to do two things:
+put point in exactly the right position; and return the location of
+point so that @code{kill-region} will know how far to kill to.
+
+The first argument to the @code{progn} is @code{search-forward}. When
address@hidden finds the string, the function leaves point
+immediately after the last character in the target string. (In this
+case the target string is just one character long.) If the search is
+backwards, @code{search-forward} leaves point just before the first
+character in the target. The movement of point is a side effect.
+
+The second and last argument to @code{progn} is the expression
address@hidden(point)}. This expression returns the value of point, which in
+this case will be the location to which it has been moved by
address@hidden (In the source, a line that tells the function
+to go to the previous character, if it is going forward, was commented
+out in 1999; I don't remember whether that feature or mis-feature was
+ever a part of the distributed source.) The value of @code{point} is
+returned by the @code{progn} expression and is passed to
address@hidden as @code{kill-region}'s second argument.
+
address@hidden Summing up zap-to-char, , progn, zap-to-char
address@hidden node-name, next, previous, up
address@hidden Summing up @code{zap-to-char}
+
+Now that we have seen how @code{search-forward} and @code{progn} work,
+we can see how the @code{zap-to-char} function works as a whole.
+
+The first argument to @code{kill-region} is the position of the cursor
+when the @code{zap-to-char} command is given---the value of point at
+that time. Within the @code{progn}, the search function then moves
+point to just after the zapped-to-character and @code{point} returns the
+value of this location. The @code{kill-region} function puts together
+these two values of point, the first one as the beginning of the region
+and the second one as the end of the region, and removes the region.
+
+The @code{progn} special form is necessary because the
address@hidden command takes two arguments; and it would fail if
address@hidden and @code{point} expressions were written in
+sequence as two additional arguments. The @code{progn} expression is
+a single argument to @code{kill-region} and returns the one value that
address@hidden needs for its second argument.
+
address@hidden kill-region, copy-region-as-kill, zap-to-char, Cutting & Storing
Text
address@hidden node-name, next, previous, up
address@hidden @code{kill-region}
address@hidden kill-region
+
+The @code{zap-to-char} function uses the @code{kill-region} function.
+This function clips text from a region and copies that text to
+the kill ring, from which it may be retrieved.
+
address@hidden
+GNU Emacs 22:
+
+(defun kill-region (beg end &optional yank-handler)
+ "Kill (\"cut\") text between point and mark.
+This deletes the text from the buffer and saves it in the kill ring.
+The command \\[yank] can retrieve it from there.
+\(If you want to kill and then yank immediately, use \\[kill-ring-save].)
+
+If you want to append the killed region to the last killed text,
+use \\[append-next-kill] before \\[kill-region].
+
+If the buffer is read-only, Emacs will beep and refrain from deleting
+the text, but put the text in the kill ring anyway. This means that
+you can use the killing commands to copy text from a read-only buffer.
+
+This is the primitive for programs to kill text (as opposed to deleting it).
+Supply two arguments, character positions indicating the stretch of text
+ to be killed.
+Any command that calls this function is a \"kill command\".
+If the previous command was also a kill command,
+the text killed this time appends to the text killed last time
+to make one entry in the kill ring.
+
+In Lisp code, optional third arg YANK-HANDLER, if non-nil,
+specifies the yank-handler text property to be set on the killed
+text. See `insert-for-yank'."
+ ;; Pass point first, then mark, because the order matters
+ ;; when calling kill-append.
+ (interactive (list (point) (mark)))
+ (unless (and beg end)
+ (error "The mark is not set now, so there is no region"))
+ (condition-case nil
+ (let ((string (filter-buffer-substring beg end t)))
+ (when string ;STRING is nil if BEG = END
+ ;; Add that string to the kill ring, one way or another.
+ (if (eq last-command 'kill-region)
+ (kill-append string (< end beg) yank-handler)
+ (kill-new string nil yank-handler)))
+ (when (or string (eq last-command 'kill-region))
+ (setq this-command 'kill-region))
+ nil)
+ ((buffer-read-only text-read-only)
+ ;; The code above failed because the buffer, or some of the characters
+ ;; in the region, are read-only.
+ ;; We should beep, in case the user just isn't aware of this.
+ ;; However, there's no harm in putting
+ ;; the region's text in the kill ring, anyway.
+ (copy-region-as-kill beg end)
+ ;; Set this-command now, so it will be set even if we get an error.
+ (setq this-command 'kill-region)
+ ;; This should barf, if appropriate, and give us the correct error.
+ (if kill-read-only-ok
+ (progn (message "Read only text copied to kill ring") nil)
+ ;; Signal an error if the buffer is read-only.
+ (barf-if-buffer-read-only)
+ ;; If the buffer isn't read-only, the text is.
+ (signal 'text-read-only (list (current-buffer)))))))
address@hidden ignore
+
+The Emacs 22 version of that function uses @code{condition-case} and
address@hidden, both of which we will explain.
address@hidden is an important special form.
+
+In essence, the @code{kill-region} function calls
address@hidden, which takes three arguments. In this function,
+the first argument does nothing. The second argument contains the
+code that does the work when all goes well. The third argument
+contains the code that is called in the event of an error.
+
address@hidden
+* Complete kill-region:: The function definition.
+* condition-case:: Dealing with a problem.
+* Lisp macro::
address@hidden menu
+
address@hidden Complete kill-region, condition-case, kill-region, kill-region
address@hidden
address@hidden The Complete @code{kill-region} Definition
address@hidden ifnottex
+
address@hidden 1200
+We will go through the @code{condition-case} code in a moment. First,
+let us look at the definition of @code{kill-region}, with comments
+added:
+
address@hidden GNU Emacs 22:
address@hidden
address@hidden
+(defun kill-region (beg end)
+ "Kill (\"cut\") text between point and mark.
+This deletes the text from the buffer and saves it in the kill ring.
+The command \\[yank] can retrieve it from there. @dots{} "
address@hidden group
+
address@hidden
+ ;; @bullet{} Since order matters, pass point first.
+ (interactive (list (point) (mark)))
+ ;; @bullet{} And tell us if we cannot cut the text.
+ ;; `unless' is an `if' without a then-part.
+ (unless (and beg end)
+ (error "The mark is not set now, so there is no region"))
address@hidden group
+
address@hidden
+ ;; @bullet{} `condition-case' takes three arguments.
+ ;; If the first argument is nil, as it is here,
+ ;; information about the error signal is not
+ ;; stored for use by another function.
+ (condition-case nil
address@hidden group
+
address@hidden
+ ;; @bullet{} The second argument to `condition-case' tells the
+ ;; Lisp interpreter what to do when all goes well.
address@hidden group
+
address@hidden
+ ;; It starts with a `let' function that extracts the string
+ ;; and tests whether it exists. If so (that is what the
+ ;; `when' checks), it calls an `if' function that determines
+ ;; whether the previous command was another call to
+ ;; `kill-region'; if it was, then the new text is appended to
+ ;; the previous text; if not, then a different function,
+ ;; `kill-new', is called.
address@hidden group
+
address@hidden
+ ;; The `kill-append' function concatenates the new string and
+ ;; the old. The `kill-new' function inserts text into a new
+ ;; item in the kill ring.
address@hidden group
+
address@hidden
+ ;; `when' is an `if' without an else-part. The second `when'
+ ;; again checks whether the current string exists; in
+ ;; addition, it checks whether the previous command was
+ ;; another call to `kill-region'. If one or the other
+ ;; condition is true, then it sets the current command to
+ ;; be `kill-region'.
address@hidden group
address@hidden
+ (let ((string (filter-buffer-substring beg end t)))
+ (when string ;STRING is nil if BEG = END
+ ;; Add that string to the kill ring, one way or another.
+ (if (eq last-command 'kill-region)
address@hidden group
address@hidden
+ ;; @minus{} `yank-handler' is an optional argument to
+ ;; `kill-region' that tells the `kill-append' and
+ ;; `kill-new' functions how deal with properties
+ ;; added to the text, such as `bold' or `italics'.
+ (kill-append string (< end beg) yank-handler)
+ (kill-new string nil yank-handler)))
+ (when (or string (eq last-command 'kill-region))
+ (setq this-command 'kill-region))
+ nil)
address@hidden group
+
address@hidden
+ ;; @bullet{} The third argument to `condition-case' tells the interpreter
+ ;; what to do with an error.
address@hidden group
address@hidden
+ ;; The third argument has a conditions part and a body part.
+ ;; If the conditions are met (in this case,
+ ;; if text or buffer are read-only)
+ ;; then the body is executed.
address@hidden group
address@hidden
+ ;; The first part of the third argument is the following:
+ ((buffer-read-only text-read-only) ;; the if-part
+ ;; @dots{} the then-part
+ (copy-region-as-kill beg end)
address@hidden group
address@hidden
+ ;; Next, also as part of the then-part, set this-command, so
+ ;; it will be set in an error
+ (setq this-command 'kill-region)
+ ;; Finally, in the then-part, send a message if you may copy
+ ;; the text to the kill ring without signally an error, but
+ ;; don't if you may not.
address@hidden group
address@hidden
+ (if kill-read-only-ok
+ (progn (message "Read only text copied to kill ring") nil)
+ (barf-if-buffer-read-only)
+ ;; If the buffer isn't read-only, the text is.
+ (signal 'text-read-only (list (current-buffer)))))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden v 21
address@hidden
address@hidden
+(defun kill-region (beg end)
+ "Kill between point and mark.
+The text is deleted but saved in the kill ring."
+ (interactive "r")
address@hidden group
+
address@hidden
+ ;; 1. `condition-case' takes three arguments.
+ ;; If the first argument is nil, as it is here,
+ ;; information about the error signal is not
+ ;; stored for use by another function.
+ (condition-case nil
address@hidden group
+
address@hidden
+ ;; 2. The second argument to `condition-case'
+ ;; tells the Lisp interpreter what to do when all goes well.
address@hidden group
+
address@hidden
+ ;; The `delete-and-extract-region' function usually does the
+ ;; work. If the beginning and ending of the region are both
+ ;; the same, then the variable `string' will be empty, or nil
+ (let ((string (delete-and-extract-region beg end)))
address@hidden group
+
address@hidden
+ ;; `when' is an `if' clause that cannot take an `else-part'.
+ ;; Emacs normally sets the value of `last-command' to the
+ ;; previous command.
address@hidden group
address@hidden
+ ;; `kill-append' concatenates the new string and the old.
+ ;; `kill-new' inserts text into a new item in the kill ring.
+ (when string
+ (if (eq last-command 'kill-region)
+ ;; if true, prepend string
+ (kill-append string (< end beg))
+ (kill-new string)))
+ (setq this-command 'kill-region))
address@hidden group
+
address@hidden
+ ;; 3. The third argument to `condition-case' tells the interpreter
+ ;; what to do with an error.
address@hidden group
address@hidden
+ ;; The third argument has a conditions part and a body part.
+ ;; If the conditions are met (in this case,
+ ;; if text or buffer are read-only)
+ ;; then the body is executed.
address@hidden group
address@hidden
+ ((buffer-read-only text-read-only) ;; this is the if-part
+ ;; then...
+ (copy-region-as-kill beg end)
address@hidden group
address@hidden
+ (if kill-read-only-ok ;; usually this variable is nil
+ (message "Read only text copied to kill ring")
+ ;; or else, signal an error if the buffer is read-only;
+ (barf-if-buffer-read-only)
+ ;; and, in any case, signal that the text is read-only.
+ (signal 'text-read-only (list (current-buffer)))))))
address@hidden group
address@hidden smallexample
address@hidden ignore
+
address@hidden condition-case, Lisp macro, Complete kill-region, kill-region
address@hidden node-name, next, previous, up
address@hidden @code{condition-case}
address@hidden condition-case
+
+As we have seen earlier (@pxref{Making Errors, , Generate an Error
+Message}), when the Emacs Lisp interpreter has trouble evaluating an
+expression, it provides you with help; in the jargon, this is called
+``signaling an error''. Usually, the computer stops the program and
+shows you a message.
+
+However, some programs undertake complicated actions. They should not
+simply stop on an error. In the @code{kill-region} function, the most
+likely error is that you will try to kill text that is read-only and
+cannot be removed. So the @code{kill-region} function contains code
+to handle this circumstance. This code, which makes up the body of
+the @code{kill-region} function, is inside of a @code{condition-case}
+special form.
+
address@hidden 800
+The template for @code{condition-case} looks like this:
+
address@hidden
address@hidden
+(condition-case
+ @var{var}
+ @var{bodyform}
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+The second argument, @var{bodyform}, is straightforward. The
address@hidden special form causes the Lisp interpreter to
+evaluate the code in @var{bodyform}. If no error occurs, the special
+form returns the code's value and produces the side-effects, if any.
+
+In short, the @var{bodyform} part of a @code{condition-case}
+expression determines what should happen when everything works
+correctly.
+
+However, if an error occurs, among its other actions, the function
+generating the error signal will define one or more error condition
+names.
+
+An error handler is the third argument to @code{condition case}.
+An error handler has two parts, a @var{condition-name} and a
address@hidden If the @var{condition-name} part of an error handler
+matches a condition name generated by an error, then the @var{body}
+part of the error handler is run.
+
+As you will expect, the @var{condition-name} part of an error handler
+may be either a single condition name or a list of condition names.
+
+Also, a complete @code{condition-case} expression may contain more
+than one error handler. When an error occurs, the first applicable
+handler is run.
+
+Lastly, the first argument to the @code{condition-case} expression,
+the @var{var} argument, is sometimes bound to a variable that
+contains information about the error. However, if that argument is
+nil, as is the case in @code{kill-region}, that information is
+discarded.
+
address@hidden 1200
+In brief, in the @code{kill-region} function, the code
address@hidden works like this:
+
address@hidden
address@hidden
address@hidden no errors}, @var{run only this code}
+ @var{but}, @var{if errors}, @var{run this other code}.
address@hidden group
address@hidden smallexample
+
address@hidden
+2006 Oct 24
+In Emacs 22,
+copy-region-as-kill is short, 12 lines, and uses
+filter-buffer-substring, which is longer, 39 lines
+and has delete-and-extract-region in it.
+delete-and-extract-region is written in C.
+
+see Initializing a Variable with @code{defvar}
+this is line 8054
+Initializing a Variable with @code{defvar} includes line 8350
address@hidden ignore
+
address@hidden Lisp macro, , condition-case, kill-region
address@hidden node-name, next, previous, up
address@hidden Lisp macro
address@hidden Macro, lisp
address@hidden Lisp macro
+
+The part of the @code{condition-case} expression that is evaluated in
+the expectation that all goes well has a @code{when}. The code uses
address@hidden to determine whether the @code{string} variable points to
+text that exists.
+
+A @code{when} expression is simply a programmers' convenience. It is
+an @code{if} without the possibility of an else clause. In your mind,
+you can replace @code{when} with @code{if} and understand what goes
+on. That is what the Lisp interpreter does.
+
+Technically speaking, @code{when} is a Lisp macro. A Lisp @dfn{macro}
+enables you to define new control constructs and other language
+features. It tells the interpreter how to compute another Lisp
+expression which will in turn compute the value. In this case, the
+`other expression' is an @code{if} expression.
+
+The @code{kill-region} function definition also has an @code{unless}
+macro; it is the converse of @code{when}. The @code{unless} macro is
+an @code{if} without a then clause
+
+For more about Lisp macros, see @ref{Macros, , Macros, elisp, The GNU
+Emacs Lisp Reference Manual}. The C programming language also
+provides macros. These are different, but also useful.
+
address@hidden
+We will briefly look at C macros in
address@hidden into C}.
address@hidden ignore
+
address@hidden 1200
+Regarding the @code{when} macro, in the @code{condition-case}
+expression, when the string has content, then another conditional
+expression is executed. This is an @code{if} with both a then-part
+and an else-part.
+
address@hidden
address@hidden
+(if (eq last-command 'kill-region)
+ (kill-append string (< end beg) yank-handler)
+ (kill-new string nil yank-handler))
address@hidden group
address@hidden smallexample
+
+The then-part is evaluated if the previous command was another call to
address@hidden; if not, the else-part is evaluated.
+
address@hidden is an optional argument to @code{kill-region} that
+tells the @code{kill-append} and @code{kill-new} functions how deal
+with properties added to the text, such as `bold' or `italics'.
+
address@hidden is a variable that comes with Emacs that we have
+not seen before. Normally, whenever a function is executed, Emacs
+sets the value of @code{last-command} to the previous command.
+
address@hidden 1200
+In this segment of the definition, the @code{if} expression checks
+whether the previous command was @code{kill-region}. If it was,
+
address@hidden
+(kill-append string (< end beg) yank-handler)
address@hidden smallexample
+
address@hidden
+concatenates a copy of the newly clipped text to the just previously
+clipped text in the kill ring.
+
address@hidden copy-region-as-kill, Digression into C, kill-region, Cutting &
Storing Text
address@hidden node-name, next, previous, up
address@hidden @code{copy-region-as-kill}
address@hidden copy-region-as-kill
address@hidden nthcdr
+
+The @code{copy-region-as-kill} function copies a region of text from a
+buffer and (via either @code{kill-append} or @code{kill-new}) saves it
+in the @code{kill-ring}.
+
+If you call @code{copy-region-as-kill} immediately after a
address@hidden command, Emacs appends the newly copied text to the
+previously copied text. This means that if you yank back the text, you
+get it all, from both this and the previous operation. On the other
+hand, if some other command precedes the @code{copy-region-as-kill},
+the function copies the text into a separate entry in the kill ring.
+
address@hidden
+* Complete copy-region-as-kill:: The complete function definition.
+* copy-region-as-kill body:: The body of @code{copy-region-as-kill}.
address@hidden menu
+
address@hidden Complete copy-region-as-kill, copy-region-as-kill body,
copy-region-as-kill, copy-region-as-kill
address@hidden
address@hidden The complete @code{copy-region-as-kill} function definition
address@hidden ifnottex
+
address@hidden 1200
+Here is the complete text of the version 22 @code{copy-region-as-kill}
+function:
+
address@hidden
address@hidden
+(defun copy-region-as-kill (beg end)
+ "Save the region as if killed, but don't kill it.
+In Transient Mark mode, deactivate the mark.
+If `interprogram-cut-function' is non-nil, also save the text for a window
+system cut and paste."
+ (interactive "r")
address@hidden group
address@hidden
+ (if (eq last-command 'kill-region)
+ (kill-append (filter-buffer-substring beg end) (< end beg))
+ (kill-new (filter-buffer-substring beg end)))
address@hidden group
address@hidden
+ (if transient-mark-mode
+ (setq deactivate-mark t))
+ nil)
address@hidden group
address@hidden smallexample
+
address@hidden 800
+As usual, this function can be divided into its component parts:
+
address@hidden
address@hidden
+(defun copy-region-as-kill (@var{argument-list})
+ "@address@hidden"
+ (interactive "r")
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+The arguments are @code{beg} and @code{end} and the function is
+interactive with @code{"r"}, so the two arguments must refer to the
+beginning and end of the region. If you have been reading though this
+document from the beginning, understanding these parts of a function is
+almost becoming routine.
+
+The documentation is somewhat confusing unless you remember that the
+word `kill' has a meaning different from usual. The `Transient Mark'
+and @code{interprogram-cut-function} comments explain certain
+side-effects.
+
+After you once set a mark, a buffer always contains a region. If you
+wish, you can use Transient Mark mode to highlight the region
+temporarily. (No one wants to highlight the region all the time, so
+Transient Mark mode highlights it only at appropriate times. Many
+people turn off Transient Mark mode, so the region is never
+highlighted.)
+
+Also, a windowing system allows you to copy, cut, and paste among
+different programs. In the X windowing system, for example, the
address@hidden function is @code{x-select-text},
+which works with the windowing system's equivalent of the Emacs kill
+ring.
+
+The body of the @code{copy-region-as-kill} function starts with an
address@hidden clause. What this clause does is distinguish between two
+different situations: whether or not this command is executed
+immediately after a previous @code{kill-region} command. In the first
+case, the new region is appended to the previously copied text.
+Otherwise, it is inserted into the beginning of the kill ring as a
+separate piece of text from the previous piece.
+
+The last two lines of the function prevent the region from lighting up
+if Transient Mark mode is turned on.
+
+The body of @code{copy-region-as-kill} merits discussion in detail.
+
address@hidden copy-region-as-kill body, , Complete copy-region-as-kill,
copy-region-as-kill
address@hidden node-name, next, previous, up
address@hidden The Body of @code{copy-region-as-kill}
+
+The @code{copy-region-as-kill} function works in much the same way as
+the @code{kill-region} function. Both are written so that two or more
+kills in a row combine their text into a single entry. If you yank
+back the text from the kill ring, you get it all in one piece.
+Moreover, kills that kill forward from the current position of the
+cursor are added to the end of the previously copied text and commands
+that copy text backwards add it to the beginning of the previously
+copied text. This way, the words in the text stay in the proper
+order.
+
+Like @code{kill-region}, the @code{copy-region-as-kill} function makes
+use of the @code{last-command} variable that keeps track of the
+previous Emacs command.
+
address@hidden
+* last-command & this-command::
+* kill-append function::
+* kill-new function::
address@hidden menu
+
address@hidden last-command & this-command, kill-append function,
copy-region-as-kill body, copy-region-as-kill body
address@hidden
address@hidden @code{last-command} and @code{this-command}
address@hidden ifnottex
+
+Normally, whenever a function is executed, Emacs sets the value of
address@hidden to the function being executed (which in this case
+would be @code{copy-region-as-kill}). At the same time, Emacs sets
+the value of @code{last-command} to the previous value of
address@hidden
+
+In the first part of the body of the @code{copy-region-as-kill}
+function, an @code{if} expression determines whether the value of
address@hidden is @code{kill-region}. If so, the then-part of
+the @code{if} expression is evaluated; it uses the @code{kill-append}
+function to concatenate the text copied at this call to the function
+with the text already in the first element (the @sc{car}) of the kill
+ring. On the other hand, if the value of @code{last-command} is not
address@hidden, then the @code{copy-region-as-kill} function
+attaches a new element to the kill ring using the @code{kill-new}
+function.
+
address@hidden 1250
+The @code{if} expression reads as follows; it uses @code{eq}:
+
address@hidden
address@hidden
+ (if (eq last-command 'kill-region)
+ ;; @r{then-part}
+ (kill-append (filter-buffer-substring beg end) (< end beg))
+ ;; @r{else-part}
+ (kill-new (filter-buffer-substring beg end)))
address@hidden group
address@hidden smallexample
+
address@hidden filter-buffer-substring
+(The @code{filter-buffer-substring} function returns a filtered
+substring of the buffer, if any. Optionally---the arguments are not
+here, so neither is done---the function may delete the initial text or
+return the text without its properties; this function is a replacement
+for the older @code{buffer-substring} function, which came before text
+properties were implemented.)
+
address@hidden eq @r{(example of use)}
address@hidden
+The @code{eq} function tests whether its first argument is the same Lisp
+object as its second argument. The @code{eq} function is similar to the
address@hidden function in that it is used to test for equality, but
+differs in that it determines whether two representations are actually
+the same object inside the computer, but with different names.
address@hidden determines whether the structure and contents of two
+expressions are the same.
+
+If the previous command was @code{kill-region}, then the Emacs Lisp
+interpreter calls the @code{kill-append} function
+
address@hidden kill-append function, kill-new function, last-command &
this-command, copy-region-as-kill body
address@hidden The @code{kill-append} function
address@hidden kill-append
+
address@hidden 800
+The @code{kill-append} function looks like this:
+
address@hidden in GNU Emacs 22
address@hidden
address@hidden
+(defun kill-append (string before-p &optional yank-handler)
+ "Append STRING to the end of the latest kill in the kill ring.
+If BEFORE-P is non-nil, prepend STRING to the kill.
address@hidden "
+ (let* ((cur (car kill-ring)))
+ (kill-new (if before-p (concat string cur) (concat cur string))
+ (or (= (length cur) 0)
+ (equal yank-handler
+ (get-text-property 0 'yank-handler cur)))
+ yank-handler)))
address@hidden group
address@hidden smallexample
+
address@hidden
+was:
+(defun kill-append (string before-p)
+ "Append STRING to the end of the latest kill in the kill ring.
+If BEFORE-P is non-nil, prepend STRING to the kill.
+If `interprogram-cut-function' is set, pass the resulting kill to
+it."
+ (kill-new (if before-p
+ (concat string (car kill-ring))
+ (concat (car kill-ring) string))
+ t))
address@hidden ignore
+
address@hidden
+The @code{kill-append} function is fairly straightforward. It uses
+the @code{kill-new} function, which we will discuss in more detail in
+a moment.
+
+(Also, the function provides an optional argument called
address@hidden; when invoked, this argument tells the function
+how to deal with properties added to the text, such as `bold' or
+`italics'.)
+
address@hidden !!! bug in GNU Emacs 22 version of kill-append ?
+It has a @code{let*} function to set the value of the first element of
+the kill ring to @code{cur}. (I do not know why the function does not
+use @code{let} instead; only one value is set in the expression.
+Perhaps this is a bug that produces no problems?)
+
+Consider the conditional that is one of the two arguments to
address@hidden It uses @code{concat} to concatenate the new text to
+the @sc{car} of the kill ring. Whether it prepends or appends the
+text depends on the results of an @code{if} expression:
+
address@hidden
address@hidden
+(if before-p ; @r{if-part}
+ (concat string cur) ; @r{then-part}
+ (concat cur string)) ; @r{else-part}
address@hidden group
address@hidden smallexample
+
address@hidden
+If the region being killed is before the region that was killed in the
+last command, then it should be prepended before the material that was
+saved in the previous kill; and conversely, if the killed text follows
+what was just killed, it should be appended after the previous text.
+The @code{if} expression depends on the predicate @code{before-p} to
+decide whether the newly saved text should be put before or after the
+previously saved text.
+
+The symbol @code{before-p} is the name of one of the arguments to
address@hidden When the @code{kill-append} function is
+evaluated, it is bound to the value returned by evaluating the actual
+argument. In this case, this is the expression @code{(< end beg)}.
+This expression does not directly determine whether the killed text in
+this command is located before or after the kill text of the last
+command; what it does is determine whether the value of the variable
address@hidden is less than the value of the variable @code{beg}. If it
+is, it means that the user is most likely heading towards the
+beginning of the buffer. Also, the result of evaluating the predicate
+expression, @code{(< end beg)}, will be true and the text will be
+prepended before the previous text. On the other hand, if the value of
+the variable @code{end} is greater than the value of the variable
address@hidden, the text will be appended after the previous text.
+
address@hidden 800
+When the newly saved text will be prepended, then the string with the new
+text will be concatenated before the old text:
+
address@hidden
+(concat string cur)
address@hidden smallexample
+
address@hidden 1200
address@hidden
+But if the text will be appended, it will be concatenated
+after the old text:
+
address@hidden
+(concat cur string))
address@hidden smallexample
+
+To understand how this works, we first need to review the
address@hidden function. The @code{concat} function links together or
+unites two strings of text. The result is a string. For example:
+
address@hidden
address@hidden
+(concat "abc" "def")
+ @result{} "abcdef"
address@hidden group
+
address@hidden
+(concat "new "
+ (car '("first element" "second element")))
+ @result{} "new first element"
+
+(concat (car
+ '("first element" "second element")) " modified")
+ @result{} "first element modified"
address@hidden group
address@hidden smallexample
+
+We can now make sense of @code{kill-append}: it modifies the contents
+of the kill ring. The kill ring is a list, each element of which is
+saved text. The @code{kill-append} function uses the @code{kill-new}
+function which in turn uses the @code{setcar} function.
+
address@hidden kill-new function, , kill-append function, copy-region-as-kill
body
address@hidden The @code{kill-new} function
address@hidden kill-new
+
address@hidden in GNU Emacs 22, additional documentation to kill-new:
address@hidden
+Optional third arguments YANK-HANDLER controls how the STRING is later
+inserted into a buffer; see `insert-for-yank' for details.
+When a yank handler is specified, STRING must be non-empty (the yank
+handler, if non-nil, is stored as a `yank-handler' text property on STRING).
+
+When the yank handler has a non-nil PARAM element, the original STRING
+argument is not used by `insert-for-yank'. However, since Lisp code
+may access and use elements from the kill ring directly, the STRING
+argument should still be a \"useful\" string for such uses."
address@hidden ignore
address@hidden 1200
+The @code{kill-new} function looks like this:
+
address@hidden
address@hidden
+(defun kill-new (string &optional replace yank-handler)
+ "Make STRING the latest kill in the kill ring.
+Set `kill-ring-yank-pointer' to point to it.
+
+If `interprogram-cut-function' is non-nil, apply it to STRING.
+Optional second argument REPLACE non-nil means that STRING will replace
+the front of the kill ring, rather than being added to the list.
address@hidden"
address@hidden group
address@hidden
+ (if (> (length string) 0)
+ (if yank-handler
+ (put-text-property 0 (length string)
+ 'yank-handler yank-handler string))
+ (if yank-handler
+ (signal 'args-out-of-range
+ (list string "yank-handler specified for empty string"))))
address@hidden group
address@hidden
+ (if (fboundp 'menu-bar-update-yank-menu)
+ (menu-bar-update-yank-menu string (and replace (car kill-ring))))
address@hidden group
address@hidden
+ (if (and replace kill-ring)
+ (setcar kill-ring string)
+ (push string kill-ring)
+ (if (> (length kill-ring) kill-ring-max)
+ (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
address@hidden group
address@hidden
+ (setq kill-ring-yank-pointer kill-ring)
+ (if interprogram-cut-function
+ (funcall interprogram-cut-function string (not replace))))
address@hidden group
address@hidden smallexample
address@hidden
+was:
+(defun kill-new (string &optional replace)
+ "Make STRING the latest kill in the kill ring.
+Set the kill-ring-yank pointer to point to it.
+If `interprogram-cut-function' is non-nil, apply it to STRING.
+Optional second argument REPLACE non-nil means that STRING will replace
+the front of the kill ring, rather than being added to the list."
+ (and (fboundp 'menu-bar-update-yank-menu)
+ (menu-bar-update-yank-menu string (and replace (car kill-ring))))
+ (if (and replace kill-ring)
+ (setcar kill-ring string)
+ (setq kill-ring (cons string kill-ring))
+ (if (> (length kill-ring) kill-ring-max)
+ (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
+ (setq kill-ring-yank-pointer kill-ring)
+ (if interprogram-cut-function
+ (funcall interprogram-cut-function string (not replace))))
address@hidden ignore
+
+(Notice that the function is not interactive.)
+
+As usual, we can look at this function in parts.
+
+The function definition has an optional @code{yank-handler} argument,
+which when invoked tells the function how to deal with properties
+added to the text, such as `bold' or `italics'. We will skip that.
+
address@hidden 1200
+The first line of the documentation makes sense:
+
address@hidden
+Make STRING the latest kill in the kill ring.
address@hidden smallexample
+
address@hidden
+Let's skip over the rest of the documentation for the moment.
+
address@hidden
+Also, let's skip over the initial @code{if} expression and those lines
+of code involving @code{menu-bar-update-yank-menu}. We will explain
+them below.
+
address@hidden 1200
+The critical lines are these:
+
address@hidden
address@hidden
+ (if (and replace kill-ring)
+ ;; @r{then}
+ (setcar kill-ring string)
address@hidden group
address@hidden
+ ;; @r{else}
+ (push string kill-ring)
address@hidden group
address@hidden
+ (setq kill-ring (cons string kill-ring))
+ (if (> (length kill-ring) kill-ring-max)
+ ;; @r{avoid overly long kill ring}
+ (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
address@hidden group
address@hidden
+ (setq kill-ring-yank-pointer kill-ring)
+ (if interprogram-cut-function
+ (funcall interprogram-cut-function string (not replace))))
address@hidden group
address@hidden smallexample
+
+The conditional test is @address@hidden(and replace kill-ring)}}.
+This will be true when two conditions are met: the kill ring has
+something in it, and the @code{replace} variable is true.
+
address@hidden 1250
+When the @code{kill-append} function sets @code{replace} to be true
+and when the kill ring has at least one item in it, the @code{setcar}
+expression is executed:
+
address@hidden
+(setcar kill-ring string)
address@hidden smallexample
+
+The @code{setcar} function actually changes the first element of the
address@hidden list to the value of @code{string}. It replaces the
+first element.
+
address@hidden 1250
+On the other hand, if the kill ring is empty, or replace is false, the
+else-part of the condition is executed:
+
address@hidden
+(push string kill-ring)
address@hidden smallexample
+
address@hidden
address@hidden 1250
address@hidden puts its first argument onto the second. It is similar to
+the older
+
address@hidden
+(setq kill-ring (cons string kill-ring))
address@hidden smallexample
+
address@hidden
address@hidden 1250
+or the newer
+
address@hidden
+(add-to-list kill-ring string)
address@hidden smallexample
+
address@hidden
+When it is false, the expression first constructs a new version of the
+kill ring by prepending @code{string} to the existing kill ring as a
+new element (that is what the @code{push} does). Then it executes a
+second @code{if} clause. This second @code{if} clause keeps the kill
+ring from growing too long.
+
+Let's look at these two expressions in order.
+
+The @code{push} line of the else-part sets the new value of the kill
+ring to what results from adding the string being killed to the old
+kill ring.
+
+We can see how this works with an example.
+
address@hidden 800
+First,
+
address@hidden
+(setq example-list '("here is a clause" "another clause"))
address@hidden smallexample
+
address@hidden 1200
address@hidden
+After evaluating this expression with @kbd{C-x C-e}, you can evaluate
address@hidden and see what it returns:
+
address@hidden
address@hidden
+example-list
+ @result{} ("here is a clause" "another clause")
address@hidden group
address@hidden smallexample
+
address@hidden 1200
address@hidden
+Now, we can add a new element on to this list by evaluating the
+following expression:
address@hidden push, @r{example}
+
address@hidden
+(push "a third clause" example-list)
address@hidden smallexample
+
address@hidden 800
address@hidden
+When we evaluate @code{example-list}, we find its value is:
+
address@hidden
address@hidden
+example-list
+ @result{} ("a third clause" "here is a clause" "another clause")
address@hidden group
address@hidden smallexample
+
address@hidden
+Thus, the third clause is added to the list by @code{push}.
+
address@hidden 1200
+Now for the second part of the @code{if} clause. This expression
+keeps the kill ring from growing too long. It looks like this:
+
address@hidden
address@hidden
+(if (> (length kill-ring) kill-ring-max)
+ (setcdr (nthcdr (1- kill-ring-max) kill-ring) nil))
address@hidden group
address@hidden smallexample
+
+The code checks whether the length of the kill ring is greater than
+the maximum permitted length. This is the value of
address@hidden (which is 60, by default). If the length of the
+kill ring is too long, then this code sets the last element of the
+kill ring to @code{nil}. It does this by using two functions,
address@hidden and @code{setcdr}.
+
+We looked at @code{setcdr} earlier (@pxref{setcdr, , @code{setcdr}}).
+It sets the @sc{cdr} of a list, just as @code{setcar} sets the
address@hidden of a list. In this case, however, @code{setcdr} will not be
+setting the @sc{cdr} of the whole kill ring; the @code{nthcdr}
+function is used to cause it to set the @sc{cdr} of the next to last
+element of the kill ring---this means that since the @sc{cdr} of the
+next to last element is the last element of the kill ring, it will set
+the last element of the kill ring.
+
address@hidden nthcdr, @r{example}
+The @code{nthcdr} function works by repeatedly taking the @sc{cdr} of a
+list---it takes the @sc{cdr} of the @sc{cdr} of the @sc{cdr}
address@hidden It does this @var{N} times and returns the results.
+(@xref{nthcdr, , @code{nthcdr}}.)
+
address@hidden setcdr, @r{example}
+Thus, if we had a four element list that was supposed to be three
+elements long, we could set the @sc{cdr} of the next to last element
+to @code{nil}, and thereby shorten the list. (If you set the last
+element to some other value than @code{nil}, which you could do, then
+you would not have shortened the list. @xref{setcdr, ,
address@hidden)
+
+You can see shortening by evaluating the following three expressions
+in turn. First set the value of @code{trees} to @code{(maple oak pine
+birch)}, then set the @sc{cdr} of its second @sc{cdr} to @code{nil}
+and then find the value of @code{trees}:
+
address@hidden
address@hidden
+(setq trees '(maple oak pine birch))
+ @result{} (maple oak pine birch)
address@hidden group
+
address@hidden
+(setcdr (nthcdr 2 trees) nil)
+ @result{} nil
+
+trees
+ @result{} (maple oak pine)
address@hidden group
address@hidden smallexample
+
address@hidden
+(The value returned by the @code{setcdr} expression is @code{nil} since
+that is what the @sc{cdr} is set to.)
+
+To repeat, in @code{kill-new}, the @code{nthcdr} function takes the
address@hidden a number of times that is one less than the maximum permitted
+size of the kill ring and @code{setcdr} sets the @sc{cdr} of that
+element (which will be the rest of the elements in the kill ring) to
address@hidden This prevents the kill ring from growing too long.
+
address@hidden 800
+The next to last expression in the @code{kill-new} function is
+
address@hidden
+(setq kill-ring-yank-pointer kill-ring)
address@hidden smallexample
+
+The @code{kill-ring-yank-pointer} is a global variable that is set to be
+the @code{kill-ring}.
+
+Even though the @code{kill-ring-yank-pointer} is called a
address@hidden, it is a variable just like the kill ring. However, the
+name has been chosen to help humans understand how the variable is used.
+
address@hidden 1200
+Now, to return to an early expression in the body of the function:
+
address@hidden
address@hidden
+ (if (fboundp 'menu-bar-update-yank-menu)
+ (menu-bar-update-yank-menu string (and replace (car kill-ring))))
address@hidden group
address@hidden smallexample
+
address@hidden
+It starts with an @code{if} expression
+
+In this case, the expression tests first to see whether
address@hidden exists as a function, and if so,
+calls it. The @code{fboundp} function returns true if the symbol it
+is testing has a function definition that `is not void'. If the
+symbol's function definition were void, we would receive an error
+message, as we did when we created errors intentionally (@pxref{Making
+Errors, , Generate an Error Message}).
+
address@hidden
+The then-part contains an expression whose first element is the
+function @code{and}.
+
address@hidden and
+The @code{and} special form evaluates each of its arguments until one
+of the arguments returns a value of @code{nil}, in which case the
address@hidden expression returns @code{nil}; however, if none of the
+arguments returns a value of @code{nil}, the value resulting from
+evaluating the last argument is returned. (Since such a value is not
address@hidden, it is considered true in Emacs Lisp.) In other words, an
address@hidden expression returns a true value only if all its arguments
+are true. (@xref{Second Buffer Related Review}.)
+
+The expression determines whether the second argument to
address@hidden is true or not.
address@hidden
+ ;; If we're supposed to be extending an existing string, and that
+ ;; string really is at the front of the menu, then update it in place.
address@hidden ignore
+
address@hidden is one of the functions that make it
+possible to use the `Select and Paste' menu in the Edit item of a menu
+bar; using a mouse, you can look at the various pieces of text you
+have saved and select one piece to paste.
+
+The last expression in the @code{kill-new} function adds the newly
+copied string to whatever facility exists for copying and pasting
+among different programs running in a windowing system. In the X
+Windowing system, for example, the @code{x-select-text} function takes
+the string and stores it in memory operated by X. You can paste the
+string in another program, such as an Xterm.
+
address@hidden 1200
+The expression looks like this:
+
address@hidden
address@hidden
+ (if interprogram-cut-function
+ (funcall interprogram-cut-function string (not replace))))
address@hidden group
address@hidden smallexample
+
+If an @code{interprogram-cut-function} exists, then Emacs executes
address@hidden, which in turn calls its first argument as a function
+and passes the remaining arguments to it. (Incidentally, as far as I
+can see, this @code{if} expression could be replaced by an @code{and}
+expression similar to the one in the first part of the function.)
+
+We are not going to discuss windowing systems and other programs
+further, but merely note that this is a mechanism that enables GNU
+Emacs to work easily and well with other programs.
+
+This code for placing text in the kill ring, either concatenated with
+an existing element or as a new element, leads us to the code for
+bringing back text that has been cut out of the buffer---the yank
+commands. However, before discussing the yank commands, it is better
+to learn how lists are implemented in a computer. This will make
+clear such mysteries as the use of the term `pointer'. But before
+that, we will digress into C.
+
address@hidden
address@hidden is this true in Emacs 22? Does not seems to be
+
+ (If the @address@hidden(< end beg))}}
+expression is true, @code{kill-append} prepends the string to the just
+previously clipped text. For a detailed discussion, see
address@hidden function, , The @code{kill-append} function}.)
+
+If you then yank back the text, i.e., `paste' it, you get both
+pieces of text at once. That way, if you delete two words in a row,
+and then yank them back, you get both words, in their proper order,
+with one yank. (The @address@hidden(< end beg))}} expression makes sure the
+order is correct.)
+
+On the other hand, if the previous command is not @code{kill-region},
+then the @code{kill-new} function is called, which adds the text to
+the kill ring as the latest item, and sets the
address@hidden variable to point to it.
address@hidden ignore
address@hidden
+
address@hidden Evidently, changed for Emacs 22. The zap-to-char command does not
address@hidden use the delete-and-extract-region function
+
+2006 Oct 26, the Digression into C is now OK but should come after
+copy-region-as-kill and filter-buffer-substring
+
+2006 Oct 24
+In Emacs 22,
+copy-region-as-kill is short, 12 lines, and uses
+filter-buffer-substring, which is longer, 39 lines
+and has delete-and-extract-region in it.
+delete-and-extract-region is written in C.
+
+see Initializing a Variable with @code{defvar}
address@hidden ignore
+
address@hidden Digression into C, defvar, copy-region-as-kill, Cutting &
Storing Text
address@hidden node-name, next, previous, up
address@hidden Digression into C
address@hidden delete-and-extract-region
address@hidden C, a digression into
address@hidden Digression into C
+
+The @code{copy-region-as-kill} function (@pxref{copy-region-as-kill, ,
address@hidden) uses the @code{filter-buffer-substring}
+function, which in turn uses the @code{delete-and-extract-region}
+function. It removes the contents of a region and you cannot get them
+back.
+
+Unlike the other code discussed here, the
address@hidden function is not written in Emacs
+Lisp; it is written in C and is one of the primitives of the GNU Emacs
+system. Since it is very simple, I will digress briefly from Lisp and
+describe it here.
+
address@hidden GNU Emacs 22 in /usr/local/src/emacs/src/editfns.c
address@hidden the DEFUN for buffer-substring-no-properties
+
address@hidden 1500
+Like many of the other Emacs primitives,
address@hidden is written as an instance of a C
+macro, a macro being a template for code. The complete macro looks
+like this:
+
address@hidden
address@hidden
+DEFUN ("buffer-substring-no-properties", Fbuffer_substring_no_properties,
+ Sbuffer_substring_no_properties, 2, 2, 0,
+ doc: /* Return the characters of part of the buffer,
+without the text properties.
+The two arguments START and END are character positions;
+they can be in either order. */)
+ (start, end)
+ Lisp_Object start, end;
address@hidden
+ register int b, e;
+
+ validate_region (&start, &end);
+ b = XINT (start);
+ e = XINT (end);
+
+ return make_buffer_string (b, e, 0);
address@hidden
address@hidden group
address@hidden smallexample
+
+Without going into the details of the macro writing process, let me
+point out that this macro starts with the word @code{DEFUN}. The word
address@hidden was chosen since the code serves the same purpose as
address@hidden does in Lisp. (The @code{DEFUN} C macro is defined in
address@hidden/src/lisp.h}.)
+
+The word @code{DEFUN} is followed by seven parts inside of
+parentheses:
+
address@hidden @bullet
address@hidden
+The first part is the name given to the function in Lisp,
address@hidden
+
address@hidden
+The second part is the name of the function in C,
address@hidden By convention, it starts with
address@hidden Since C does not use hyphens in names, underscores are used
+instead.
+
address@hidden
+The third part is the name for the C constant structure that records
+information on this function for internal use. It is the name of the
+function in C but begins with an @samp{S} instead of an @samp{F}.
+
address@hidden
+The fourth and fifth parts specify the minimum and maximum number of
+arguments the function can have. This function demands exactly 2
+arguments.
+
address@hidden
+The sixth part is nearly like the argument that follows the
address@hidden declaration in a function written in Lisp: a letter
+followed, perhaps, by a prompt. The only difference from the Lisp is
+when the macro is called with no arguments. Then you write a @code{0}
+(which is a `null string'), as in this macro.
+
+If you were to specify arguments, you would place them between
+quotation marks. The C macro for @code{goto-char} includes
address@hidden"NGoto char: "} in this position to indicate that the function
+expects a raw prefix, in this case, a numerical location in a buffer,
+and provides a prompt.
+
address@hidden
+The seventh part is a documentation string, just like the one for a
+function written in Emacs Lisp, except that every newline must be
+written explicitly as @samp{\n} followed by a backslash and carriage
+return.
+
address@hidden 1000
+Thus, the first two lines of documentation for @code{goto-char} are
+written like this:
+
address@hidden
address@hidden
+ "Set point to POSITION, a number or marker.\n\
+Beginning of buffer is position (point-min), end is (point-max)."
address@hidden group
address@hidden smallexample
address@hidden itemize
+
address@hidden 1200
+In a C macro, the formal parameters come next, with a statement of
+what kind of object they are, followed by what might be called the `body'
+of the macro. For @code{delete-and-extract-region} the `body'
+consists of the following four lines:
+
address@hidden
address@hidden
+validate_region (&start, &end);
+if (XINT (start) == XINT (end))
+ return build_string ("");
+return del_range_1 (XINT (start), XINT (end), 1, 1);
address@hidden group
address@hidden smallexample
+
+The @code{validate_region} function checks whether the values
+passed as the beginning and end of the region are the proper type and
+are within range. If the beginning and end positions are the same,
+then return and empty string.
+
+The @code{del_range_1} function actually deletes the text. It is a
+complex function we will not look into. It updates the buffer and
+does other things. However, it is worth looking at the two arguments
+passed to @code{del_range}. These are @address@hidden (start)}} and
address@hidden@code{XINT (end)}}.
+
+As far as the C language is concerned, @code{start} and @code{end} are
+two integers that mark the beginning and end of the region to be
address@hidden precisely, and requiring more expert knowledge
+to understand, the two integers are of type `Lisp_Object', which can
+also be a C union instead of an integer type.}.
+
+In early versions of Emacs, these two numbers were thirty-two bits
+long, but the code is slowly being generalized to handle other
+lengths. Three of the available bits are used to specify the type of
+information; the remaining bits are used as `content'.
+
address@hidden is a C macro that extracts the relevant number from the
+longer collection of bits; the three other bits are discarded.
+
address@hidden 800
+The command in @code{delete-and-extract-region} looks like this:
+
address@hidden
+del_range_1 (XINT (start), XINT (end), 1, 1);
address@hidden smallexample
+
address@hidden
+It deletes the region between the beginning position, @code{start},
+and the ending position, @code{end}.
+
+From the point of view of the person writing Lisp, Emacs is all very
+simple; but hidden underneath is a great deal of complexity to make it
+all work.
+
address@hidden defvar, cons & search-fwd Review, Digression into C, Cutting &
Storing Text
address@hidden node-name, next, previous, up
address@hidden Initializing a Variable with @code{defvar}
address@hidden defvar
address@hidden Initializing a variable
address@hidden Variable initialization
+
address@hidden
+2006 Oct 24
+In Emacs 22,
+copy-region-as-kill is short, 12 lines, and uses
+filter-buffer-substring, which is longer, 39 lines
+and has delete-and-extract-region in it.
+delete-and-extract-region is written in C.
+
+see Initializing a Variable with @code{defvar}
+
address@hidden ignore
+
+The @code{copy-region-as-kill} function is written in Emacs Lisp. Two
+functions within it, @code{kill-append} and @code{kill-new}, copy a
+region in a buffer and save it in a variable called the
address@hidden This section describes how the @code{kill-ring}
+variable is created and initialized using the @code{defvar} special
+form.
+
+(Again we note that the term @code{kill-ring} is a misnomer. The text
+that is clipped out of the buffer can be brought back; it is not a ring
+of corpses, but a ring of resurrectable text.)
+
+In Emacs Lisp, a variable such as the @code{kill-ring} is created and
+given an initial value by using the @code{defvar} special form. The
+name comes from ``define variable''.
+
+The @code{defvar} special form is similar to @code{setq} in that it sets
+the value of a variable. It is unlike @code{setq} in two ways: first,
+it only sets the value of the variable if the variable does not already
+have a value. If the variable already has a value, @code{defvar} does
+not override the existing value. Second, @code{defvar} has a
+documentation string.
+
+(Another special form, @code{defcustom}, is designed for variables
+that people customize. It has more features than @code{defvar}.
+(@xref{defcustom, , Setting Variables with @code{defcustom}}.)
+
address@hidden
+* See variable current value::
+* defvar and asterisk::
address@hidden menu
+
address@hidden See variable current value, defvar and asterisk, defvar, defvar
address@hidden
address@hidden Seeing the Current Value of a Variable
address@hidden ifnottex
+
+You can see the current value of a variable, any variable, by using
+the @code{describe-variable} function, which is usually invoked by
+typing @kbd{C-h v}. If you type @kbd{C-h v} and then @code{kill-ring}
+(followed by @key{RET}) when prompted, you will see what is in your
+current kill ring---this may be quite a lot! Conversely, if you have
+been doing nothing this Emacs session except read this document, you
+may have nothing in it. Also, you will see the documentation for
address@hidden:
+
address@hidden
address@hidden
+Documentation:
+List of killed text sequences.
+Since the kill ring is supposed to interact nicely with cut-and-paste
+facilities offered by window systems, use of this variable should
address@hidden group
address@hidden
+interact nicely with `interprogram-cut-function' and
+`interprogram-paste-function'. The functions `kill-new',
+`kill-append', and `current-kill' are supposed to implement this
+interaction; you may want to use them instead of manipulating the kill
+ring directly.
address@hidden group
address@hidden smallexample
+
address@hidden 800
+The kill ring is defined by a @code{defvar} in the following way:
+
address@hidden
address@hidden
+(defvar kill-ring nil
+ "List of killed text sequences.
address@hidden")
address@hidden group
address@hidden smallexample
+
address@hidden
+In this variable definition, the variable is given an initial value of
address@hidden, which makes sense, since if you have saved nothing, you want
+nothing back if you give a @code{yank} command. The documentation
+string is written just like the documentation string of a @code{defun}.
+As with the documentation string of the @code{defun}, the first line of
+the documentation should be a complete sentence, since some commands,
+like @code{apropos}, print only the first line of documentation.
+Succeeding lines should not be indented; otherwise they look odd when
+you use @kbd{C-h v} (@code{describe-variable}).
+
address@hidden defvar and asterisk, , See variable current value, defvar
address@hidden @code{defvar} and an asterisk
address@hidden defvar @r{for a user customizable variable}
address@hidden defvar @r{with an asterisk}
+
+In the past, Emacs used the @code{defvar} special form both for
+internal variables that you would not expect a user to change and for
+variables that you do expect a user to change. Although you can still
+use @code{defvar} for user customizable variables, please use
address@hidden instead, since that special form provides a path into
+the Customization commands. (@xref{defcustom, , Specifying Variables
+using @code{defcustom}}.)
+
+When you specified a variable using the @code{defvar} special form,
+you could distinguish a readily settable variable from others by
+typing an asterisk, @samp{*}, in the first column of its documentation
+string. For example:
+
address@hidden
address@hidden
+(defvar shell-command-default-error-buffer nil
+ "*Buffer name for `shell-command' @dots{} error output.
address@hidden ")
address@hidden group
address@hidden smallexample
+
address@hidden set-variable
address@hidden
+You could (and still can) use the @code{set-variable} command to
+change the value of @code{shell-command-default-error-buffer}
+temporarily. However, options set using @code{set-variable} are set
+only for the duration of your editing session. The new values are not
+saved between sessions. Each time Emacs starts, it reads the original
+value, unless you change the value within your @file{.emacs} file,
+either by setting it manually or by using @code{customize}.
address@hidden Initialization, , Your @file{.emacs} File}.
+
+For me, the major use of the @code{set-variable} command is to suggest
+variables that I might want to set in my @file{.emacs} file. There
+are now more than 700 such variables --- far too many to remember
+readily. Fortunately, you can press @key{TAB} after calling the
address@hidden set-variable} command to see the list of variables.
+(@xref{Examining, , Examining and Setting Variables, emacs,
+The GNU Emacs Manual}.)
+
address@hidden 1250
address@hidden cons & search-fwd Review, search Exercises, defvar, Cutting &
Storing Text
address@hidden node-name, next, previous, up
address@hidden Review
+
+Here is a brief summary of some recently introduced functions.
+
address@hidden @code
address@hidden car
address@hidden cdr
address@hidden returns the first element of a list; @code{cdr} returns the
+second and subsequent elements of a list.
+
address@hidden 1250
+For example:
+
address@hidden
address@hidden
+(car '(1 2 3 4 5 6 7))
+ @result{} 1
+(cdr '(1 2 3 4 5 6 7))
+ @result{} (2 3 4 5 6 7)
address@hidden group
address@hidden smallexample
+
address@hidden cons
address@hidden constructs a list by prepending its first argument to its
+second argument.
+
address@hidden 1250
+For example:
+
address@hidden
address@hidden
+(cons 1 '(2 3 4))
+ @result{} (1 2 3 4)
address@hidden group
address@hidden smallexample
+
address@hidden funcall
address@hidden evaluates its first argument as a function. It passes
+its remaining arguments to its first argument.
+
address@hidden nthcdr
+Return the result of taking @sc{cdr} `n' times on a list.
address@hidden
+The
address@hidden
+$n^{th}$
address@hidden tex
address@hidden
address@hidden iftex
+The `rest of the rest', as it were.
+
address@hidden 1250
+For example:
+
address@hidden
address@hidden
+(nthcdr 3 '(1 2 3 4 5 6 7))
+ @result{} (4 5 6 7)
address@hidden group
address@hidden smallexample
+
address@hidden setcar
address@hidden setcdr
address@hidden changes the first element of a list; @code{setcdr}
+changes the second and subsequent elements of a list.
+
address@hidden 1250
+For example:
+
address@hidden
address@hidden
+(setq triple '(1 2 3))
+
+(setcar triple '37)
+
+triple
+ @result{} (37 2 3)
+
+(setcdr triple '("foo" "bar"))
+
+triple
+ @result{} (37 "foo" "bar")
address@hidden group
address@hidden smallexample
+
address@hidden progn
+Evaluate each argument in sequence and then return the value of the
+last.
+
address@hidden 1250
+For example:
+
address@hidden
address@hidden
+(progn 1 2 3 4)
+ @result{} 4
address@hidden group
address@hidden smallexample
+
address@hidden save-restriction
+Record whatever narrowing is in effect in the current buffer, if any,
+and restore that narrowing after evaluating the arguments.
+
address@hidden search-forward
+Search for a string, and if the string is found, move point. With a
+regular expression, use the similar @code{re-search-forward}.
+(@xref{Regexp Search, , Regular Expression Searches}, for an
+explanation of regular expression patterns and searches.)
+
address@hidden 1250
address@hidden
address@hidden and @code{re-search-forward} take four
+arguments:
+
address@hidden
address@hidden
+The string or regular expression to search for.
+
address@hidden
+Optionally, the limit of the search.
+
address@hidden
+Optionally, what to do if the search fails, return @code{nil} or an
+error message.
+
address@hidden
+Optionally, how many times to repeat the search; if negative, the
+search goes backwards.
address@hidden enumerate
+
address@hidden kill-region
address@hidden delete-and-extract-region
address@hidden copy-region-as-kill
+
address@hidden cuts the text between point and mark from the
+buffer and stores that text in the kill ring, so you can get it back
+by yanking.
+
address@hidden copies the text between point and mark into
+the kill ring, from which you can get it by yanking. The function
+does not cut or remove the text from the buffer.
address@hidden table
+
address@hidden removes the text between point and
+mark from the buffer and throws it away. You cannot get it back.
+(This is not an interactive command.)
+
address@hidden 1500
address@hidden search Exercises, , cons & search-fwd Review, Cutting & Storing
Text
address@hidden Searching Exercises
+
address@hidden @bullet
address@hidden
+Write an interactive function that searches for a string. If the
+search finds the string, leave point after it and display a message
+that says ``Found!''. (Do not use @code{search-forward} for the name
+of this function; if you do, you will overwrite the existing version of
address@hidden that comes with Emacs. Use a name such as
address@hidden instead.)
+
address@hidden
+Write a function that prints the third element of the kill ring in the
+echo area, if any; if the kill ring does not contain a third element,
+print an appropriate message.
address@hidden itemize
+
address@hidden List Implementation, Yanking, Cutting & Storing Text, Top
address@hidden node-name, next, previous, up
address@hidden How Lists are Implemented
address@hidden Lists in a computer
+
+In Lisp, atoms are recorded in a straightforward fashion; if the
+implementation is not straightforward in practice, it is, nonetheless,
+straightforward in theory. The atom @samp{rose}, for example, is
+recorded as the four contiguous letters @samp{r}, @samp{o}, @samp{s},
address@hidden A list, on the other hand, is kept differently. The mechanism
+is equally simple, but it takes a moment to get used to the idea. A
+list is kept using a series of pairs of pointers. In the series, the
+first pointer in each pair points to an atom or to another list, and the
+second pointer in each pair points to the next pair, or to the symbol
address@hidden, which marks the end of the list.
+
+A pointer itself is quite simply the electronic address of what is
+pointed to. Hence, a list is kept as a series of electronic addresses.
+
address@hidden
+* Lists diagrammed::
+* Symbols as Chest:: Exploring a powerful metaphor.
+* List Exercise::
address@hidden menu
+
address@hidden Lists diagrammed, Symbols as Chest, List Implementation, List
Implementation
address@hidden
address@hidden Lists diagrammed
address@hidden ifnottex
+
+For example, the list @code{(rose violet buttercup)} has three elements,
address@hidden, @samp{violet}, and @samp{buttercup}. In the computer, the
+electronic address of @samp{rose} is recorded in a segment of computer
+memory along with the address that gives the electronic address of where
+the atom @samp{violet} is located; and that address (the one that tells
+where @samp{violet} is located) is kept along with an address that tells
+where the address for the atom @samp{buttercup} is located.
+
address@hidden 1200
+This sounds more complicated than it is and is easier seen in a diagram:
+
address@hidden clear print-postscript-figures
address@hidden !!! cons-cell-diagram #1
address@hidden
address@hidden
address@hidden
+ ___ ___ ___ ___ ___ ___
+ |___|___|--> |___|___|--> |___|___|--> nil
+ | | |
+ | | |
+ --> rose --> violet --> buttercup
address@hidden group
address@hidden smallexample
address@hidden ifnottex
address@hidden print-postscript-figures
address@hidden 1
address@hidden
address@hidden @image{cons-1}
+%%%% old method of including an image
+% \input /usr/local/lib/tex/inputs/psfig.tex
+% \centerline{\psfig{figure=/usr/local/lib/emacs/man/cons-1.eps}}
+% address@hidden %
address@hidden tex
address@hidden 1
address@hidden ifset
address@hidden print-postscript-figures
address@hidden
address@hidden
address@hidden
+ ___ ___ ___ ___ ___ ___
+ |___|___|--> |___|___|--> |___|___|--> nil
+ | | |
+ | | |
+ --> rose --> violet --> buttercup
address@hidden group
address@hidden smallexample
address@hidden iftex
address@hidden ifclear
+
address@hidden
+In the diagram, each box represents a word of computer memory that
+holds a Lisp object, usually in the form of a memory address. The boxes,
+i.e.@: the addresses, are in pairs. Each arrow points to what the address
+is the address of, either an atom or another pair of addresses. The
+first box is the electronic address of @samp{rose} and the arrow points
+to @samp{rose}; the second box is the address of the next pair of boxes,
+the first part of which is the address of @samp{violet} and the second
+part of which is the address of the next pair. The very last box
+points to the symbol @code{nil}, which marks the end of the list.
+
address@hidden 1200
+When a variable is set to a list with a function such as @code{setq},
+it stores the address of the first box in the variable. Thus,
+evaluation of the expression
+
address@hidden
+(setq bouquet '(rose violet buttercup))
address@hidden smallexample
+
address@hidden 1250
address@hidden
+creates a situation like this:
+
address@hidden cons-cell-diagram #2
address@hidden
address@hidden
address@hidden
+bouquet
+ |
+ | ___ ___ ___ ___ ___ ___
+ --> |___|___|--> |___|___|--> |___|___|--> nil
+ | | |
+ | | |
+ --> rose --> violet --> buttercup
address@hidden group
address@hidden smallexample
address@hidden ifnottex
address@hidden print-postscript-figures
address@hidden 1
address@hidden
address@hidden @image{cons-2}
+%%%% old method of including an image
+% \input /usr/local/lib/tex/inputs/psfig.tex
+% \centerline{\psfig{figure=/usr/local/lib/emacs/man/cons-2.eps}}
+% address@hidden %
address@hidden tex
address@hidden 1
address@hidden ifset
address@hidden print-postscript-figures
address@hidden
address@hidden
address@hidden
+bouquet
+ |
+ | ___ ___ ___ ___ ___ ___
+ --> |___|___|--> |___|___|--> |___|___|--> nil
+ | | |
+ | | |
+ --> rose --> violet --> buttercup
address@hidden group
address@hidden smallexample
address@hidden iftex
address@hidden ifclear
+
address@hidden
+In this example, the symbol @code{bouquet} holds the address of the first
+pair of boxes.
+
address@hidden 1200
+This same list can be illustrated in a different sort of box notation
+like this:
+
address@hidden cons-cell-diagram #2a
address@hidden
address@hidden
address@hidden
+bouquet
+ |
+ | -------------- --------------- ----------------
+ | | car | cdr | | car | cdr | | car | cdr |
+ -->| rose | o------->| violet | o------->| butter- | nil |
+ | | | | | | | cup | |
+ -------------- --------------- ----------------
address@hidden group
address@hidden smallexample
address@hidden ifnottex
address@hidden print-postscript-figures
address@hidden 1
address@hidden
address@hidden @image{cons-2a}
+%%%% old method of including an image
+% \input /usr/local/lib/tex/inputs/psfig.tex
+% \centerline{\psfig{figure=/usr/local/lib/emacs/man/cons-2a.eps}}
+% address@hidden %
address@hidden tex
address@hidden 1
address@hidden ifset
address@hidden print-postscript-figures
address@hidden
address@hidden
address@hidden
+bouquet
+ |
+ | -------------- --------------- ----------------
+ | | car | cdr | | car | cdr | | car | cdr |
+ -->| rose | o------->| violet | o------->| butter- | nil |
+ | | | | | | | cup | |
+ -------------- --------------- ----------------
address@hidden group
address@hidden smallexample
address@hidden iftex
address@hidden ifclear
+
+(Symbols consist of more than pairs of addresses, but the structure of
+a symbol is made up of addresses. Indeed, the symbol @code{bouquet}
+consists of a group of address-boxes, one of which is the address of
+the printed word @samp{bouquet}, a second of which is the address of a
+function definition attached to the symbol, if any, a third of which
+is the address of the first pair of address-boxes for the list
address@hidden(rose violet buttercup)}, and so on. Here we are showing that
+the symbol's third address-box points to the first pair of
+address-boxes for the list.)
+
+If a symbol is set to the @sc{cdr} of a list, the list itself is not
+changed; the symbol simply has an address further down the list. (In
+the jargon, @sc{car} and @sc{cdr} are `non-destructive'.) Thus,
+evaluation of the following expression
+
address@hidden
+(setq flowers (cdr bouquet))
address@hidden smallexample
+
address@hidden 800
address@hidden
+produces this:
+
address@hidden cons-cell-diagram #3
address@hidden
address@hidden 1
address@hidden
address@hidden
+bouquet flowers
+ | |
+ | ___ ___ | ___ ___ ___ ___
+ --> | | | --> | | | | | |
+ |___|___|----> |___|___|--> |___|___|--> nil
+ | | |
+ | | |
+ --> rose --> violet --> buttercup
address@hidden group
address@hidden smallexample
address@hidden 1
address@hidden ifnottex
address@hidden print-postscript-figures
address@hidden 1
address@hidden
address@hidden @image{cons-3}
+%%%% old method of including an image
+% \input /usr/local/lib/tex/inputs/psfig.tex
+% \centerline{\psfig{figure=/usr/local/lib/emacs/man/cons-3.eps}}
+% address@hidden %
address@hidden tex
address@hidden 1
address@hidden ifset
address@hidden print-postscript-figures
address@hidden
address@hidden 1
address@hidden
address@hidden
+bouquet flowers
+ | |
+ | ___ ___ | ___ ___ ___ ___
+ --> | | | --> | | | | | |
+ |___|___|----> |___|___|--> |___|___|--> nil
+ | | |
+ | | |
+ --> rose --> violet --> buttercup
address@hidden group
address@hidden smallexample
address@hidden 1
address@hidden iftex
address@hidden ifclear
+
address@hidden
+The value of @code{flowers} is @code{(violet buttercup)}, which is
+to say, the symbol @code{flowers} holds the address of the pair of
+address-boxes, the first of which holds the address of @code{violet},
+and the second of which holds the address of @code{buttercup}.
+
+A pair of address-boxes is called a @dfn{cons cell} or @dfn{dotted
+pair}. @xref{Cons Cell Type, , Cons Cell and List Types, elisp, The GNU Emacs
Lisp
+Reference Manual}, and @ref{Dotted Pair Notation, , Dotted Pair
+Notation, elisp, The GNU Emacs Lisp Reference Manual}, for more
+information about cons cells and dotted pairs.
+
address@hidden 1200
+The function @code{cons} adds a new pair of addresses to the front of
+a series of addresses like that shown above. For example, evaluating
+the expression
+
address@hidden
+(setq bouquet (cons 'lily bouquet))
address@hidden smallexample
+
address@hidden 1500
address@hidden
+produces:
+
address@hidden cons-cell-diagram #4
address@hidden
address@hidden 1
address@hidden
address@hidden
+bouquet flowers
+ | |
+ | ___ ___ ___ ___ | ___ ___ ___ ___
+ --> | | | | | | --> | | | | | |
+ |___|___|----> |___|___|----> |___|___|---->|___|___|--> nil
+ | | | |
+ | | | |
+ --> lily --> rose --> violet --> buttercup
address@hidden group
address@hidden smallexample
address@hidden 1
address@hidden ifnottex
address@hidden print-postscript-figures
address@hidden 1
address@hidden
address@hidden @image{cons-4}
+%%%% old method of including an image
+% \input /usr/local/lib/tex/inputs/psfig.tex
+% \centerline{\psfig{figure=/usr/local/lib/emacs/man/cons-4.eps}}
+% address@hidden %
address@hidden tex
address@hidden 1
address@hidden ifset
address@hidden print-postscript-figures
address@hidden
address@hidden 1
address@hidden
address@hidden
+bouquet flowers
+ | |
+ | ___ ___ ___ ___ | ___ ___ ___ ___
+ --> | | | | | | --> | | | | | |
+ |___|___|----> |___|___|----> |___|___|---->|___|___|--> nil
+ | | | |
+ | | | |
+ --> lily --> rose --> violet --> buttercup
address@hidden group
address@hidden smallexample
address@hidden 1
address@hidden iftex
address@hidden ifclear
+
address@hidden 1200
address@hidden
+However, this does not change the value of the symbol
address@hidden, as you can see by evaluating the following,
+
address@hidden
+(eq (cdr (cdr bouquet)) flowers)
address@hidden smallexample
+
address@hidden
+which returns @code{t} for true.
+
+Until it is reset, @code{flowers} still has the value
address@hidden(violet buttercup)}; that is, it has the address of the cons
+cell whose first address is of @code{violet}. Also, this does not
+alter any of the pre-existing cons cells; they are all still there.
+
+Thus, in Lisp, to get the @sc{cdr} of a list, you just get the address
+of the next cons cell in the series; to get the @sc{car} of a list,
+you get the address of the first element of the list; to @code{cons} a
+new element on a list, you add a new cons cell to the front of the list.
+That is all there is to it! The underlying structure of Lisp is
+brilliantly simple!
+
+And what does the last address in a series of cons cells refer to? It
+is the address of the empty list, of @code{nil}.
+
+In summary, when a Lisp variable is set to a value, it is provided with
+the address of the list to which the variable refers.
+
address@hidden Symbols as Chest, List Exercise, Lists diagrammed, List
Implementation
address@hidden Symbols as a Chest of Drawers
address@hidden Symbols as a Chest of Drawers
address@hidden Chest of Drawers, metaphor for a symbol
address@hidden Drawers, Chest of, metaphor for a symbol
+
+In an earlier section, I suggested that you might imagine a symbol as
+being a chest of drawers. The function definition is put in one
+drawer, the value in another, and so on. What is put in the drawer
+holding the value can be changed without affecting the contents of the
+drawer holding the function definition, and vice-verse.
+
+Actually, what is put in each drawer is the address of the value or
+function definition. It is as if you found an old chest in the attic,
+and in one of its drawers you found a map giving you directions to
+where the buried treasure lies.
+
+(In addition to its name, symbol definition, and variable value, a
+symbol has a `drawer' for a @dfn{property list} which can be used to
+record other information. Property lists are not discussed here; see
address@hidden Lists, , Property Lists, elisp, The GNU Emacs Lisp
+Reference Manual}.)
+
address@hidden 1500
+Here is a fanciful representation:
+
address@hidden chest-of-drawers diagram
address@hidden
address@hidden 1
address@hidden
address@hidden
+ Chest of Drawers Contents of Drawers
+
+ __ o0O0o __
+ / \
+ ---------------------
+ | directions to | [map to]
+ | symbol name | bouquet
+ | |
+ +---------------------+
+ | directions to |
+ | symbol definition | [none]
+ | |
+ +---------------------+
+ | directions to | [map to]
+ | variable value | (rose violet buttercup)
+ | |
+ +---------------------+
+ | directions to |
+ | property list | [not described here]
+ | |
+ +---------------------+
+ |/ \|
address@hidden group
address@hidden smallexample
address@hidden 1
address@hidden ifnottex
address@hidden print-postscript-figures
address@hidden 1
address@hidden
address@hidden @image{drawers}
+%%%% old method of including an image
+% \input /usr/local/lib/tex/inputs/psfig.tex
+% \centerline{\psfig{figure=/usr/local/lib/emacs/man/drawers.eps}}
+% address@hidden %
address@hidden tex
address@hidden 1
address@hidden ifset
address@hidden print-postscript-figures
address@hidden
address@hidden 1
address@hidden
address@hidden
+ Chest of Drawers Contents of Drawers
+
+ __ o0O0o __
+ / \
+ ---------------------
+ | directions to | [map to]
+ | symbol name | bouquet
+ | |
+ +---------------------+
+ | directions to |
+ | symbol definition | [none]
+ | |
+ +---------------------+
+ | directions to | [map to]
+ | variable value | (rose violet buttercup)
+ | |
+ +---------------------+
+ | directions to |
+ | property list | [not described here]
+ | |
+ +---------------------+
+ |/ \|
address@hidden group
address@hidden smallexample
address@hidden 1
address@hidden iftex
address@hidden ifclear
+
address@hidden List Exercise, , Symbols as Chest, List Implementation
address@hidden Exercise
+
+Set @code{flowers} to @code{violet} and @code{buttercup}. Cons two
+more flowers on to this list and set this new list to
address@hidden Set the @sc{car} of @code{flowers} to a fish.
+What does the @code{more-flowers} list now contain?
+
address@hidden Yanking, Loops & Recursion, List Implementation, Top
address@hidden node-name, next, previous, up
address@hidden Yanking Text Back
address@hidden yank
address@hidden Text retrieval
address@hidden Retrieving text
address@hidden Pasting text
+
+Whenever you cut text out of a buffer with a `kill' command in GNU Emacs,
+you can bring it back with a `yank' command. The text that is cut out of
+the buffer is put in the kill ring and the yank commands insert the
+appropriate contents of the kill ring back into a buffer (not necessarily
+the original buffer).
+
+A simple @kbd{C-y} (@code{yank}) command inserts the first item from
+the kill ring into the current buffer. If the @kbd{C-y} command is
+followed immediately by @kbd{M-y}, the first element is replaced by
+the second element. Successive @kbd{M-y} commands replace the second
+element with the third, fourth, or fifth element, and so on. When the
+last element in the kill ring is reached, it is replaced by the first
+element and the cycle is repeated. (Thus the kill ring is called a
+`ring' rather than just a `list'. However, the actual data structure
+that holds the text is a list.
address@hidden Ring, , Handling the Kill Ring}, for the details of how the
+list is handled as a ring.)
+
address@hidden
+* Kill Ring Overview::
+* kill-ring-yank-pointer:: The kill ring is a list.
+* yank nthcdr Exercises:: The @code{kill-ring-yank-pointer} variable.
address@hidden menu
+
address@hidden Kill Ring Overview, kill-ring-yank-pointer, Yanking, Yanking
address@hidden node-name, next, previous, up
address@hidden Kill Ring Overview
address@hidden Kill ring overview
+
+The kill ring is a list of textual strings. This is what it looks like:
+
address@hidden
+("some text" "a different piece of text" "yet more text")
address@hidden smallexample
+
+If this were the contents of my kill ring and I pressed @kbd{C-y}, the
+string of characters saying @samp{some text} would be inserted in this
+buffer where my cursor is located.
+
+The @code{yank} command is also used for duplicating text by copying it.
+The copied text is not cut from the buffer, but a copy of it is put on the
+kill ring and is inserted by yanking it back.
+
+Three functions are used for bringing text back from the kill ring:
address@hidden, which is usually bound to @kbd{C-y}; @code{yank-pop},
+which is usually bound to @kbd{M-y}; and @code{rotate-yank-pointer},
+which is used by the two other functions.
+
+These functions refer to the kill ring through a variable called the
address@hidden Indeed, the insertion code for both the
address@hidden and @code{yank-pop} functions is:
+
address@hidden
+(insert (car kill-ring-yank-pointer))
address@hidden smallexample
+
address@hidden
+(Well, no more. In GNU Emacs 22, the function has been replaced by
address@hidden which calls @code{insert-for-yank-1}
+repetitively for each @code{yank-handler} segment. In turn,
address@hidden strips text properties from the inserted text
+according to @code{yank-excluded-properties}. Otherwise, it is just
+like @code{insert}. We will stick with plain @code{insert} since it
+is easier to understand.)
+
+To begin to understand how @code{yank} and @code{yank-pop} work, it is
+first necessary to look at the @code{kill-ring-yank-pointer} variable.
+
address@hidden kill-ring-yank-pointer, yank nthcdr Exercises, Kill Ring
Overview, Yanking
address@hidden node-name, next, previous, up
address@hidden The @code{kill-ring-yank-pointer} Variable
+
address@hidden is a variable, just as @code{kill-ring} is
+a variable. It points to something by being bound to the value of what
+it points to, like any other Lisp variable.
+
address@hidden 1000
+Thus, if the value of the kill ring is:
+
address@hidden
+("some text" "a different piece of text" "yet more text")
address@hidden smallexample
+
address@hidden 1250
address@hidden
+and the @code{kill-ring-yank-pointer} points to the second clause, the
+value of @code{kill-ring-yank-pointer} is:
+
address@hidden
+("a different piece of text" "yet more text")
address@hidden smallexample
+
+As explained in the previous chapter (@pxref{List Implementation}), the
+computer does not keep two different copies of the text being pointed to
+by both the @code{kill-ring} and the @code{kill-ring-yank-pointer}. The
+words ``a different piece of text'' and ``yet more text'' are not
+duplicated. Instead, the two Lisp variables point to the same pieces of
+text. Here is a diagram:
+
address@hidden cons-cell-diagram #5
address@hidden
address@hidden
address@hidden
+kill-ring kill-ring-yank-pointer
+ | |
+ | ___ ___ | ___ ___ ___ ___
+ ---> | | | --> | | | | | |
+ |___|___|----> |___|___|--> |___|___|--> nil
+ | | |
+ | | |
+ | | --> "yet more text"
+ | |
+ | --> "a different piece of text"
+ |
+ --> "some text"
address@hidden group
address@hidden smallexample
address@hidden 1
address@hidden ifnottex
address@hidden print-postscript-figures
address@hidden 1
address@hidden
address@hidden @image{cons-5}
+%%%% old method of including an image
+% \input /usr/local/lib/tex/inputs/psfig.tex
+% \centerline{\psfig{figure=/usr/local/lib/emacs/man/cons-5.eps}}
+% address@hidden %
address@hidden tex
address@hidden 1
address@hidden ifset
address@hidden print-postscript-figures
address@hidden
address@hidden
address@hidden
+kill-ring kill-ring-yank-pointer
+ | |
+ | ___ ___ | ___ ___ ___ ___
+ ---> | | | --> | | | | | |
+ |___|___|----> |___|___|--> |___|___|--> nil
+ | | |
+ | | |
+ | | --> "yet more text"
+ | |
+ | --> "a different piece of text
+ |
+ --> "some text"
address@hidden group
address@hidden smallexample
address@hidden 1
address@hidden iftex
address@hidden ifclear
+
+Both the variable @code{kill-ring} and the variable
address@hidden are pointers. But the kill ring itself is
+usually described as if it were actually what it is composed of. The
address@hidden is spoken of as if it were the list rather than that it
+points to the list. Conversely, the @code{kill-ring-yank-pointer} is
+spoken of as pointing to a list.
+
+These two ways of talking about the same thing sound confusing at first but
+make sense on reflection. The kill ring is generally thought of as the
+complete structure of data that holds the information of what has recently
+been cut out of the Emacs buffers. The @code{kill-ring-yank-pointer}
+on the other hand, serves to indicate---that is, to `point to'---that part
+of the kill ring of which the first element (the @sc{car}) will be
+inserted.
+
address@hidden
+In GNU Emacs 22, the @code{kill-new} function calls
+
address@hidden(setq kill-ring-yank-pointer kill-ring)}
+
+(defun rotate-yank-pointer (arg)
+ "Rotate the yanking point in the kill ring.
+With argument, rotate that many kills forward (or backward, if negative)."
+ (interactive "p")
+ (current-kill arg))
+
+(defun current-kill (n &optional do-not-move)
+ "Rotate the yanking point by N places, and then return that kill.
+If N is zero, `interprogram-paste-function' is set, and calling it
+returns a string, then that string is added to the front of the
+kill ring and returned as the latest kill.
+If optional arg DO-NOT-MOVE is non-nil, then don't actually move the
+yanking point; just return the Nth kill forward."
+ (let ((interprogram-paste (and (= n 0)
+ interprogram-paste-function
+ (funcall interprogram-paste-function))))
+ (if interprogram-paste
+ (progn
+ ;; Disable the interprogram cut function when we add the new
+ ;; text to the kill ring, so Emacs doesn't try to own the
+ ;; selection, with identical text.
+ (let ((interprogram-cut-function nil))
+ (kill-new interprogram-paste))
+ interprogram-paste)
+ (or kill-ring (error "Kill ring is empty"))
+ (let ((ARGth-kill-element
+ (nthcdr (mod (- n (length kill-ring-yank-pointer))
+ (length kill-ring))
+ kill-ring)))
+ (or do-not-move
+ (setq kill-ring-yank-pointer ARGth-kill-element))
+ (car ARGth-kill-element)))))
+
address@hidden ignore
+
address@hidden 1500
address@hidden yank nthcdr Exercises, , kill-ring-yank-pointer, Yanking
address@hidden Exercises with @code{yank} and @code{nthcdr}
+
address@hidden @bullet
address@hidden
+Using @kbd{C-h v} (@code{describe-variable}), look at the value of
+your kill ring. Add several items to your kill ring; look at its
+value again. Using @kbd{M-y} (@code{yank-pop)}, move all the way
+around the kill ring. How many items were in your kill ring? Find
+the value of @code{kill-ring-max}. Was your kill ring full, or could
+you have kept more blocks of text within it?
+
address@hidden
+Using @code{nthcdr} and @code{car}, construct a series of expressions
+to return the first, second, third, and fourth elements of a list.
address@hidden itemize
+
address@hidden Loops & Recursion, Regexp Search, Yanking, Top
address@hidden node-name, next, previous, up
address@hidden Loops and Recursion
address@hidden Loops and recursion
address@hidden Recursion and loops
address@hidden Repetition (loops)
+
+Emacs Lisp has two primary ways to cause an expression, or a series of
+expressions, to be evaluated repeatedly: one uses a @code{while}
+loop, and the other uses @dfn{recursion}.
+
+Repetition can be very valuable. For example, to move forward four
+sentences, you need only write a program that will move forward one
+sentence and then repeat the process four times. Since a computer does
+not get bored or tired, such repetitive action does not have the
+deleterious effects that excessive or the wrong kinds of repetition can
+have on humans.
+
+People mostly write Emacs Lisp functions using @code{while} loops and
+their kin; but you can use recursion, which provides a very powerful
+way to think about and then to solve address@hidden can write
+recursive functions to be frugal or wasteful of mental or computer
+resources; as it happens, methods that people find easy---that are
+frugal of `mental resources'---sometimes use considerable computer
+resources. Emacs was designed to run on machines that we now consider
+limited and its default settings are conservative. You may want to
+increase the values of @code{max-specpdl-size} and
address@hidden In my @file{.emacs} file, I set them to
+15 and 30 times their default value.}.
+
address@hidden
+* while:: Causing a stretch of code to repeat.
+* dolist dotimes::
+* Recursion:: Causing a function to call itself.
+* Looping exercise::
address@hidden menu
+
address@hidden while, dolist dotimes, Loops & Recursion, Loops & Recursion
address@hidden node-name, next, previous, up
address@hidden @code{while}
address@hidden Loops
address@hidden while
+
+The @code{while} special form tests whether the value returned by
+evaluating its first argument is true or false. This is similar to what
+the Lisp interpreter does with an @code{if}; what the interpreter does
+next, however, is different.
+
+In a @code{while} expression, if the value returned by evaluating the
+first argument is false, the Lisp interpreter skips the rest of the
+expression (the @dfn{body} of the expression) and does not evaluate it.
+However, if the value is true, the Lisp interpreter evaluates the body
+of the expression and then again tests whether the first argument to
address@hidden is true or false. If the value returned by evaluating the
+first argument is again true, the Lisp interpreter again evaluates the
+body of the expression.
+
address@hidden 1200
+The template for a @code{while} expression looks like this:
+
address@hidden
address@hidden
+(while @var{true-or-false-test}
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
address@hidden
+* Looping with while:: Repeat so long as test returns true.
+* Loop Example:: A @code{while} loop that uses a list.
+* print-elements-of-list:: Uses @code{while}, @code{car}, @code{cdr}.
+* Incrementing Loop:: A loop with an incrementing counter.
+* Incrementing Loop Details::
+* Decrementing Loop:: A loop with a decrementing counter.
address@hidden menu
+
address@hidden Looping with while, Loop Example, while, while
address@hidden
address@hidden Looping with @code{while}
address@hidden ifnottex
+
+So long as the true-or-false-test of the @code{while} expression
+returns a true value when it is evaluated, the body is repeatedly
+evaluated. This process is called a loop since the Lisp interpreter
+repeats the same thing again and again, like an airplane doing a loop.
+When the result of evaluating the true-or-false-test is false, the
+Lisp interpreter does not evaluate the rest of the @code{while}
+expression and `exits the loop'.
+
+Clearly, if the value returned by evaluating the first argument to
address@hidden is always true, the body following will be evaluated
+again and again @dots{} and again @dots{} forever. Conversely, if the
+value returned is never true, the expressions in the body will never
+be evaluated. The craft of writing a @code{while} loop consists of
+choosing a mechanism such that the true-or-false-test returns true
+just the number of times that you want the subsequent expressions to
+be evaluated, and then have the test return false.
+
+The value returned by evaluating a @code{while} is the value of the
+true-or-false-test. An interesting consequence of this is that a
address@hidden loop that evaluates without error will return @code{nil}
+or false regardless of whether it has looped 1 or 100 times or none at
+all. A @code{while} expression that evaluates successfully never
+returns a true value! What this means is that @code{while} is always
+evaluated for its side effects, which is to say, the consequences of
+evaluating the expressions within the body of the @code{while} loop.
+This makes sense. It is not the mere act of looping that is desired,
+but the consequences of what happens when the expressions in the loop
+are repeatedly evaluated.
+
address@hidden Loop Example, print-elements-of-list, Looping with while, while
address@hidden node-name, next, previous, up
address@hidden A @code{while} Loop and a List
+
+A common way to control a @code{while} loop is to test whether a list
+has any elements. If it does, the loop is repeated; but if it does not,
+the repetition is ended. Since this is an important technique, we will
+create a short example to illustrate it.
+
+A simple way to test whether a list has elements is to evaluate the
+list: if it has no elements, it is an empty list and will return the
+empty list, @code{()}, which is a synonym for @code{nil} or false. On
+the other hand, a list with elements will return those elements when it
+is evaluated. Since Emacs Lisp considers as true any value that is not
address@hidden, a list that returns elements will test true in a
address@hidden loop.
+
address@hidden 1200
+For example, you can set the variable @code{empty-list} to @code{nil} by
+evaluating the following @code{setq} expression:
+
address@hidden
+(setq empty-list ())
address@hidden smallexample
+
address@hidden
+After evaluating the @code{setq} expression, you can evaluate the
+variable @code{empty-list} in the usual way, by placing the cursor after
+the symbol and typing @kbd{C-x C-e}; @code{nil} will appear in your
+echo area:
+
address@hidden
+empty-list
address@hidden smallexample
+
+On the other hand, if you set a variable to be a list with elements, the
+list will appear when you evaluate the variable, as you can see by
+evaluating the following two expressions:
+
address@hidden
address@hidden
+(setq animals '(gazelle giraffe lion tiger))
+
+animals
address@hidden group
address@hidden smallexample
+
+Thus, to create a @code{while} loop that tests whether there are any
+items in the list @code{animals}, the first part of the loop will be
+written like this:
+
address@hidden
address@hidden
+(while animals
+ @dots{}
address@hidden group
address@hidden smallexample
+
address@hidden
+When the @code{while} tests its first argument, the variable
address@hidden is evaluated. It returns a list. So long as the list
+has elements, the @code{while} considers the results of the test to be
+true; but when the list is empty, it considers the results of the test
+to be false.
+
+To prevent the @code{while} loop from running forever, some mechanism
+needs to be provided to empty the list eventually. An oft-used
+technique is to have one of the subsequent forms in the @code{while}
+expression set the value of the list to be the @sc{cdr} of the list.
+Each time the @code{cdr} function is evaluated, the list will be made
+shorter, until eventually only the empty list will be left. At this
+point, the test of the @code{while} loop will return false, and the
+arguments to the @code{while} will no longer be evaluated.
+
+For example, the list of animals bound to the variable @code{animals}
+can be set to be the @sc{cdr} of the original list with the
+following expression:
+
address@hidden
+(setq animals (cdr animals))
address@hidden smallexample
+
address@hidden
+If you have evaluated the previous expressions and then evaluate this
+expression, you will see @code{(giraffe lion tiger)} appear in the echo
+area. If you evaluate the expression again, @code{(lion tiger)} will
+appear in the echo area. If you evaluate it again and yet again,
address@hidden(tiger)} appears and then the empty list, shown by @code{nil}.
+
+A template for a @code{while} loop that uses the @code{cdr} function
+repeatedly to cause the true-or-false-test eventually to test false
+looks like this:
+
address@hidden
address@hidden
+(while @var{test-whether-list-is-empty}
+ @address@hidden
+ @var{set-list-to-cdr-of-list})
address@hidden group
address@hidden smallexample
+
+This test and use of @code{cdr} can be put together in a function that
+goes through a list and prints each element of the list on a line of its
+own.
+
address@hidden print-elements-of-list, Incrementing Loop, Loop Example, while
address@hidden An Example: @code{print-elements-of-list}
address@hidden print-elements-of-list
+
+The @code{print-elements-of-list} function illustrates a @code{while}
+loop with a list.
+
address@hidden @file{*scratch*} buffer
+The function requires several lines for its output. If you are
+reading this in a recent instance of GNU Emacs,
address@hidden GNU Emacs 21, GNU Emacs 22, or a later version,
+you can evaluate the following expression inside of Info, as usual.
+
+If you are using an earlier version of Emacs, you need to copy the
+necessary expressions to your @file{*scratch*} buffer and evaluate
+them there. This is because the echo area had only one line in the
+earlier versions.
+
+You can copy the expressions by marking the beginning of the region
+with @address@hidden (@code{set-mark-command}), moving the cursor to
+the end of the region and then copying the region using @kbd{M-w}
+(@code{kill-ring-save}, which calls @code{copy-region-as-kill} and
+then provides visual feedback). In the @file{*scratch*}
+buffer, you can yank the expressions back by typing @kbd{C-y}
+(@code{yank}).
+
+After you have copied the expressions to the @file{*scratch*} buffer,
+evaluate each expression in turn. Be sure to evaluate the last
+expression, @code{(print-elements-of-list animals)}, by typing
address@hidden C-x C-e}, that is, by giving an argument to
address@hidden This will cause the result of the evaluation
+to be printed in the @file{*scratch*} buffer instead of being printed
+in the echo area. (Otherwise you will see something like this in your
+echo area: @code{^Jgazelle^J^Jgiraffe^J^Jlion^J^Jtiger^Jnil}, in which
+each @samp{^J} stands for a `newline'.)
+
address@hidden 1500
+In a recent instance of GNU Emacs, you can evaluate these expressions
+directly in the Info buffer, and the echo area will grow to show the
+results.
+
address@hidden
address@hidden
+(setq animals '(gazelle giraffe lion tiger))
+
+(defun print-elements-of-list (list)
+ "Print each element of LIST on a line of its own."
+ (while list
+ (print (car list))
+ (setq list (cdr list))))
+
+(print-elements-of-list animals)
address@hidden group
address@hidden smallexample
+
address@hidden 1200
address@hidden
+When you evaluate the three expressions in sequence, you will see
+this:
+
address@hidden
address@hidden
+gazelle
+
+giraffe
+
+lion
+
+tiger
+nil
address@hidden group
address@hidden smallexample
+
+Each element of the list is printed on a line of its own (that is what
+the function @code{print} does) and then the value returned by the
+function is printed. Since the last expression in the function is the
address@hidden loop, and since @code{while} loops always return
address@hidden, a @code{nil} is printed after the last element of the list.
+
address@hidden Incrementing Loop, Incrementing Loop Details,
print-elements-of-list, while
address@hidden node-name, next, previous, up
address@hidden A Loop with an Incrementing Counter
+
+A loop is not useful unless it stops when it ought. Besides
+controlling a loop with a list, a common way of stopping a loop is to
+write the first argument as a test that returns false when the correct
+number of repetitions are complete. This means that the loop must
+have a counter---an expression that counts how many times the loop
+repeats itself.
+
address@hidden Incrementing Loop Details, Decrementing Loop, Incrementing Loop,
while
address@hidden
address@hidden Details of an Incrementing Loop
address@hidden ifnottex
+
+The test for a loop with an incrementing counter can be an expression
+such as @code{(< count desired-number)} which returns @code{t} for
+true if the value of @code{count} is less than the
address@hidden of repetitions and @code{nil} for false if the
+value of @code{count} is equal to or is greater than the
address@hidden The expression that increments the count can
+be a simple @code{setq} such as @code{(setq count (1+ count))}, where
address@hidden is a built-in function in Emacs Lisp that adds 1 to its
+argument. (The expression @address@hidden(1+ count)}} has the same result
+as @address@hidden(+ count 1)}}, but is easier for a human to read.)
+
address@hidden 1250
+The template for a @code{while} loop controlled by an incrementing
+counter looks like this:
+
address@hidden
address@hidden
address@hidden
+(while (< count desired-number) ; @r{true-or-false-test}
+ @address@hidden
+ (setq count (1+ count))) ; @r{incrementer}
address@hidden group
address@hidden smallexample
+
address@hidden
+Note that you need to set the initial value of @code{count}; usually it
+is set to 1.
+
address@hidden
+* Incrementing Example:: Counting pebbles in a triangle.
+* Inc Example parts:: The parts of the function definition.
+* Inc Example altogether:: Putting the function definition together.
address@hidden menu
+
address@hidden Incrementing Example, Inc Example parts, Incrementing Loop
Details, Incrementing Loop Details
address@hidden Example with incrementing counter
+
+Suppose you are playing on the beach and decide to make a triangle of
+pebbles, putting one pebble in the first row, two in the second row,
+three in the third row and so on, like this:
+
address@hidden 1
address@hidden pebble diagram
address@hidden
address@hidden
address@hidden
+ *
+ * *
+ * * *
+ * * * *
address@hidden group
address@hidden smallexample
address@hidden ifnottex
address@hidden
address@hidden
address@hidden
+ @bullet{}
+ @bullet{} @bullet{}
+ @bullet{} @bullet{} @bullet{}
+ @bullet{} @bullet{} @bullet{} @bullet{}
address@hidden group
address@hidden smallexample
address@hidden iftex
address@hidden 1
+
address@hidden
+(About 2500 years ago, Pythagoras and others developed the beginnings of
+number theory by considering questions such as this.)
+
+Suppose you want to know how many pebbles you will need to make a
+triangle with 7 rows?
+
+Clearly, what you need to do is add up the numbers from 1 to 7. There
+are two ways to do this; start with the smallest number, one, and add up
+the list in sequence, 1, 2, 3, 4 and so on; or start with the largest
+number and add the list going down: 7, 6, 5, 4 and so on. Because both
+mechanisms illustrate common ways of writing @code{while} loops, we will
+create two examples, one counting up and the other counting down. In
+this first example, we will start with 1 and add 2, 3, 4 and so on.
+
+If you are just adding up a short list of numbers, the easiest way to do
+it is to add up all the numbers at once. However, if you do not know
+ahead of time how many numbers your list will have, or if you want to be
+prepared for a very long list, then you need to design your addition so
+that what you do is repeat a simple process many times instead of doing
+a more complex process once.
+
+For example, instead of adding up all the pebbles all at once, what you
+can do is add the number of pebbles in the first row, 1, to the number
+in the second row, 2, and then add the total of those two rows to the
+third row, 3. Then you can add the number in the fourth row, 4, to the
+total of the first three rows; and so on.
+
+The critical characteristic of the process is that each repetitive
+action is simple. In this case, at each step we add only two numbers,
+the number of pebbles in the row and the total already found. This
+process of adding two numbers is repeated again and again until the last
+row has been added to the total of all the preceding rows. In a more
+complex loop the repetitive action might not be so simple, but it will
+be simpler than doing everything all at once.
+
address@hidden Inc Example parts, Inc Example altogether, Incrementing Example,
Incrementing Loop Details
address@hidden The parts of the function definition
+
+The preceding analysis gives us the bones of our function definition:
+first, we will need a variable that we can call @code{total} that will
+be the total number of pebbles. This will be the value returned by
+the function.
+
+Second, we know that the function will require an argument: this
+argument will be the total number of rows in the triangle. It can be
+called @code{number-of-rows}.
+
+Finally, we need a variable to use as a counter. We could call this
+variable @code{counter}, but a better name is @code{row-number}. That
+is because what the counter does in this function is count rows, and a
+program should be written to be as understandable as possible.
+
+When the Lisp interpreter first starts evaluating the expressions in the
+function, the value of @code{total} should be set to zero, since we have
+not added anything to it. Then the function should add the number of
+pebbles in the first row to the total, and then add the number of
+pebbles in the second to the total, and then add the number of
+pebbles in the third row to the total, and so on, until there are no
+more rows left to add.
+
+Both @code{total} and @code{row-number} are used only inside the
+function, so they can be declared as local variables with @code{let}
+and given initial values. Clearly, the initial value for @code{total}
+should be 0. The initial value of @code{row-number} should be 1,
+since we start with the first row. This means that the @code{let}
+statement will look like this:
+
address@hidden
address@hidden
+ (let ((total 0)
+ (row-number 1))
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+After the internal variables are declared and bound to their initial
+values, we can begin the @code{while} loop. The expression that serves
+as the test should return a value of @code{t} for true so long as the
address@hidden is less than or equal to the @code{number-of-rows}.
+(If the expression tests true only so long as the row number is less
+than the number of rows in the triangle, the last row will never be
+added to the total; hence the row number has to be either less than or
+equal to the number of rows.)
+
address@hidden 1500
address@hidden <= @r{(less than or equal)}
+Lisp provides the @code{<=} function that returns true if the value of
+its first argument is less than or equal to the value of its second
+argument and false otherwise. So the expression that the @code{while}
+will evaluate as its test should look like this:
+
address@hidden
+(<= row-number number-of-rows)
address@hidden smallexample
+
+The total number of pebbles can be found by repeatedly adding the number
+of pebbles in a row to the total already found. Since the number of
+pebbles in the row is equal to the row number, the total can be found by
+adding the row number to the total. (Clearly, in a more complex
+situation, the number of pebbles in the row might be related to the row
+number in a more complicated way; if this were the case, the row number
+would be replaced by the appropriate expression.)
+
address@hidden
+(setq total (+ total row-number))
address@hidden smallexample
+
address@hidden
+What this does is set the new value of @code{total} to be equal to the
+sum of adding the number of pebbles in the row to the previous total.
+
+After setting the value of @code{total}, the conditions need to be
+established for the next repetition of the loop, if there is one. This
+is done by incrementing the value of the @code{row-number} variable,
+which serves as a counter. After the @code{row-number} variable has
+been incremented, the true-or-false-test at the beginning of the
address@hidden loop tests whether its value is still less than or equal to
+the value of the @code{number-of-rows} and if it is, adds the new value
+of the @code{row-number} variable to the @code{total} of the previous
+repetition of the loop.
+
address@hidden 1200
+The built-in Emacs Lisp function @code{1+} adds 1 to a number, so the
address@hidden variable can be incremented with this expression:
+
address@hidden
+(setq row-number (1+ row-number))
address@hidden smallexample
+
address@hidden Inc Example altogether, , Inc Example parts, Incrementing Loop
Details
address@hidden Putting the function definition together
+
+We have created the parts for the function definition; now we need to
+put them together.
+
address@hidden 800
+First, the contents of the @code{while} expression:
+
address@hidden
address@hidden
+(while (<= row-number number-of-rows) ; @r{true-or-false-test}
+ (setq total (+ total row-number))
+ (setq row-number (1+ row-number))) ; @r{incrementer}
address@hidden group
address@hidden smallexample
+
+Along with the @code{let} expression varlist, this very nearly
+completes the body of the function definition. However, it requires
+one final element, the need for which is somewhat subtle.
+
+The final touch is to place the variable @code{total} on a line by
+itself after the @code{while} expression. Otherwise, the value returned
+by the whole function is the value of the last expression that is
+evaluated in the body of the @code{let}, and this is the value
+returned by the @code{while}, which is always @code{nil}.
+
+This may not be evident at first sight. It almost looks as if the
+incrementing expression is the last expression of the whole function.
+But that expression is part of the body of the @code{while}; it is the
+last element of the list that starts with the symbol @code{while}.
+Moreover, the whole of the @code{while} loop is a list within the body
+of the @code{let}.
+
address@hidden 1250
+In outline, the function will look like this:
+
address@hidden
address@hidden
+(defun @var{name-of-function} (@var{argument-list})
+ "@address@hidden"
+ (let (@var{varlist})
+ (while (@var{true-or-false-test})
+ @address@hidden )
+ @dots{} )) ; @r{Need final expression here.}
address@hidden group
address@hidden smallexample
+
+The result of evaluating the @code{let} is what is going to be returned
+by the @code{defun} since the @code{let} is not embedded within any
+containing list, except for the @code{defun} as a whole. However, if
+the @code{while} is the last element of the @code{let} expression, the
+function will always return @code{nil}. This is not what we want!
+Instead, what we want is the value of the variable @code{total}. This
+is returned by simply placing the symbol as the last element of the list
+starting with @code{let}. It gets evaluated after the preceding
+elements of the list are evaluated, which means it gets evaluated after
+it has been assigned the correct value for the total.
+
+It may be easier to see this by printing the list starting with
address@hidden all on one line. This format makes it evident that the
address@hidden and @code{while} expressions are the second and third
+elements of the list starting with @code{let}, and the @code{total} is
+the last element:
+
address@hidden
address@hidden
+(let (@var{varlist}) (while (@var{true-or-false-test}) @address@hidden ) total)
address@hidden group
address@hidden smallexample
+
address@hidden 1200
+Putting everything together, the @code{triangle} function definition
+looks like this:
+
address@hidden
address@hidden
+(defun triangle (number-of-rows) ; @r{Version with}
+ ; @r{ incrementing counter.}
+ "Add up the number of pebbles in a triangle.
+The first row has one pebble, the second row two pebbles,
+the third row three pebbles, and so on.
+The argument is NUMBER-OF-ROWS."
address@hidden group
address@hidden
+ (let ((total 0)
+ (row-number 1))
+ (while (<= row-number number-of-rows)
+ (setq total (+ total row-number))
+ (setq row-number (1+ row-number)))
+ total))
address@hidden group
address@hidden smallexample
+
address@hidden 1200
+After you have installed @code{triangle} by evaluating the function, you
+can try it out. Here are two examples:
+
address@hidden
address@hidden
+(triangle 4)
+
+(triangle 7)
address@hidden group
address@hidden smallexample
+
address@hidden
+The sum of the first four numbers is 10 and the sum of the first seven
+numbers is 28.
+
address@hidden Decrementing Loop, , Incrementing Loop Details, while
address@hidden node-name, next, previous, up
address@hidden Loop with a Decrementing Counter
+
+Another common way to write a @code{while} loop is to write the test
+so that it determines whether a counter is greater than zero. So long
+as the counter is greater than zero, the loop is repeated. But when
+the counter is equal to or less than zero, the loop is stopped. For
+this to work, the counter has to start out greater than zero and then
+be made smaller and smaller by a form that is evaluated
+repeatedly.
+
+The test will be an expression such as @code{(> counter 0)} which
+returns @code{t} for true if the value of @code{counter} is greater
+than zero, and @code{nil} for false if the value of @code{counter} is
+equal to or less than zero. The expression that makes the number
+smaller and smaller can be a simple @code{setq} such as @code{(setq
+counter (1- counter))}, where @code{1-} is a built-in function in
+Emacs Lisp that subtracts 1 from its argument.
+
address@hidden 1250
+The template for a decrementing @code{while} loop looks like this:
+
address@hidden
address@hidden
+(while (> counter 0) ; @r{true-or-false-test}
+ @address@hidden
+ (setq counter (1- counter))) ; @r{decrementer}
address@hidden group
address@hidden smallexample
+
address@hidden
+* Decrementing Example:: More pebbles on the beach.
+* Dec Example parts:: The parts of the function definition.
+* Dec Example altogether:: Putting the function definition together.
address@hidden menu
+
address@hidden Decrementing Example, Dec Example parts, Decrementing Loop,
Decrementing Loop
address@hidden Example with decrementing counter
+
+To illustrate a loop with a decrementing counter, we will rewrite the
address@hidden function so the counter decreases to zero.
+
+This is the reverse of the earlier version of the function. In this
+case, to find out how many pebbles are needed to make a triangle with
+3 rows, add the number of pebbles in the third row, 3, to the number
+in the preceding row, 2, and then add the total of those two rows to
+the row that precedes them, which is 1.
+
+Likewise, to find the number of pebbles in a triangle with 7 rows, add
+the number of pebbles in the seventh row, 7, to the number in the
+preceding row, which is 6, and then add the total of those two rows to
+the row that precedes them, which is 5, and so on. As in the previous
+example, each addition only involves adding two numbers, the total of
+the rows already added up and the number of pebbles in the row that is
+being added to the total. This process of adding two numbers is
+repeated again and again until there are no more pebbles to add.
+
+We know how many pebbles to start with: the number of pebbles in the
+last row is equal to the number of rows. If the triangle has seven
+rows, the number of pebbles in the last row is 7. Likewise, we know how
+many pebbles are in the preceding row: it is one less than the number in
+the row.
+
address@hidden Dec Example parts, Dec Example altogether, Decrementing Example,
Decrementing Loop
address@hidden The parts of the function definition
+
+We start with three variables: the total number of rows in the
+triangle; the number of pebbles in a row; and the total number of
+pebbles, which is what we want to calculate. These variables can be
+named @code{number-of-rows}, @code{number-of-pebbles-in-row}, and
address@hidden, respectively.
+
+Both @code{total} and @code{number-of-pebbles-in-row} are used only
+inside the function and are declared with @code{let}. The initial
+value of @code{total} should, of course, be zero. However, the
+initial value of @code{number-of-pebbles-in-row} should be equal to
+the number of rows in the triangle, since the addition will start with
+the longest row.
+
address@hidden 1250
+This means that the beginning of the @code{let} expression will look
+like this:
+
address@hidden
address@hidden
+(let ((total 0)
+ (number-of-pebbles-in-row number-of-rows))
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+The total number of pebbles can be found by repeatedly adding the number
+of pebbles in a row to the total already found, that is, by repeatedly
+evaluating the following expression:
+
address@hidden
+(setq total (+ total number-of-pebbles-in-row))
address@hidden smallexample
+
address@hidden
+After the @code{number-of-pebbles-in-row} is added to the @code{total},
+the @code{number-of-pebbles-in-row} should be decremented by one, since
+the next time the loop repeats, the preceding row will be
+added to the total.
+
+The number of pebbles in a preceding row is one less than the number of
+pebbles in a row, so the built-in Emacs Lisp function @code{1-} can be
+used to compute the number of pebbles in the preceding row. This can be
+done with the following expression:
+
address@hidden
address@hidden
+(setq number-of-pebbles-in-row
+ (1- number-of-pebbles-in-row))
address@hidden group
address@hidden smallexample
+
+Finally, we know that the @code{while} loop should stop making repeated
+additions when there are no pebbles in a row. So the test for
+the @code{while} loop is simply:
+
address@hidden
+(while (> number-of-pebbles-in-row 0)
address@hidden smallexample
+
address@hidden Dec Example altogether, , Dec Example parts, Decrementing Loop
address@hidden Putting the function definition together
+
+We can put these expressions together to create a function definition
+that works. However, on examination, we find that one of the local
+variables is unneeded!
+
address@hidden 1250
+The function definition looks like this:
+
address@hidden
address@hidden
+;;; @r{First subtractive version.}
+(defun triangle (number-of-rows)
+ "Add up the number of pebbles in a triangle."
+ (let ((total 0)
+ (number-of-pebbles-in-row number-of-rows))
+ (while (> number-of-pebbles-in-row 0)
+ (setq total (+ total number-of-pebbles-in-row))
+ (setq number-of-pebbles-in-row
+ (1- number-of-pebbles-in-row)))
+ total))
address@hidden group
address@hidden smallexample
+
+As written, this function works.
+
+However, we do not need @code{number-of-pebbles-in-row}.
+
address@hidden Argument as local variable
+When the @code{triangle} function is evaluated, the symbol
address@hidden will be bound to a number, giving it an initial
+value. That number can be changed in the body of the function as if
+it were a local variable, without any fear that such a change will
+effect the value of the variable outside of the function. This is a
+very useful characteristic of Lisp; it means that the variable
address@hidden can be used anywhere in the function where
address@hidden is used.
+
address@hidden 800
+Here is a second version of the function written a bit more cleanly:
+
address@hidden
address@hidden
+(defun triangle (number) ; @r{Second version.}
+ "Return sum of numbers 1 through NUMBER inclusive."
+ (let ((total 0))
+ (while (> number 0)
+ (setq total (+ total number))
+ (setq number (1- number)))
+ total))
address@hidden group
address@hidden smallexample
+
+In brief, a properly written @code{while} loop will consist of three parts:
+
address@hidden
address@hidden
+A test that will return false after the loop has repeated itself the
+correct number of times.
+
address@hidden
+An expression the evaluation of which will return the value desired
+after being repeatedly evaluated.
+
address@hidden
+An expression to change the value passed to the true-or-false-test so
+that the test returns false after the loop has repeated itself the right
+number of times.
address@hidden enumerate
+
address@hidden dolist dotimes, Recursion, while, Loops & Recursion
address@hidden node-name, next, previous, up
address@hidden Save your time: @code{dolist} and @code{dotimes}
+
+In addition to @code{while}, both @code{dolist} and @code{dotimes}
+provide for looping. Sometimes these are quicker to write than the
+equivalent @code{while} loop. Both are Lisp macros. (@xref{Macros, ,
+Macros, elisp, The GNU Emacs Lisp Reference Manual}. )
+
address@hidden works like a @code{while} loop that address@hidden down a
+list': @code{dolist} automatically shortens the list each time it
+loops---takes the @sc{cdr} of the list---and binds the @sc{car} of
+each shorter version of the list to the first of its arguments.
+
address@hidden loops a specific number of times: you specify the number.
+
address@hidden
+* dolist::
+* dotimes::
address@hidden menu
+
address@hidden dolist, dotimes, dolist dotimes, dolist dotimes
address@hidden The @code{dolist} Macro
address@hidden dolist
+
+Suppose, for example, you want to reverse a list, so that
+``first'' ``second'' ``third'' becomes ``third'' ``second'' ``first''.
+
address@hidden 1250
+In practice, you would use the @code{reverse} function, like this:
+
address@hidden
address@hidden
+(setq animals '(gazelle giraffe lion tiger))
+
+(reverse animals)
address@hidden group
address@hidden smallexample
+
address@hidden 800
address@hidden
+Here is how you could reverse the list using a @code{while} loop:
+
address@hidden
address@hidden
+(setq animals '(gazelle giraffe lion tiger))
+
+(defun reverse-list-with-while (list)
+ "Using while, reverse the order of LIST."
+ (let (value) ; make sure list starts empty
+ (while list
+ (setq value (cons (car list) value))
+ (setq list (cdr list)))
+ value))
+
+(reverse-list-with-while animals)
address@hidden group
address@hidden smallexample
+
address@hidden 800
address@hidden
+And here is how you could use the @code{dolist} macro:
+
address@hidden
address@hidden
+(setq animals '(gazelle giraffe lion tiger))
+
+(defun reverse-list-with-dolist (list)
+ "Using dolist, reverse the order of LIST."
+ (let (value) ; make sure list starts empty
+ (dolist (element list value)
+ (setq value (cons element value)))))
+
+(reverse-list-with-dolist animals)
address@hidden group
address@hidden smallexample
+
address@hidden 1250
address@hidden
+In Info, you can place your cursor after the closing parenthesis of
+each expression and type @kbd{C-x C-e}; in each case, you should see
+
address@hidden
+(tiger lion giraffe gazelle)
address@hidden smallexample
+
address@hidden
+in the echo area.
+
+For this example, the existing @code{reverse} function is obviously best.
+The @code{while} loop is just like our first example (@pxref{Loop
+Example, , A @code{while} Loop and a List}). The @code{while} first
+checks whether the list has elements; if so, it constructs a new list
+by adding the first element of the list to the existing list (which in
+the first iteration of the loop is @code{nil}). Since the second
+element is prepended in front of the first element, and the third
+element is prepended in front of the second element, the list is reversed.
+
+In the expression using a @code{while} loop,
+the @address@hidden(setq list (cdr list))}}
+expression shortens the list, so the @code{while} loop eventually
+stops. In addition, it provides the @code{cons} expression with a new
+first element by creating a new and shorter list at each repetition of
+the loop.
+
+The @code{dolist} expression does very much the same as the
address@hidden expression, except that the @code{dolist} macro does some
+of the work you have to do when writing a @code{while} expression.
+
+Like a @code{while} loop, a @code{dolist} loops. What is different is
+that it automatically shortens the list each time it loops --- it
address@hidden down the list' on its own --- and it automatically binds
+the @sc{car} of each shorter version of the list to the first of its
+arguments.
+
+In the example, the @sc{car} of each shorter version of the list is
+referred to using the symbol @samp{element}, the list itself is called
address@hidden, and the value returned is called @samp{value}. The
+remainder of the @code{dolist} expression is the body.
+
+The @code{dolist} expression binds the @sc{car} of each shorter
+version of the list to @code{element} and then evaluates the body of
+the expression; and repeats the loop. The result is returned in
address@hidden
+
address@hidden dotimes, , dolist, dolist dotimes
address@hidden The @code{dotimes} Macro
address@hidden dotimes
+
+The @code{dotimes} macro is similar to @code{dolist}, except that it
+loops a specific number of times.
+
+The first argument to @code{dotimes} is assigned the numbers 0, 1, 2
+and so forth each time around the loop, and the value of the third
+argument is returned. You need to provide the value of the second
+argument, which is how many times the macro loops.
+
address@hidden 1250
+For example, the following binds the numbers from 0 up to, but not
+including, the number 3 to the first argument, @var{number}, and then
+constructs a list of the three numbers. (The first number is 0, the
+second number is 1, and the third number is 2; this makes a total of
+three numbers in all, starting with zero as the first number.)
+
address@hidden
address@hidden
+(let (value) ; otherwise a value is a void variable
+ (dotimes (number 3 value)
+ (setq value (cons number value))))
+
address@hidden (2 1 0)
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden returns @code{value}, so the way to use
address@hidden is to operate on some expression @var{number} number of
+times and then return the result, either as a list or an atom.
+
address@hidden 1250
+Here is an example of a @code{defun} that uses @code{dotimes} to add
+up the number of pebbles in a triangle.
+
address@hidden
address@hidden
+(defun triangle-using-dotimes (number-of-rows)
+ "Using dotimes, add up the number of pebbles in a triangle."
+(let ((total 0)) ; otherwise a total is a void variable
+ (dotimes (number number-of-rows total)
+ (setq total (+ total (1+ number))))))
+
+(triangle-using-dotimes 4)
address@hidden group
address@hidden smallexample
+
address@hidden Recursion, Looping exercise, dolist dotimes, Loops & Recursion
address@hidden node-name, next, previous, up
address@hidden Recursion
address@hidden Recursion
+
+A recursive function contains code that tells the Lisp interpreter to
+call a program that runs exactly like itself, but with slightly
+different arguments. The code runs exactly the same because it has
+the same name. However, even though the program has the same name, it
+is not the same entity. It is different. In the jargon, it is a
+different `instance'.
+
+Eventually, if the program is written correctly, the `slightly
+different arguments' will become sufficiently different from the first
+arguments that the final instance will stop.
+
address@hidden
+* Building Robots:: Same model, different serial number ...
+* Recursive Definition Parts:: Walk until you stop ...
+* Recursion with list:: Using a list as the test whether to recurse.
+* Recursive triangle function::
+* Recursion with cond::
+* Recursive Patterns:: Often used templates.
+* No Deferment:: Don't store up work ...
+* No deferment solution::
address@hidden menu
+
address@hidden Building Robots, Recursive Definition Parts, Recursion, Recursion
address@hidden node-name, next, previous, up
address@hidden Building Robots: Extending the Metaphor
address@hidden Building robots
address@hidden Robots, building
+
+It is sometimes helpful to think of a running program as a robot that
+does a job. In doing its job, a recursive function calls on a second
+robot to help it. The second robot is identical to the first in every
+way, except that the second robot helps the first and has been
+passed different arguments than the first.
+
+In a recursive function, the second robot may call a third; and the
+third may call a fourth, and so on. Each of these is a different
+entity; but all are clones.
+
+Since each robot has slightly different instructions---the arguments
+will differ from one robot to the next---the last robot should know
+when to stop.
+
+Let's expand on the metaphor in which a computer program is a robot.
+
+A function definition provides the blueprints for a robot. When you
+install a function definition, that is, when you evaluate a
address@hidden special form, you install the necessary equipment to
+build robots. It is as if you were in a factory, setting up an
+assembly line. Robots with the same name are built according to the
+same blueprints. So they have, as it were, the same `model number',
+but a different `serial number'.
+
+We often say that a recursive function `calls itself'. What we mean
+is that the instructions in a recursive function cause the Lisp
+interpreter to run a different function that has the same name and
+does the same job as the first, but with different arguments.
+
+It is important that the arguments differ from one instance to the
+next; otherwise, the process will never stop.
+
address@hidden Recursive Definition Parts, Recursion with list, Building
Robots, Recursion
address@hidden node-name, next, previous, up
address@hidden The Parts of a Recursive Definition
address@hidden Parts of a Recursive Definition
address@hidden Recursive Definition Parts
+
+A recursive function typically contains a conditional expression which
+has three parts:
+
address@hidden
address@hidden
+A true-or-false-test that determines whether the function is called
+again, here called the @dfn{do-again-test}.
+
address@hidden
+The name of the function. When this name is called, a new instance of
+the function---a new robot, as it were---is created and told what to do.
+
address@hidden
+An expression that returns a different value each time the function is
+called, here called the @dfn{next-step-expression}. Consequently, the
+argument (or arguments) passed to the new instance of the function
+will be different from that passed to the previous instance. This
+causes the conditional expression, the @dfn{do-again-test}, to test
+false after the correct number of repetitions.
address@hidden enumerate
+
+Recursive functions can be much simpler than any other kind of
+function. Indeed, when people first start to use them, they often look
+so mysteriously simple as to be incomprehensible. Like riding a
+bicycle, reading a recursive function definition takes a certain knack
+which is hard at first but then seems simple.
+
address@hidden 1200
+There are several different common recursive patterns. A very simple
+pattern looks like this:
+
address@hidden
address@hidden
+(defun @var{name-of-recursive-function} (@var{argument-list})
+ "@address@hidden"
+ (if @var{do-again-test}
+ @address@hidden
+ (@var{name-of-recursive-function}
+ @var{next-step-expression})))
address@hidden group
address@hidden smallexample
+
+Each time a recursive function is evaluated, a new instance of it is
+created and told what to do. The arguments tell the instance what to do.
+
+An argument is bound to the value of the next-step-expression. Each
+instance runs with a different value of the next-step-expression.
+
+The value in the next-step-expression is used in the do-again-test.
+
+The value returned by the next-step-expression is passed to the new
+instance of the function, which evaluates it (or some
+transmogrification of it) to determine whether to continue or stop.
+The next-step-expression is designed so that the do-again-test returns
+false when the function should no longer be repeated.
+
+The do-again-test is sometimes called the @dfn{stop condition},
+since it stops the repetitions when it tests false.
+
address@hidden Recursion with list, Recursive triangle function, Recursive
Definition Parts, Recursion
address@hidden node-name, next, previous, up
address@hidden Recursion with a List
+
+The example of a @code{while} loop that printed the elements of a list
+of numbers can be written recursively. Here is the code, including
+an expression to set the value of the variable @code{animals} to a list.
+
+If you are using GNU Emacs 20 or before, this example must be copied
+to the @file{*scratch*} buffer and each expression must be evaluated
+there. Use @kbd{C-u C-x C-e} to evaluate the
address@hidden(print-elements-recursively animals)} expression so that the
+results are printed in the buffer; otherwise the Lisp interpreter will
+try to squeeze the results into the one line of the echo area.
+
+Also, place your cursor immediately after the last closing parenthesis
+of the @code{print-elements-recursively} function, before the comment.
+Otherwise, the Lisp interpreter will try to evaluate the comment.
+
+If you are using a more recent version of Emacs, you can evaluate this
+expression directly in Info.
+
address@hidden print-elements-recursively
address@hidden
address@hidden
+(setq animals '(gazelle giraffe lion tiger))
+
+(defun print-elements-recursively (list)
+ "Print each element of LIST on a line of its own.
+Uses recursion."
+ (when list ; @r{do-again-test}
+ (print (car list)) ; @r{body}
+ (print-elements-recursively ; @r{recursive call}
+ (cdr list)))) ; @r{next-step-expression}
+
+(print-elements-recursively animals)
address@hidden group
address@hidden smallexample
+
+The @code{print-elements-recursively} function first tests whether
+there is any content in the list; if there is, the function prints the
+first element of the list, the @sc{car} of the list. Then the
+function `invokes itself', but gives itself as its argument, not the
+whole list, but the second and subsequent elements of the list, the
address@hidden of the list.
+
+Put another way, if the list is not empty, the function invokes
+another instance of code that is similar to the initial code, but is a
+different thread of execution, with different arguments than the first
+instance.
+
+Put in yet another way, if the list is not empty, the first robot
+assemblies a second robot and tells it what to do; the second robot is
+a different individual from the first, but is the same model.
+
+When the second evaluation occurs, the @code{when} expression is
+evaluated and if true, prints the first element of the list it
+receives as its argument (which is the second element of the original
+list). Then the function `calls itself' with the @sc{cdr} of the list
+it is invoked with, which (the second time around) is the @sc{cdr} of
+the @sc{cdr} of the original list.
+
+Note that although we say that the function `calls itself', what we
+mean is that the Lisp interpreter assembles and instructs a new
+instance of the program. The new instance is a clone of the first,
+but is a separate individual.
+
+Each time the function `invokes itself', it invokes itself on a
+shorter version of the original list. It creates a new instance that
+works on a shorter list.
+
+Eventually, the function invokes itself on an empty list. It creates
+a new instance whose argument is @code{nil}. The conditional expression
+tests the value of @code{list}. Since the value of @code{list} is
address@hidden, the @code{when} expression tests false so the then-part is
+not evaluated. The function as a whole then returns @code{nil}.
+
address@hidden 1200
+When you evaluate @code{(print-elements-recursively animals)} in the
address@hidden buffer, you see this result:
+
address@hidden
address@hidden
+gazelle
+
+giraffe
+
+lion
+
+tiger
+nil
address@hidden group
address@hidden smallexample
+
address@hidden 2000
address@hidden Recursive triangle function, Recursion with cond, Recursion with
list, Recursion
address@hidden node-name, next, previous, up
address@hidden Recursion in Place of a Counter
address@hidden triangle-recursively
+
address@hidden 1200
+The @code{triangle} function described in a previous section can also
+be written recursively. It looks like this:
+
address@hidden
address@hidden
+(defun triangle-recursively (number)
+ "Return the sum of the numbers 1 through NUMBER inclusive.
+Uses recursion."
+ (if (= number 1) ; @r{do-again-test}
+ 1 ; @r{then-part}
+ (+ number ; @r{else-part}
+ (triangle-recursively ; @r{recursive call}
+ (1- number))))) ; @r{next-step-expression}
+
+(triangle-recursively 7)
address@hidden group
address@hidden smallexample
+
address@hidden
+You can install this function by evaluating it and then try it by
+evaluating @code{(triangle-recursively 7)}. (Remember to put your
+cursor immediately after the last parenthesis of the function
+definition, before the comment.) The function evaluates to 28.
+
+To understand how this function works, let's consider what happens in the
+various cases when the function is passed 1, 2, 3, or 4 as the value of
+its argument.
+
address@hidden
+* Recursive Example arg of 1 or 2::
+* Recursive Example arg of 3 or 4::
address@hidden menu
+
address@hidden Recursive Example arg of 1 or 2, Recursive Example arg of 3 or
4, Recursive triangle function, Recursive triangle function
address@hidden
address@hidden An argument of 1 or 2
address@hidden ifnottex
+
+First, what happens if the value of the argument is 1?
+
+The function has an @code{if} expression after the documentation
+string. It tests whether the value of @code{number} is equal to 1; if
+so, Emacs evaluates the then-part of the @code{if} expression, which
+returns the number 1 as the value of the function. (A triangle with
+one row has one pebble in it.)
+
+Suppose, however, that the value of the argument is 2. In this case,
+Emacs evaluates the else-part of the @code{if} expression.
+
address@hidden 1200
+The else-part consists of an addition, the recursive call to
address@hidden and a decrementing action; and it looks like
+this:
+
address@hidden
+(+ number (triangle-recursively (1- number)))
address@hidden smallexample
+
+When Emacs evaluates this expression, the innermost expression is
+evaluated first; then the other parts in sequence. Here are the steps
+in detail:
+
address@hidden @i
address@hidden Step 1 @w{ } Evaluate the innermost expression.
+
+The innermost expression is @code{(1- number)} so Emacs decrements the
+value of @code{number} from 2 to 1.
+
address@hidden Step 2 @w{ } Evaluate the @code{triangle-recursively} function.
+
+The Lisp interpreter creates an individual instance of
address@hidden It does not matter that this function is
+contained within itself. Emacs passes the result Step 1 as the
+argument used by this instance of the @code{triangle-recursively}
+function
+
+In this case, Emacs evaluates @code{triangle-recursively} with an
+argument of 1. This means that this evaluation of
address@hidden returns 1.
+
address@hidden Step 3 @w{ } Evaluate the value of @code{number}.
+
+The variable @code{number} is the second element of the list that
+starts with @code{+}; its value is 2.
+
address@hidden Step 4 @w{ } Evaluate the @code{+} expression.
+
+The @code{+} expression receives two arguments, the first
+from the evaluation of @code{number} (Step 3) and the second from the
+evaluation of @code{triangle-recursively} (Step 2).
+
+The result of the addition is the sum of 2 plus 1, and the number 3 is
+returned, which is correct. A triangle with two rows has three
+pebbles in it.
address@hidden table
+
address@hidden Recursive Example arg of 3 or 4, , Recursive Example arg of 1
or 2, Recursive triangle function
address@hidden An argument of 3 or 4
+
+Suppose that @code{triangle-recursively} is called with an argument of
+3.
+
address@hidden @i
address@hidden Step 1 @w{ } Evaluate the do-again-test.
+
+The @code{if} expression is evaluated first. This is the do-again
+test and returns false, so the else-part of the @code{if} expression
+is evaluated. (Note that in this example, the do-again-test causes
+the function to call itself when it tests false, not when it tests
+true.)
+
address@hidden Step 2 @w{ } Evaluate the innermost expression of the else-part.
+
+The innermost expression of the else-part is evaluated, which decrements
+3 to 2. This is the next-step-expression.
+
address@hidden Step 3 @w{ } Evaluate the @code{triangle-recursively} function.
+
+The number 2 is passed to the @code{triangle-recursively} function.
+
+We know what happens when Emacs evaluates @code{triangle-recursively} with
+an argument of 2. After going through the sequence of actions described
+earlier, it returns a value of 3. So that is what will happen here.
+
address@hidden Step 4 @w{ } Evaluate the addition.
+
+3 will be passed as an argument to the addition and will be added to the
+number with which the function was called, which is 3.
address@hidden table
+
address@hidden
+The value returned by the function as a whole will be 6.
+
+Now that we know what will happen when @code{triangle-recursively} is
+called with an argument of 3, it is evident what will happen if it is
+called with an argument of 4:
+
address@hidden
address@hidden 800
+In the recursive call, the evaluation of
+
address@hidden
+(triangle-recursively (1- 4))
address@hidden smallexample
+
address@hidden 800
address@hidden
+will return the value of evaluating
+
address@hidden
+(triangle-recursively 3)
address@hidden smallexample
+
address@hidden
+which is 6 and this value will be added to 4 by the addition in the
+third line.
address@hidden quotation
+
address@hidden
+The value returned by the function as a whole will be 10.
+
+Each time @code{triangle-recursively} is evaluated, it evaluates a
+version of itself---a different instance of itself---with a smaller
+argument, until the argument is small enough so that it does not
+evaluate itself.
+
+Note that this particular design for a recursive function
+requires that operations be deferred.
+
+Before @code{(triangle-recursively 7)} can calculate its answer, it
+must call @code{(triangle-recursively 6)}; and before
address@hidden(triangle-recursively 6)} can calculate its answer, it must call
address@hidden(triangle-recursively 5)}; and so on. That is to say, the
+calculation that @code{(triangle-recursively 7)} makes must be
+deferred until @code{(triangle-recursively 6)} makes its calculation;
+and @code{(triangle-recursively 6)} must defer until
address@hidden(triangle-recursively 5)} completes; and so on.
+
+If each of these instances of @code{triangle-recursively} are thought
+of as different robots, the first robot must wait for the second to
+complete its job, which must wait until the third completes, and so
+on.
+
+There is a way around this kind of waiting, which we will discuss in
address@hidden Deferment, , Recursion without Deferments}.
+
address@hidden Recursion with cond, Recursive Patterns, Recursive triangle
function, Recursion
address@hidden node-name, next, previous, up
address@hidden Recursion Example Using @code{cond}
address@hidden cond
+
+The version of @code{triangle-recursively} described earlier is written
+with the @code{if} special form. It can also be written using another
+special form called @code{cond}. The name of the special form
address@hidden is an abbreviation of the word @samp{conditional}.
+
+Although the @code{cond} special form is not used as often in the
+Emacs Lisp sources as @code{if}, it is used often enough to justify
+explaining it.
+
address@hidden 800
+The template for a @code{cond} expression looks like this:
+
address@hidden
address@hidden
+(cond
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
address@hidden
+where the @var{body} is a series of lists.
+
address@hidden 800
+Written out more fully, the template looks like this:
+
address@hidden
address@hidden
+(cond
+ (@var{first-true-or-false-test} @var{first-consequent})
+ (@var{second-true-or-false-test} @var{second-consequent})
+ (@var{third-true-or-false-test} @var{third-consequent})
+ @dots{})
address@hidden group
address@hidden smallexample
+
+When the Lisp interpreter evaluates the @code{cond} expression, it
+evaluates the first element (the @sc{car} or true-or-false-test) of
+the first expression in a series of expressions within the body of the
address@hidden
+
+If the true-or-false-test returns @code{nil} the rest of that
+expression, the consequent, is skipped and the true-or-false-test of the
+next expression is evaluated. When an expression is found whose
+true-or-false-test returns a value that is not @code{nil}, the
+consequent of that expression is evaluated. The consequent can be one
+or more expressions. If the consequent consists of more than one
+expression, the expressions are evaluated in sequence and the value of
+the last one is returned. If the expression does not have a consequent,
+the value of the true-or-false-test is returned.
+
+If none of the true-or-false-tests test true, the @code{cond} expression
+returns @code{nil}.
+
address@hidden 1250
+Written using @code{cond}, the @code{triangle} function looks like this:
+
address@hidden
address@hidden
+(defun triangle-using-cond (number)
+ (cond ((<= number 0) 0)
+ ((= number 1) 1)
+ ((> number 1)
+ (+ number (triangle-using-cond (1- number))))))
address@hidden group
address@hidden smallexample
+
address@hidden
+In this example, the @code{cond} returns 0 if the number is less than or
+equal to 0, it returns 1 if the number is 1 and it evaluates @code{(+
+number (triangle-using-cond (1- number)))} if the number is greater than
+1.
+
address@hidden Recursive Patterns, No Deferment, Recursion with cond, Recursion
address@hidden node-name, next, previous, up
address@hidden Recursive Patterns
address@hidden Recursive Patterns
+
+Here are three common recursive patterns. Each involves a list.
+Recursion does not need to involve lists, but Lisp is designed for lists
+and this provides a sense of its primal capabilities.
+
address@hidden
+* Every::
+* Accumulate::
+* Keep::
address@hidden menu
+
address@hidden Every, Accumulate, Recursive Patterns, Recursive Patterns
address@hidden node-name, next, previous, up
address@hidden Recursive Pattern: @emph{every}
address@hidden Every, type of recursive pattern
address@hidden Recursive pattern: every
+
+In the @code{every} recursive pattern, an action is performed on every
+element of a list.
+
address@hidden 1500
+The basic pattern is:
+
address@hidden @bullet
address@hidden
+If a list be empty, return @code{nil}.
address@hidden
+Else, act on the beginning of the list (the @sc{car} of the list)
+ @itemize @minus
+ @item
+ through a recursive call by the function on the rest (the
+ @sc{cdr}) of the list,
+ @item
+ and, optionally, combine the acted-on element, using @code{cons},
+ with the results of acting on the rest.
+ @end itemize
address@hidden itemize
+
address@hidden 1500
+Here is example:
+
address@hidden
address@hidden
+(defun square-each (numbers-list)
+ "Square each of a NUMBERS LIST, recursively."
+ (if (not numbers-list) ; do-again-test
+ nil
+ (cons
+ (* (car numbers-list) (car numbers-list))
+ (square-each (cdr numbers-list))))) ; next-step-expression
address@hidden group
+
address@hidden
+(square-each '(1 2 3))
+ @result{} (1 4 9)
address@hidden group
address@hidden smallexample
+
address@hidden 1200
address@hidden
+If @code{numbers-list} is empty, do nothing. But if it has content,
+construct a list combining the square of the first number in the list
+with the result of the recursive call.
+
+(The example follows the pattern exactly: @code{nil} is returned if
+the numbers' list is empty. In practice, you would write the
+conditional so it carries out the action when the numbers' list is not
+empty.)
+
+The @code{print-elements-recursively} function (@pxref{Recursion with
+list, , Recursion with a List}) is another example of an @code{every}
+pattern, except in this case, rather than bring the results together
+using @code{cons}, we print each element of output.
+
address@hidden 1250
+The @code{print-elements-recursively} function looks like this:
+
address@hidden
address@hidden
+(setq animals '(gazelle giraffe lion tiger))
address@hidden group
+
address@hidden
+(defun print-elements-recursively (list)
+ "Print each element of LIST on a line of its own.
+Uses recursion."
+ (when list ; @r{do-again-test}
+ (print (car list)) ; @r{body}
+ (print-elements-recursively ; @r{recursive call}
+ (cdr list)))) ; @r{next-step-expression}
+
+(print-elements-recursively animals)
address@hidden group
address@hidden smallexample
+
address@hidden 1500
+The pattern for @code{print-elements-recursively} is:
+
address@hidden @bullet
address@hidden
+When the list is empty, do nothing.
address@hidden
+But when the list has at least one element,
+ @itemize @minus
+ @item
+ act on the beginning of the list (the @sc{car} of the list),
+ @item
+ and make a recursive call on the rest (the @sc{cdr}) of the list.
+ @end itemize
address@hidden itemize
+
address@hidden Accumulate, Keep, Every, Recursive Patterns
address@hidden node-name, next, previous, up
address@hidden Recursive Pattern: @emph{accumulate}
address@hidden Accumulate, type of recursive pattern
address@hidden Recursive pattern: accumulate
+
+Another recursive pattern is called the @code{accumulate} pattern. In
+the @code{accumulate} recursive pattern, an action is performed on
+every element of a list and the result of that action is accumulated
+with the results of performing the action on the other elements.
+
+This is very like the `every' pattern using @code{cons}, except that
address@hidden is not used, but some other combiner.
+
address@hidden 1500
+The pattern is:
+
address@hidden @bullet
address@hidden
+If a list be empty, return zero or some other constant.
address@hidden
+Else, act on the beginning of the list (the @sc{car} of the list),
+ @itemize @minus
+ @item
+ and combine that acted-on element, using @code{+} or
+ some other combining function, with
+ @item
+ a recursive call by the function on the rest (the @sc{cdr}) of the list.
+ @end itemize
address@hidden itemize
+
address@hidden 1500
+Here is an example:
+
address@hidden
address@hidden
+(defun add-elements (numbers-list)
+ "Add the elements of NUMBERS-LIST together."
+ (if (not numbers-list)
+ 0
+ (+ (car numbers-list) (add-elements (cdr numbers-list)))))
address@hidden group
+
address@hidden
+(add-elements '(1 2 3 4))
+ @result{} 10
address@hidden group
address@hidden smallexample
+
address@hidden List, , Making a List of Files}, for an example of the
+accumulate pattern.
+
address@hidden Keep, , Accumulate, Recursive Patterns
address@hidden node-name, next, previous, up
address@hidden Recursive Pattern: @emph{keep}
address@hidden Keep, type of recursive pattern
address@hidden Recursive pattern: keep
+
+A third recursive pattern is called the @code{keep} pattern.
+In the @code{keep} recursive pattern, each element of a list is tested;
+the element is acted on and the results are kept only if the element
+meets a criterion.
+
+Again, this is very like the `every' pattern, except the element is
+skipped unless it meets a criterion.
+
address@hidden 1500
+The pattern has three parts:
+
address@hidden @bullet
address@hidden
+If a list be empty, return @code{nil}.
address@hidden
+Else, if the beginning of the list (the @sc{car} of the list) passes
+ a test
+ @itemize @minus
+ @item
+ act on that element and combine it, using @code{cons} with
+ @item
+ a recursive call by the function on the rest (the @sc{cdr}) of the list.
+ @end itemize
address@hidden
+Otherwise, if the beginning of the list (the @sc{car} of the list) fails
+the test
+ @itemize @minus
+ @item
+ skip on that element,
+ @item
+ and, recursively call the function on the rest (the @sc{cdr}) of the list.
+ @end itemize
address@hidden itemize
+
address@hidden 1500
+Here is an example that uses @code{cond}:
+
address@hidden
address@hidden
+(defun keep-three-letter-words (word-list)
+ "Keep three letter words in WORD-LIST."
+ (cond
+ ;; First do-again-test: stop-condition
+ ((not word-list) nil)
+
+ ;; Second do-again-test: when to act
+ ((eq 3 (length (symbol-name (car word-list))))
+ ;; combine acted-on element with recursive call on shorter list
+ (cons (car word-list) (keep-three-letter-words (cdr word-list))))
+
+ ;; Third do-again-test: when to skip element;
+ ;; recursively call shorter list with next-step expression
+ (t (keep-three-letter-words (cdr word-list)))))
address@hidden group
+
address@hidden
+(keep-three-letter-words '(one two three four five six))
+ @result{} (one two six)
address@hidden group
address@hidden smallexample
+
+It goes without saying that you need not use @code{nil} as the test for
+when to stop; and you can, of course, combine these patterns.
+
address@hidden No Deferment, No deferment solution, Recursive Patterns,
Recursion
address@hidden Recursion without Deferments
address@hidden Deferment in recursion
address@hidden Recursion without Deferments
+
+Let's consider again what happens with the @code{triangle-recursively}
+function. We will find that the intermediate calculations are
+deferred until all can be done.
+
address@hidden 800
+Here is the function definition:
+
address@hidden
address@hidden
+(defun triangle-recursively (number)
+ "Return the sum of the numbers 1 through NUMBER inclusive.
+Uses recursion."
+ (if (= number 1) ; @r{do-again-test}
+ 1 ; @r{then-part}
+ (+ number ; @r{else-part}
+ (triangle-recursively ; @r{recursive call}
+ (1- number))))) ; @r{next-step-expression}
address@hidden group
address@hidden smallexample
+
+What happens when we call this function with a argument of 7?
+
+The first instance of the @code{triangle-recursively} function adds
+the number 7 to the value returned by a second instance of
address@hidden, an instance that has been passed an
+argument of 6. That is to say, the first calculation is:
+
address@hidden
+(+ 7 (triangle-recursively 6))
address@hidden smallexample
+
address@hidden
+The first instance of @code{triangle-recursively}---you may want to
+think of it as a little robot---cannot complete its job. It must hand
+off the calculation for @code{(triangle-recursively 6)} to a second
+instance of the program, to a second robot. This second individual is
+completely different from the first one; it is, in the jargon, a
+`different instantiation'. Or, put another way, it is a different
+robot. It is the same model as the first; it calculates triangle
+numbers recursively; but it has a different serial number.
+
+And what does @code{(triangle-recursively 6)} return? It returns the
+number 6 added to the value returned by evaluating
address@hidden with an argument of 5. Using the robot
+metaphor, it asks yet another robot to help it.
+
address@hidden 800
+Now the total is:
+
address@hidden
+(+ 7 6 (triangle-recursively 5))
address@hidden smallexample
+
address@hidden 800
+And what happens next?
+
address@hidden
+(+ 7 6 5 (triangle-recursively 4))
address@hidden smallexample
+
+Each time @code{triangle-recursively} is called, except for the last
+time, it creates another instance of the program---another robot---and
+asks it to make a calculation.
+
address@hidden 800
+Eventually, the full addition is set up and performed:
+
address@hidden
+(+ 7 6 5 4 3 2 1)
address@hidden smallexample
+
+This design for the function defers the calculation of the first step
+until the second can be done, and defers that until the third can be
+done, and so on. Each deferment means the computer must remember what
+is being waited on. This is not a problem when there are only a few
+steps, as in this example. But it can be a problem when there are
+more steps.
+
address@hidden No deferment solution, , No Deferment, Recursion
address@hidden No Deferment Solution
address@hidden No deferment solution
address@hidden Defermentless solution
address@hidden Solution without deferment
+
+The solution to the problem of deferred operations is to write in a
+manner that does not defer address@hidden phrase @dfn{tail
+recursive} is used to describe such a process, one that uses
+`constant space'.}. This requires
+writing to a different pattern, often one that involves writing two
+function definitions, an `initialization' function and a `helper'
+function.
+
+The `initialization' function sets up the job; the `helper' function
+does the work.
+
address@hidden 1200
+Here are the two function definitions for adding up numbers. They are
+so simple, I find them hard to understand.
+
address@hidden
address@hidden
+(defun triangle-initialization (number)
+ "Return the sum of the numbers 1 through NUMBER inclusive.
+This is the `initialization' component of a two function
+duo that uses recursion."
+ (triangle-recursive-helper 0 0 number))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun triangle-recursive-helper (sum counter number)
+ "Return SUM, using COUNTER, through NUMBER inclusive.
+This is the `helper' component of a two function duo
+that uses recursion."
+ (if (> counter number)
+ sum
+ (triangle-recursive-helper (+ sum counter) ; @r{sum}
+ (1+ counter) ; @r{counter}
+ number))) ; @r{number}
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+Install both function definitions by evaluating them, then call
address@hidden with 2 rows:
+
address@hidden
address@hidden
+(triangle-initialization 2)
+ @result{} 3
address@hidden group
address@hidden smallexample
+
+The `initialization' function calls the first instance of the `helper'
+function with three arguments: zero, zero, and a number which is the
+number of rows in the triangle.
+
+The first two arguments passed to the `helper' function are
+initialization values. These values are changed when
address@hidden invokes new address@hidden
+jargon is mildly confusing: @code{triangle-recursive-helper} uses a
+process that is iterative in a procedure that is recursive. The
+process is called iterative because the computer need only record the
+three values, @code{sum}, @code{counter}, and @code{number}; the
+procedure is recursive because the function `calls itself'. On the
+other hand, both the process and the procedure used by
address@hidden are called recursive. The word
+`recursive' has different meanings in the two contexts.}
+
+Let's see what happens when we have a triangle that has one row. (This
+triangle will have one pebble in it!)
+
address@hidden 1200
address@hidden will call its helper with
+the arguments @address@hidden 0 1}}. That function will run the conditional
+test whether @code{(> counter number)}:
+
address@hidden
+(> 0 1)
address@hidden smallexample
+
address@hidden 1200
address@hidden
+and find that the result is false, so it will invoke
+the else-part of the @code{if} clause:
+
address@hidden
address@hidden
+ (triangle-recursive-helper
+ (+ sum counter) ; @r{sum plus counter} @result{} @r{sum}
+ (1+ counter) ; @r{increment counter} @result{} @r{counter}
+ number) ; @r{number stays the same}
address@hidden group
address@hidden smallexample
+
address@hidden 800
address@hidden
+which will first compute:
+
address@hidden
address@hidden
+(triangle-recursive-helper (+ 0 0) ; @r{sum}
+ (1+ 0) ; @r{counter}
+ 1) ; @r{number}
address@hidden which is:
+
+(triangle-recursive-helper 0 1 1)
address@hidden group
address@hidden smallexample
+
+Again, @code{(> counter number)} will be false, so again, the Lisp
+interpreter will evaluate @code{triangle-recursive-helper}, creating a
+new instance with new arguments.
+
address@hidden 800
+This new instance will be;
+
address@hidden
address@hidden
+ (triangle-recursive-helper
+ (+ sum counter) ; @r{sum plus counter} @result{} @r{sum}
+ (1+ counter) ; @r{increment counter} @result{} @r{counter}
+ number) ; @r{number stays the same}
+
address@hidden which is:
+
+(triangle-recursive-helper 1 2 1)
address@hidden group
address@hidden smallexample
+
+In this case, the @code{(> counter number)} test will be true! So the
+instance will return the value of the sum, which will be 1, as
+expected.
+
+Now, let's pass @code{triangle-initialization} an argument
+of 2, to find out how many pebbles there are in a triangle with two rows.
+
+That function calls @code{(triangle-recursive-helper 0 0 2)}.
+
address@hidden 800
+In stages, the instances called will be:
+
address@hidden
address@hidden
+ @r{sum counter number}
+(triangle-recursive-helper 0 1 2)
+
+(triangle-recursive-helper 1 2 2)
+
+(triangle-recursive-helper 3 3 2)
address@hidden group
address@hidden smallexample
+
+When the last instance is called, the @code{(> counter number)} test
+will be true, so the instance will return the value of @code{sum},
+which will be 3.
+
+This kind of pattern helps when you are writing functions that can use
+many resources in a computer.
+
address@hidden 1500
address@hidden Looping exercise, , Recursion, Loops & Recursion
address@hidden Looping Exercise
+
address@hidden @bullet
address@hidden
+Write a function similar to @code{triangle} in which each row has a
+value which is the square of the row number. Use a @code{while} loop.
+
address@hidden
+Write a function similar to @code{triangle} that multiplies instead of
+adds the values.
+
address@hidden
+Rewrite these two functions recursively. Rewrite these functions
+using @code{cond}.
+
address@hidden comma in printed title causes problem in Info cross reference
address@hidden
+Write a function for Texinfo mode that creates an index entry at the
+beginning of a paragraph for every @samp{@@dfn} within the paragraph.
+(In a Texinfo file, @samp{@@dfn} marks a definition. This book is
+written in Texinfo.)
+
+Many of the functions you will need are described in two of the
+previous chapters, @ref{Cutting & Storing Text, , Cutting and Storing
+Text}, and @ref{Yanking, , Yanking Text Back}. If you use
address@hidden to put the index entry at the beginning of
+the paragraph, you will have to use @address@hidden f}}
+(@code{describe-function}) to find out how to make the command go
+backwards.
+
+For more information, see
address@hidden
address@hidden, , Indicating Definitions, texinfo}.
address@hidden ifinfo
address@hidden
address@hidden, , Indicating, texinfo, Texinfo Manual}, which goes to
+a Texinfo manual in the current directory. Or, if you are on the
+Internet, see
address@hidden://www.gnu.org/software/texinfo/manual/texinfo/}
address@hidden ifhtml
address@hidden
+``Indicating Definitions, Commands, etc.'' in @cite{Texinfo, The GNU
+Documentation Format}.
address@hidden iftex
address@hidden itemize
+
address@hidden Regexp Search, Counting Words, Loops & Recursion, Top
address@hidden node-name, next, previous, up
address@hidden Regular Expression Searches
address@hidden Searches, illustrating
address@hidden Regular expression searches
address@hidden Patterns, searching for
address@hidden Motion by sentence and paragraph
address@hidden Sentences, movement by
address@hidden Paragraphs, movement by
+
+Regular expression searches are used extensively in GNU Emacs. The
+two functions, @code{forward-sentence} and @code{forward-paragraph},
+illustrate these searches well. They use regular expressions to find
+where to move point. The phrase `regular expression' is often written
+as `regexp'.
+
+Regular expression searches are described in @ref{Regexp Search, ,
+Regular Expression Search, emacs, The GNU Emacs Manual}, as well as in
address@hidden Expressions, , , elisp, The GNU Emacs Lisp Reference
+Manual}. In writing this chapter, I am presuming that you have at
+least a mild acquaintance with them. The major point to remember is
+that regular expressions permit you to search for patterns as well as
+for literal strings of characters. For example, the code in
address@hidden searches for the pattern of possible
+characters that could mark the end of a sentence, and moves point to
+that spot.
+
+Before looking at the code for the @code{forward-sentence} function, it
+is worth considering what the pattern that marks the end of a sentence
+must be. The pattern is discussed in the next section; following that
+is a description of the regular expression search function,
address@hidden The @code{forward-sentence} function
+is described in the section following. Finally, the
address@hidden function is described in the last section of
+this chapter. @code{forward-paragraph} is a complex function that
+introduces several new features.
+
address@hidden
+* sentence-end:: The regular expression for @code{sentence-end}.
+* re-search-forward:: Very similar to @code{search-forward}.
+* forward-sentence:: A straightforward example of regexp search.
+* forward-paragraph:: A somewhat complex example.
+* etags:: How to create your own @file{TAGS} table.
+* Regexp Review::
+* re-search Exercises::
address@hidden menu
+
address@hidden sentence-end, re-search-forward, Regexp Search, Regexp Search
address@hidden node-name, next, previous, up
address@hidden The Regular Expression for @code{sentence-end}
address@hidden sentence-end
+
+The symbol @code{sentence-end} is bound to the pattern that marks the
+end of a sentence. What should this regular expression be?
+
+Clearly, a sentence may be ended by a period, a question mark, or an
+exclamation mark. Indeed, in English, only clauses that end with one
+of those three characters should be considered the end of a sentence.
+This means that the pattern should include the character set:
+
address@hidden
+[.?!]
address@hidden smallexample
+
+However, we do not want @code{forward-sentence} merely to jump to a
+period, a question mark, or an exclamation mark, because such a character
+might be used in the middle of a sentence. A period, for example, is
+used after abbreviations. So other information is needed.
+
+According to convention, you type two spaces after every sentence, but
+only one space after a period, a question mark, or an exclamation mark in
+the body of a sentence. So a period, a question mark, or an exclamation
+mark followed by two spaces is a good indicator of an end of sentence.
+However, in a file, the two spaces may instead be a tab or the end of a
+line. This means that the regular expression should include these three
+items as alternatives.
+
address@hidden 800
+This group of alternatives will look like this:
+
address@hidden
address@hidden
+\\($\\| \\| \\)
+ ^ ^^
+ TAB SPC
address@hidden group
address@hidden smallexample
+
address@hidden
+Here, @samp{$} indicates the end of the line, and I have pointed out
+where the tab and two spaces are inserted in the expression. Both are
+inserted by putting the actual characters into the expression.
+
+Two backslashes, @samp{\\}, are required before the parentheses and
+vertical bars: the first backslash quotes the following backslash in
+Emacs; and the second indicates that the following character, the
+parenthesis or the vertical bar, is special.
+
address@hidden 1000
+Also, a sentence may be followed by one or more carriage returns, like
+this:
+
address@hidden
address@hidden
+[
+]*
address@hidden group
address@hidden smallexample
+
address@hidden
+Like tabs and spaces, a carriage return is inserted into a regular
+expression by inserting it literally. The asterisk indicates that the
address@hidden is repeated zero or more times.
+
+But a sentence end does not consist only of a period, a question mark or
+an exclamation mark followed by appropriate space: a closing quotation
+mark or a closing brace of some kind may precede the space. Indeed more
+than one such mark or brace may precede the space. These require a
+expression that looks like this:
+
address@hidden
+[]\"')@}]*
address@hidden smallexample
+
+In this expression, the first @samp{]} is the first character in the
+expression; the second character is @samp{"}, which is preceded by a
address@hidden to tell Emacs the @samp{"} is @emph{not} special. The last
+three characters are @samp{'}, @samp{)}, and @address@hidden
+
+All this suggests what the regular expression pattern for matching the
+end of a sentence should be; and, indeed, if we evaluate
address@hidden we find that it returns the following value:
+
address@hidden
address@hidden
+sentence-end
+ @result{} "[.?!][]\"')@}]*\\($\\| \\| \\)[
+]*"
address@hidden group
address@hidden smallexample
+
address@hidden
+(Well, not in GNU Emacs 22; that is because of an effort to make the
+process simpler and to handle more glyphs and languages. When the
+value of @code{sentence-end} is @code{nil}, then use the value defined
+by the function @code{sentence-end}. (Here is a use of the difference
+between a value and a function in Emacs Lisp.) The function returns a
+value constructed from the variables @code{sentence-end-base},
address@hidden, @code{sentence-end-without-period},
+and @code{sentence-end-without-space}. The critical variable is
address@hidden; its global value is similar to the one
+described above but it also contains two additional quotation marks.
+These have differing degrees of curliness. The
address@hidden variable, when true, tells Emacs
+that a sentence may end without a period, such as text in Thai.)
+
address@hidden
address@hidden
+(Note that here the @key{TAB}, two spaces, and @key{RET} are shown
+literally in the pattern.)
+
+This regular expression can be deciphered as follows:
+
address@hidden @code
address@hidden [.?!]
+The first part of the pattern is the three characters, a period, a question
+mark and an exclamation mark, within square brackets. The pattern must
+begin with one or other of these characters.
+
address@hidden []\"')@}]*
+The second part of the pattern is the group of closing braces and
+quotation marks, which can appear zero or more times. These may follow
+the period, question mark or exclamation mark. In a regular expression,
+the backslash, @samp{\}, followed by the double quotation mark,
address@hidden"}, indicates the class of string-quote characters. Usually, the
+double quotation mark is the only character in this class. The
+asterisk, @samp{*}, indicates that the items in the previous group (the
+group surrounded by square brackets, @samp{[]}) may be repeated zero or
+more times.
+
address@hidden \\($\\| \\| \\)
+The third part of the pattern is one or other of: either the end of a
+line, or two blank spaces, or a tab. The double back-slashes are used
+to prevent Emacs from reading the parentheses and vertical bars as part
+of the search pattern; the parentheses are used to mark the group and
+the vertical bars are used to indicated that the patterns to either side
+of them are alternatives. The dollar sign is used to indicate the end
+of a line and both the two spaces and the tab are each inserted as is to
+indicate what they are.
+
address@hidden address@hidden
+Finally, the last part of the pattern indicates that the end of the line
+or the whitespace following the period, question mark or exclamation
+mark may, but need not, be followed by one or more carriage returns. In
+the pattern, the carriage return is inserted as an actual carriage
+return between square brackets but here it is shown as @key{RET}.
address@hidden table
address@hidden ignore
+
address@hidden re-search-forward, forward-sentence, sentence-end, Regexp Search
address@hidden node-name, next, previous, up
address@hidden The @code{re-search-forward} Function
address@hidden re-search-forward
+
+The @code{re-search-forward} function is very like the
address@hidden function. (@xref{search-forward, , The
address@hidden Function}.)
+
address@hidden searches for a regular expression. If the
+search is successful, it leaves point immediately after the last
+character in the target. If the search is backwards, it leaves point
+just before the first character in the target. You may tell
address@hidden to return @code{t} for true. (Moving point
+is therefore a `side effect'.)
+
+Like @code{search-forward}, the @code{re-search-forward} function takes
+four arguments:
+
address@hidden
address@hidden
+The first argument is the regular expression that the function searches
+for. The regular expression will be a string between quotations marks.
+
address@hidden
+The optional second argument limits how far the function will search; it is a
+bound, which is specified as a position in the buffer.
+
address@hidden
+The optional third argument specifies how the function responds to
+failure: @code{nil} as the third argument causes the function to
+signal an error (and print a message) when the search fails; any other
+value causes it to return @code{nil} if the search fails and @code{t}
+if the search succeeds.
+
address@hidden
+The optional fourth argument is the repeat count. A negative repeat
+count causes @code{re-search-forward} to search backwards.
address@hidden enumerate
+
address@hidden 800
+The template for @code{re-search-forward} looks like this:
+
address@hidden
address@hidden
+(re-search-forward "@var{regular-expression}"
+ @var{limit-of-search}
+ @var{what-to-do-if-search-fails}
+ @var{repeat-count})
address@hidden group
address@hidden smallexample
+
+The second, third, and fourth arguments are optional. However, if you
+want to pass a value to either or both of the last two arguments, you
+must also pass a value to all the preceding arguments. Otherwise, the
+Lisp interpreter will mistake which argument you are passing the value
+to.
+
address@hidden 1200
+In the @code{forward-sentence} function, the regular expression will be
+the value of the variable @code{sentence-end}. In simple form, that is:
+
address@hidden
address@hidden
+"[.?!][]\"')@}]*\\($\\| \\| \\)[
+]*"
address@hidden group
address@hidden smallexample
+
address@hidden
+The limit of the search will be the end of the paragraph (since a
+sentence cannot go beyond a paragraph). If the search fails, the
+function will return @code{nil}; and the repeat count will be provided
+by the argument to the @code{forward-sentence} function.
+
address@hidden forward-sentence, forward-paragraph, re-search-forward, Regexp
Search
address@hidden node-name, next, previous, up
address@hidden @code{forward-sentence}
address@hidden forward-sentence
+
+The command to move the cursor forward a sentence is a straightforward
+illustration of how to use regular expression searches in Emacs Lisp.
+Indeed, the function looks longer and more complicated than it is; this
+is because the function is designed to go backwards as well as forwards;
+and, optionally, over more than one sentence. The function is usually
+bound to the key command @kbd{M-e}.
+
address@hidden
+* Complete forward-sentence::
+* fwd-sentence while loops:: Two @code{while} loops.
+* fwd-sentence re-search:: A regular expression search.
address@hidden menu
+
address@hidden Complete forward-sentence, fwd-sentence while loops,
forward-sentence, forward-sentence
address@hidden
address@hidden Complete @code{forward-sentence} function definition
address@hidden ifnottex
+
address@hidden 1250
+Here is the code for @code{forward-sentence}:
+
address@hidden in GNU Emacs 22
address@hidden
address@hidden
+(defun forward-sentence (&optional arg)
+ "Move forward to next `sentence-end'. With argument, repeat.
+With negative argument, move backward repeatedly to `sentence-beginning'.
+
+The variable `sentence-end' is a regular expression that matches ends of
+sentences. Also, every paragraph boundary terminates sentences as well."
address@hidden group
address@hidden
+ (interactive "p")
+ (or arg (setq arg 1))
+ (let ((opoint (point))
+ (sentence-end (sentence-end)))
+ (while (< arg 0)
+ (let ((pos (point))
+ (par-beg (save-excursion (start-of-paragraph-text) (point))))
+ (if (and (re-search-backward sentence-end par-beg t)
+ (or (< (match-end 0) pos)
+ (re-search-backward sentence-end par-beg t)))
+ (goto-char (match-end 0))
+ (goto-char par-beg)))
+ (setq arg (1+ arg)))
address@hidden group
address@hidden
+ (while (> arg 0)
+ (let ((par-end (save-excursion (end-of-paragraph-text) (point))))
+ (if (re-search-forward sentence-end par-end t)
+ (skip-chars-backward " \t\n")
+ (goto-char par-end)))
+ (setq arg (1- arg)))
+ (constrain-to-field nil opoint t)))
address@hidden group
address@hidden smallexample
+
address@hidden
+GNU Emacs 21
address@hidden
address@hidden
+(defun forward-sentence (&optional arg)
+ "Move forward to next sentence-end. With argument, repeat.
+With negative argument, move backward repeatedly to sentence-beginning.
+Sentence ends are identified by the value of sentence-end
+treated as a regular expression. Also, every paragraph boundary
+terminates sentences as well."
address@hidden group
address@hidden
+ (interactive "p")
+ (or arg (setq arg 1))
+ (while (< arg 0)
+ (let ((par-beg
+ (save-excursion (start-of-paragraph-text) (point))))
+ (if (re-search-backward
+ (concat sentence-end "[^ \t\n]") par-beg t)
+ (goto-char (1- (match-end 0)))
+ (goto-char par-beg)))
+ (setq arg (1+ arg)))
+ (while (> arg 0)
+ (let ((par-end
+ (save-excursion (end-of-paragraph-text) (point))))
+ (if (re-search-forward sentence-end par-end t)
+ (skip-chars-backward " \t\n")
+ (goto-char par-end)))
+ (setq arg (1- arg))))
address@hidden group
address@hidden smallexample
address@hidden ignore
+
+The function looks long at first sight and it is best to look at its
+skeleton first, and then its muscle. The way to see the skeleton is to
+look at the expressions that start in the left-most columns:
+
address@hidden
address@hidden
+(defun forward-sentence (&optional arg)
+ "@address@hidden"
+ (interactive "p")
+ (or arg (setq arg 1))
+ (let ((opoint (point)) (sentence-end (sentence-end)))
+ (while (< arg 0)
+ (let ((pos (point))
+ (par-beg (save-excursion (start-of-paragraph-text) (point))))
+ @var{rest-of-body-of-while-loop-when-going-backwards}
+ (while (> arg 0)
+ (let ((par-end (save-excursion (end-of-paragraph-text) (point))))
+ @var{rest-of-body-of-while-loop-when-going-forwards}
+ @var{handle-forms-and-equivalent}
address@hidden group
address@hidden smallexample
+
+This looks much simpler! The function definition consists of
+documentation, an @code{interactive} expression, an @code{or}
+expression, a @code{let} expression, and @code{while} loops.
+
+Let's look at each of these parts in turn.
+
+We note that the documentation is thorough and understandable.
+
+The function has an @code{interactive "p"} declaration. This means
+that the processed prefix argument, if any, is passed to the
+function as its argument. (This will be a number.) If the function
+is not passed an argument (it is optional) then the argument
address@hidden will be bound to 1.
+
+When @code{forward-sentence} is called non-interactively without an
+argument, @code{arg} is bound to @code{nil}. The @code{or} expression
+handles this. What it does is either leave the value of @code{arg} as
+it is, but only if @code{arg} is bound to a value; or it sets the
+value of @code{arg} to 1, in the case when @code{arg} is bound to
address@hidden
+
+Next is a @code{let}. That specifies the values of two local
+variables, @code{point} and @code{sentence-end}. The local value of
+point, from before the search, is used in the
address@hidden function which handles forms and
+equivalents. The @code{sentence-end} variable is set by the
address@hidden function.
+
address@hidden fwd-sentence while loops, fwd-sentence re-search, Complete
forward-sentence, forward-sentence
address@hidden The @code{while} loops
+
+Two @code{while} loops follow. The first @code{while} has a
+true-or-false-test that tests true if the prefix argument for
address@hidden is a negative number. This is for going
+backwards. The body of this loop is similar to the body of the second
address@hidden clause, but it is not exactly the same. We will skip
+this @code{while} loop and concentrate on the second @code{while}
+loop.
+
address@hidden 1500
+The second @code{while} loop is for moving point forward. Its skeleton
+looks like this:
+
address@hidden
address@hidden
+(while (> arg 0) ; @r{true-or-false-test}
+ (let @var{varlist}
+ (if (@var{true-or-false-test})
+ @var{then-part}
+ @var{else-part}
+ (setq arg (1- arg)))) ; @code{while} @r{loop decrementer}
address@hidden group
address@hidden smallexample
+
+The @code{while} loop is of the decrementing kind.
+(@xref{Decrementing Loop, , A Loop with a Decrementing Counter}.) It
+has a true-or-false-test that tests true so long as the counter (in
+this case, the variable @code{arg}) is greater than zero; and it has a
+decrementer that subtracts 1 from the value of the counter every time
+the loop repeats.
+
+If no prefix argument is given to @code{forward-sentence}, which is
+the most common way the command is used, this @code{while} loop will
+run once, since the value of @code{arg} will be 1.
+
+The body of the @code{while} loop consists of a @code{let} expression,
+which creates and binds a local variable, and has, as its body, an
address@hidden expression.
+
address@hidden 1250
+The body of the @code{while} loop looks like this:
+
address@hidden
address@hidden
+(let ((par-end
+ (save-excursion (end-of-paragraph-text) (point))))
+ (if (re-search-forward sentence-end par-end t)
+ (skip-chars-backward " \t\n")
+ (goto-char par-end)))
address@hidden group
address@hidden smallexample
+
+The @code{let} expression creates and binds the local variable
address@hidden As we shall see, this local variable is designed to
+provide a bound or limit to the regular expression search. If the
+search fails to find a proper sentence ending in the paragraph, it will
+stop on reaching the end of the paragraph.
+
+But first, let us examine how @code{par-end} is bound to the value of
+the end of the paragraph. What happens is that the @code{let} sets the
+value of @code{par-end} to the value returned when the Lisp interpreter
+evaluates the expression
+
address@hidden
address@hidden
+(save-excursion (end-of-paragraph-text) (point))
address@hidden group
address@hidden smallexample
+
address@hidden
+In this expression, @code{(end-of-paragraph-text)} moves point to the
+end of the paragraph, @code{(point)} returns the value of point, and then
address@hidden restores point to its original position. Thus,
+the @code{let} binds @code{par-end} to the value returned by the
address@hidden expression, which is the position of the end of
+the paragraph. (The @code{end-of-paragraph-text} function uses
address@hidden, which we will discuss shortly.)
+
address@hidden 1200
+Emacs next evaluates the body of the @code{let}, which is an @code{if}
+expression that looks like this:
+
address@hidden
address@hidden
+(if (re-search-forward sentence-end par-end t) ; @r{if-part}
+ (skip-chars-backward " \t\n") ; @r{then-part}
+ (goto-char par-end))) ; @r{else-part}
address@hidden group
address@hidden smallexample
+
+The @code{if} tests whether its first argument is true and if so,
+evaluates its then-part; otherwise, the Emacs Lisp interpreter
+evaluates the else-part. The true-or-false-test of the @code{if}
+expression is the regular expression search.
+
+It may seem odd to have what looks like the `real work' of
+the @code{forward-sentence} function buried here, but this is a common
+way this kind of operation is carried out in Lisp.
+
address@hidden fwd-sentence re-search, , fwd-sentence while loops,
forward-sentence
address@hidden The regular expression search
+
+The @code{re-search-forward} function searches for the end of the
+sentence, that is, for the pattern defined by the @code{sentence-end}
+regular expression. If the pattern is found---if the end of the sentence is
+found---then the @code{re-search-forward} function does two things:
+
address@hidden
address@hidden
+The @code{re-search-forward} function carries out a side effect, which
+is to move point to the end of the occurrence found.
+
address@hidden
+The @code{re-search-forward} function returns a value of true. This is
+the value received by the @code{if}, and means that the search was
+successful.
address@hidden enumerate
+
address@hidden
+The side effect, the movement of point, is completed before the
address@hidden function is handed the value returned by the successful
+conclusion of the search.
+
+When the @code{if} function receives the value of true from a successful
+call to @code{re-search-forward}, the @code{if} evaluates the then-part,
+which is the expression @code{(skip-chars-backward " \t\n")}. This
+expression moves backwards over any blank spaces, tabs or carriage
+returns until a printed character is found and then leaves point after
+the character. Since point has already been moved to the end of the
+pattern that marks the end of the sentence, this action leaves point
+right after the closing printed character of the sentence, which is
+usually a period.
+
+On the other hand, if the @code{re-search-forward} function fails to
+find a pattern marking the end of the sentence, the function returns
+false. The false then causes the @code{if} to evaluate its third
+argument, which is @code{(goto-char par-end)}: it moves point to the
+end of the paragraph.
+
+(And if the text is in a form or equivalent, and point may not move
+fully, then the @code{constrain-to-field} function comes into play.)
+
+Regular expression searches are exceptionally useful and the pattern
+illustrated by @code{re-search-forward}, in which the search is the
+test of an @code{if} expression, is handy. You will see or write code
+incorporating this pattern often.
+
address@hidden forward-paragraph, etags, forward-sentence, Regexp Search
address@hidden node-name, next, previous, up
address@hidden @code{forward-paragraph}: a Goldmine of Functions
address@hidden forward-paragraph
+
address@hidden
address@hidden in GNU Emacs 22
+(defun forward-paragraph (&optional arg)
+ "Move forward to end of paragraph.
+With argument ARG, do it ARG times;
+a negative argument ARG = -N means move backward N paragraphs.
+
+A line which `paragraph-start' matches either separates paragraphs
+\(if `paragraph-separate' matches it also) or is the first line of a paragraph.
+A paragraph end is the beginning of a line which is not part of the paragraph
+to which the end of the previous line belongs, or the end of the buffer.
+Returns the count of paragraphs left to move."
+ (interactive "p")
+ (or arg (setq arg 1))
+ (let* ((opoint (point))
+ (fill-prefix-regexp
+ (and fill-prefix (not (equal fill-prefix ""))
+ (not paragraph-ignore-fill-prefix)
+ (regexp-quote fill-prefix)))
+ ;; Remove ^ from paragraph-start and paragraph-sep if they are there.
+ ;; These regexps shouldn't be anchored, because we look for them
+ ;; starting at the left-margin. This allows paragraph commands to
+ ;; work normally with indented text.
+ ;; This hack will not find problem cases like "whatever\\|^something".
+ (parstart (if (and (not (equal "" paragraph-start))
+ (equal ?^ (aref paragraph-start 0)))
+ (substring paragraph-start 1)
+ paragraph-start))
+ (parsep (if (and (not (equal "" paragraph-separate))
+ (equal ?^ (aref paragraph-separate 0)))
+ (substring paragraph-separate 1)
+ paragraph-separate))
+ (parsep
+ (if fill-prefix-regexp
+ (concat parsep "\\|"
+ fill-prefix-regexp "[ \t]*$")
+ parsep))
+ ;; This is used for searching.
+ (sp-parstart (concat "^[ \t]*\\(?:" parstart "\\|" parsep "\\)"))
+ start found-start)
+ (while (and (< arg 0) (not (bobp)))
+ (if (and (not (looking-at parsep))
+ (re-search-backward "^\n" (max (1- (point)) (point-min)) t)
+ (looking-at parsep))
+ (setq arg (1+ arg))
+ (setq start (point))
+ ;; Move back over paragraph-separating lines.
+ (forward-char -1) (beginning-of-line)
+ (while (and (not (bobp))
+ (progn (move-to-left-margin)
+ (looking-at parsep)))
+ (forward-line -1))
+ (if (bobp)
+ nil
+ (setq arg (1+ arg))
+ ;; Go to end of the previous (non-separating) line.
+ (end-of-line)
+ ;; Search back for line that starts or separates paragraphs.
+ (if (if fill-prefix-regexp
+ ;; There is a fill prefix; it overrides parstart.
+ (let (multiple-lines)
+ (while (and (progn (beginning-of-line) (not (bobp)))
+ (progn (move-to-left-margin)
+ (not (looking-at parsep)))
+ (looking-at fill-prefix-regexp))
+ (unless (= (point) start)
+ (setq multiple-lines t))
+ (forward-line -1))
+ (move-to-left-margin)
+ ;; This deleted code caused a long hanging-indent line
+ ;; not to be filled together with the following lines.
+ ;; ;; Don't move back over a line before the paragraph
+ ;; ;; which doesn't start with fill-prefix
+ ;; ;; unless that is the only line we've moved over.
+ ;; (and (not (looking-at fill-prefix-regexp))
+ ;; multiple-lines
+ ;; (forward-line 1))
+ (not (bobp)))
+ (while (and (re-search-backward sp-parstart nil 1)
+ (setq found-start t)
+ ;; Found a candidate, but need to check if it is a
+ ;; REAL parstart.
+ (progn (setq start (point))
+ (move-to-left-margin)
+ (not (looking-at parsep)))
+ (not (and (looking-at parstart)
+ (or (not use-hard-newlines)
+ (bobp)
+ (get-text-property
+ (1- start) 'hard)))))
+ (setq found-start nil)
+ (goto-char start))
+ found-start)
+ ;; Found one.
+ (progn
+ ;; Move forward over paragraph separators.
+ ;; We know this cannot reach the place we started
+ ;; because we know we moved back over a non-separator.
+ (while (and (not (eobp))
+ (progn (move-to-left-margin)
+ (looking-at parsep)))
+ (forward-line 1))
+ ;; If line before paragraph is just margin, back up to there.
+ (end-of-line 0)
+ (if (> (current-column) (current-left-margin))
+ (forward-char 1)
+ (skip-chars-backward " \t")
+ (if (not (bolp))
+ (forward-line 1))))
+ ;; No starter or separator line => use buffer beg.
+ (goto-char (point-min))))))
+
+ (while (and (> arg 0) (not (eobp)))
+ ;; Move forward over separator lines...
+ (while (and (not (eobp))
+ (progn (move-to-left-margin) (not (eobp)))
+ (looking-at parsep))
+ (forward-line 1))
+ (unless (eobp) (setq arg (1- arg)))
+ ;; ... and one more line.
+ (forward-line 1)
+ (if fill-prefix-regexp
+ ;; There is a fill prefix; it overrides parstart.
+ (while (and (not (eobp))
+ (progn (move-to-left-margin) (not (eobp)))
+ (not (looking-at parsep))
+ (looking-at fill-prefix-regexp))
+ (forward-line 1))
+ (while (and (re-search-forward sp-parstart nil 1)
+ (progn (setq start (match-beginning 0))
+ (goto-char start)
+ (not (eobp)))
+ (progn (move-to-left-margin)
+ (not (looking-at parsep)))
+ (or (not (looking-at parstart))
+ (and use-hard-newlines
+ (not (get-text-property (1- start) 'hard)))))
+ (forward-char 1))
+ (if (< (point) (point-max))
+ (goto-char start))))
+ (constrain-to-field nil opoint t)
+ ;; Return the number of steps that could not be done.
+ arg))
address@hidden ignore
+
+The @code{forward-paragraph} function moves point forward to the end
+of the paragraph. It is usually bound to @address@hidden and makes use of a
+number of functions that are important in themselves, including
address@hidden, @code{match-beginning}, and @code{looking-at}.
+
+The function definition for @code{forward-paragraph} is considerably
+longer than the function definition for @code{forward-sentence}
+because it works with a paragraph, each line of which may begin with a
+fill prefix.
+
+A fill prefix consists of a string of characters that are repeated at
+the beginning of each line. For example, in Lisp code, it is a
+convention to start each line of a paragraph-long comment with
address@hidden;;; }. In Text mode, four blank spaces make up another common
+fill prefix, creating an indented paragraph. (@xref{Fill Prefix, , ,
+emacs, The GNU Emacs Manual}, for more information about fill
+prefixes.)
+
+The existence of a fill prefix means that in addition to being able to
+find the end of a paragraph whose lines begin on the left-most
+column, the @code{forward-paragraph} function must be able to find the
+end of a paragraph when all or many of the lines in the buffer begin
+with the fill prefix.
+
+Moreover, it is sometimes practical to ignore a fill prefix that
+exists, especially when blank lines separate paragraphs.
+This is an added complication.
+
address@hidden
+* forward-paragraph in brief:: Key parts of the function definition.
+* fwd-para let:: The @code{let*} expression.
+* fwd-para while:: The forward motion @code{while} loop.
address@hidden menu
+
address@hidden forward-paragraph in brief, fwd-para let, forward-paragraph,
forward-paragraph
address@hidden
address@hidden Shortened @code{forward-paragraph} function definition
address@hidden ifnottex
+
+Rather than print all of the @code{forward-paragraph} function, we
+will only print parts of it. Read without preparation, the function
+can be daunting!
+
address@hidden 800
+In outline, the function looks like this:
+
address@hidden
address@hidden
+(defun forward-paragraph (&optional arg)
+ "@address@hidden"
+ (interactive "p")
+ (or arg (setq arg 1))
+ (let*
+ @var{varlist}
+ (while (and (< arg 0) (not (bobp))) ; @r{backward-moving-code}
+ @dots{}
+ (while (and (> arg 0) (not (eobp))) ; @r{forward-moving-code}
+ @dots{}
address@hidden group
address@hidden smallexample
+
+The first parts of the function are routine: the function's argument
+list consists of one optional argument. Documentation follows.
+
+The lower case @samp{p} in the @code{interactive} declaration means
+that the processed prefix argument, if any, is passed to the function.
+This will be a number, and is the repeat count of how many paragraphs
+point will move. The @code{or} expression in the next line handles
+the common case when no argument is passed to the function, which occurs
+if the function is called from other code rather than interactively.
+This case was described earlier. (@xref{forward-sentence, The
address@hidden function}.) Now we reach the end of the
+familiar part of this function.
+
address@hidden fwd-para let, fwd-para while, forward-paragraph in brief,
forward-paragraph
address@hidden The @code{let*} expression
+
+The next line of the @code{forward-paragraph} function begins a
address@hidden expression. This is a different than @code{let}. The
+symbol is @code{let*} not @code{let}.
+
+The @code{let*} special form is like @code{let} except that Emacs sets
+each variable in sequence, one after another, and variables in the
+latter part of the varlist can make use of the values to which Emacs
+set variables in the earlier part of the varlist.
+
address@hidden
+( refappend save-excursion, , code save-excursion in code append-to-buffer .)
address@hidden ignore
+
+(@ref{append save-excursion, , @code{save-excursion} in
@code{append-to-buffer}}.)
+
+In the @code{let*} expression in this function, Emacs binds a total of
+seven variables: @code{opoint}, @code{fill-prefix-regexp},
address@hidden, @code{parsep}, @code{sp-parstart}, @code{start}, and
address@hidden
+
+The variable @code{parsep} appears twice, first, to remove instances
+of @samp{^}, and second, to handle fill prefixes.
+
+The variable @code{opoint} is just the value of @code{point}. As you
+can guess, it is used in a @code{constrain-to-field} expression, just
+as in @code{forward-sentence}.
+
+The variable @code{fill-prefix-regexp} is set to the value returned by
+evaluating the following list:
+
address@hidden
address@hidden
+(and fill-prefix
+ (not (equal fill-prefix ""))
+ (not paragraph-ignore-fill-prefix)
+ (regexp-quote fill-prefix))
address@hidden group
address@hidden smallexample
+
address@hidden
+This is an expression whose first element is the @code{and} special form.
+
+As we learned earlier (@pxref{kill-new function, , The @code{kill-new}
+function}), the @code{and} special form evaluates each of its
+arguments until one of the arguments returns a value of @code{nil}, in
+which case the @code{and} expression returns @code{nil}; however, if
+none of the arguments returns a value of @code{nil}, the value
+resulting from evaluating the last argument is returned. (Since such
+a value is not @code{nil}, it is considered true in Lisp.) In other
+words, an @code{and} expression returns a true value only if all its
+arguments are true.
address@hidden and
+
+In this case, the variable @code{fill-prefix-regexp} is bound to a
address@hidden value only if the following four expressions produce a
+true (i.e., a address@hidden) value when they are evaluated; otherwise,
address@hidden is bound to @code{nil}.
+
address@hidden @code
address@hidden fill-prefix
+When this variable is evaluated, the value of the fill prefix, if any,
+is returned. If there is no fill prefix, this variable returns
address@hidden
+
address@hidden (not (equal fill-prefix "")
+This expression checks whether an existing fill prefix is an empty
+string, that is, a string with no characters in it. An empty string is
+not a useful fill prefix.
+
address@hidden (not paragraph-ignore-fill-prefix)
+This expression returns @code{nil} if the variable
address@hidden has been turned on by being set to a
+true value such as @code{t}.
+
address@hidden (regexp-quote fill-prefix)
+This is the last argument to the @code{and} special form. If all the
+arguments to the @code{and} are true, the value resulting from
+evaluating this expression will be returned by the @code{and} expression
+and bound to the variable @code{fill-prefix-regexp},
address@hidden table
+
address@hidden regexp-quote
address@hidden
+The result of evaluating this @code{and} expression successfully is that
address@hidden will be bound to the value of
address@hidden as modified by the @code{regexp-quote} function.
+What @code{regexp-quote} does is read a string and return a regular
+expression that will exactly match the string and match nothing else.
+This means that @code{fill-prefix-regexp} will be set to a value that
+will exactly match the fill prefix if the fill prefix exists.
+Otherwise, the variable will be set to @code{nil}.
+
+The next two local variables in the @code{let*} expression are
+designed to remove instances of @samp{^} from @code{parstart} and
address@hidden, the local variables which indicate the paragraph start
+and the paragraph separator. The next expression sets @code{parsep}
+again. That is to handle fill prefixes.
+
+This is the setting that requires the definition call @code{let*}
+rather than @code{let}. The true-or-false-test for the @code{if}
+depends on whether the variable @code{fill-prefix-regexp} evaluates to
address@hidden or some other value.
+
+If @code{fill-prefix-regexp} does not have a value, Emacs evaluates
+the else-part of the @code{if} expression and binds @code{parsep} to
+its local value. (@code{parsep} is a regular expression that matches
+what separates paragraphs.)
+
+But if @code{fill-prefix-regexp} does have a value, Emacs evaluates
+the then-part of the @code{if} expression and binds @code{parsep} to a
+regular expression that includes the @code{fill-prefix-regexp} as part
+of the pattern.
+
+Specifically, @code{parsep} is set to the original value of the
+paragraph separate regular expression concatenated with an alternative
+expression that consists of the @code{fill-prefix-regexp} followed by
+optional whitespace to the end of the line. The whitespace is defined
+by @address@hidden"[ \t]*$"}}.) The @samp{\\|} defines this portion of the
+regexp as an alternative to @code{parsep}.
+
+According to a comment in the code, the next local variable,
address@hidden, is used for searching, and then the final two,
address@hidden and @code{found-start}, are set to @code{nil}.
+
+Now we get into the body of the @code{let*}. The first part of the body
+of the @code{let*} deals with the case when the function is given a
+negative argument and is therefore moving backwards. We will skip this
+section.
+
address@hidden fwd-para while, , fwd-para let, forward-paragraph
address@hidden The forward motion @code{while} loop
+
+The second part of the body of the @code{let*} deals with forward
+motion. It is a @code{while} loop that repeats itself so long as the
+value of @code{arg} is greater than zero. In the most common use of
+the function, the value of the argument is 1, so the body of the
address@hidden loop is evaluated exactly once, and the cursor moves
+forward one paragraph.
+
address@hidden
+(while (and (> arg 0) (not (eobp)))
+
+ ;; Move forward over separator lines...
+ (while (and (not (eobp))
+ (progn (move-to-left-margin) (not (eobp)))
+ (looking-at parsep))
+ (forward-line 1))
+ (unless (eobp) (setq arg (1- arg)))
+ ;; ... and one more line.
+ (forward-line 1)
+
+ (if fill-prefix-regexp
+ ;; There is a fill prefix; it overrides parstart.
+ (while (and (not (eobp))
+ (progn (move-to-left-margin) (not (eobp)))
+ (not (looking-at parsep))
+ (looking-at fill-prefix-regexp))
+ (forward-line 1))
+
+ (while (and (re-search-forward sp-parstart nil 1)
+ (progn (setq start (match-beginning 0))
+ (goto-char start)
+ (not (eobp)))
+ (progn (move-to-left-margin)
+ (not (looking-at parsep)))
+ (or (not (looking-at parstart))
+ (and use-hard-newlines
+ (not (get-text-property (1- start) 'hard)))))
+ (forward-char 1))
+
+ (if (< (point) (point-max))
+ (goto-char start))))
address@hidden ignore
+
+This part handles three situations: when point is between paragraphs,
+when there is a fill prefix and when there is no fill prefix.
+
address@hidden 800
+The @code{while} loop looks like this:
+
address@hidden
address@hidden
+;; @r{going forwards and not at the end of the buffer}
+(while (and (> arg 0) (not (eobp)))
+
+ ;; @r{between paragraphs}
+ ;; Move forward over separator lines...
+ (while (and (not (eobp))
+ (progn (move-to-left-margin) (not (eobp)))
+ (looking-at parsep))
+ (forward-line 1))
+ ;; @r{This decrements the loop}
+ (unless (eobp) (setq arg (1- arg)))
+ ;; ... and one more line.
+ (forward-line 1)
address@hidden group
+
address@hidden
+ (if fill-prefix-regexp
+ ;; There is a fill prefix; it overrides parstart;
+ ;; we go forward line by line
+ (while (and (not (eobp))
+ (progn (move-to-left-margin) (not (eobp)))
+ (not (looking-at parsep))
+ (looking-at fill-prefix-regexp))
+ (forward-line 1))
address@hidden group
+
address@hidden
+ ;; There is no fill prefix;
+ ;; we go forward character by character
+ (while (and (re-search-forward sp-parstart nil 1)
+ (progn (setq start (match-beginning 0))
+ (goto-char start)
+ (not (eobp)))
+ (progn (move-to-left-margin)
+ (not (looking-at parsep)))
+ (or (not (looking-at parstart))
+ (and use-hard-newlines
+ (not (get-text-property (1- start) 'hard)))))
+ (forward-char 1))
address@hidden group
+
address@hidden
+ ;; and if there is no fill prefix and if we are not at the end,
+ ;; go to whatever was found in the regular expression search
+ ;; for sp-parstart
+ (if (< (point) (point-max))
+ (goto-char start))))
address@hidden group
address@hidden smallexample
+
address@hidden eobp
+We can see that this is a decrementing counter @code{while} loop,
+using the expression @code{(setq arg (1- arg))} as the decrementer.
+That expression is not far from the @code{while}, but is hidden in
+another Lisp macro, an @code{unless} macro. Unless we are at the end
+of the buffer --- that is what the @code{eobp} function determines; it
+is an abbreviation of @samp{End Of Buffer P} --- we decrease the value
+of @code{arg} by one.
+
+(If we are at the end of the buffer, we cannot go forward any more and
+the next loop of the @code{while} expression will test false since the
+test is an @code{and} with @code{(not (eobp))}. The @code{not}
+function means exactly as you expect; it is another name for
address@hidden, a function that returns true when its argument is false.)
+
+Interestingly, the loop count is not decremented until we leave the
+space between paragraphs, unless we come to the end of buffer or stop
+seeing the local value of the paragraph separator.
+
+That second @code{while} also has a @code{(move-to-left-margin)}
+expression. The function is self-explanatory. It is inside a
address@hidden expression and not the last element of its body, so it is
+only invoked for its side effect, which is to move point to the left
+margin of the current line.
+
address@hidden looking-at
+The @code{looking-at} function is also self-explanatory; it returns
+true if the text after point matches the regular expression given as
+its argument.
+
+The rest of the body of the loop looks difficult at first, but makes
+sense as you come to understand it.
+
address@hidden 800
+First consider what happens if there is a fill prefix:
+
address@hidden
address@hidden
+ (if fill-prefix-regexp
+ ;; There is a fill prefix; it overrides parstart;
+ ;; we go forward line by line
+ (while (and (not (eobp))
+ (progn (move-to-left-margin) (not (eobp)))
+ (not (looking-at parsep))
+ (looking-at fill-prefix-regexp))
+ (forward-line 1))
address@hidden group
address@hidden smallexample
+
address@hidden
+This expression moves point forward line by line so long
+as four conditions are true:
+
address@hidden
address@hidden
+Point is not at the end of the buffer.
+
address@hidden
+We can move to the left margin of the text and are
+not at the end of the buffer.
+
address@hidden
+The text following point does not separate paragraphs.
+
address@hidden
+The pattern following point is the fill prefix regular expression.
address@hidden enumerate
+
+The last condition may be puzzling, until you remember that point was
+moved to the beginning of the line early in the @code{forward-paragraph}
+function. This means that if the text has a fill prefix, the
address@hidden function will see it.
+
address@hidden 1250
+Consider what happens when there is no fill prefix.
+
address@hidden
address@hidden
+ (while (and (re-search-forward sp-parstart nil 1)
+ (progn (setq start (match-beginning 0))
+ (goto-char start)
+ (not (eobp)))
+ (progn (move-to-left-margin)
+ (not (looking-at parsep)))
+ (or (not (looking-at parstart))
+ (and use-hard-newlines
+ (not (get-text-property (1- start) 'hard)))))
+ (forward-char 1))
address@hidden group
address@hidden smallexample
+
address@hidden
+This @code{while} loop has us searching forward for
address@hidden, which is the combination of possible whitespace
+with a the local value of the start of a paragraph or of a paragraph
+separator. (The latter two are within an expression starting
address@hidden(?:} so that they are not referenced by the
address@hidden function.)
+
address@hidden 800
+The two expressions,
+
address@hidden
address@hidden
+(setq start (match-beginning 0))
+(goto-char start)
address@hidden group
address@hidden smallexample
+
address@hidden
+mean go to the start of the text matched by the regular expression
+search.
+
+The @code{(match-beginning 0)} expression is new. It returns a number
+specifying the location of the start of the text that was matched by
+the last search.
+
+The @code{match-beginning} function is used here because of a
+characteristic of a forward search: a successful forward search,
+regardless of whether it is a plain search or a regular expression
+search, moves point to the end of the text that is found. In this
+case, a successful search moves point to the end of the pattern for
address@hidden
+
+However, we want to put point at the end of the current paragraph, not
+somewhere else. Indeed, since the search possibly includes the
+paragraph separator, point may end up at the beginning of the next one
+unless we use an expression that includes @code{match-beginning}.
+
address@hidden match-beginning
+When given an argument of 0, @code{match-beginning} returns the
+position that is the start of the text matched by the most recent
+search. In this case, the most recent search looks for
address@hidden The @code{(match-beginning 0)} expression returns
+the beginning position of that pattern, rather than the end position
+of that pattern.
+
+(Incidentally, when passed a positive number as an argument, the
address@hidden function returns the location of point at that
+parenthesized expression in the last search unless that parenthesized
+expression begins with @code{\(?:}. I don't know why @code{\(?:}
+appears here since the argument is 0.)
+
address@hidden 1250
+The last expression when there is no fill prefix is
+
address@hidden
address@hidden
+(if (< (point) (point-max))
+ (goto-char start))))
address@hidden group
address@hidden smallexample
+
address@hidden
+This says that if there is no fill prefix and if we are not at the
+end, point should move to the beginning of whatever was found by the
+regular expression search for @code{sp-parstart}.
+
+The full definition for the @code{forward-paragraph} function not only
+includes code for going forwards, but also code for going backwards.
+
+If you are reading this inside of GNU Emacs and you want to see the
+whole function, you can type @kbd{C-h f} (@code{describe-function})
+and the name of the function. This gives you the function
+documentation and the name of the library containing the function's
+source. Place point over the name of the library and press the RET
+key; you will be taken directly to the source. (Be sure to install
+your sources! Without them, you are like a person who tries to drive
+a car with his eyes shut!)
+
address@hidden etags, Regexp Review, forward-paragraph, Regexp Search
address@hidden Create Your Own @file{TAGS} File
address@hidden etags
address@hidden @file{TAGS} file, create own
+
+Besides @kbd{C-h f} (@code{describe-function}), another way to see the
+source of a function is to type @kbd{M-.} (@code{find-tag}) and the
+name of the function when prompted for it. This is a good habit to
+get into. The @kbd{M-.} (@code{find-tag}) command takes you directly
+to the source for a function, variable, or node. The function depends
+on tags tables to tell it where to go.
+
+If the @code{find-tag} function first asks you for the name of a
address@hidden table, give it the name of a @file{TAGS} file such as
address@hidden/usr/local/src/emacs/src/TAGS}. (The exact path to your
address@hidden file depends on how your copy of Emacs was installed. I
+just told you the location that provides both my C and my Emacs Lisp
+sources.)
+
+You can also create your own @file{TAGS} file for directories that
+lack one.
+
+You often need to build and install tags tables yourself. They are
+not built automatically. A tags table is called a @file{TAGS} file;
+the name is in upper case letters.
+
+You can create a @file{TAGS} file by calling the @code{etags} program
+that comes as a part of the Emacs distribution. Usually, @code{etags}
+is compiled and installed when Emacs is built. (@code{etags} is not
+an Emacs Lisp function or a part of Emacs; it is a C program.)
+
address@hidden 1250
+To create a @file{TAGS} file, first switch to the directory in which
+you want to create the file. In Emacs you can do this with the
address@hidden cd} command, or by visiting a file in the directory, or by
+listing the directory with @kbd{C-x d} (@code{dired}). Then run the
+compile command, with @address@hidden *.el}} as the command to execute
+
address@hidden
+M-x compile RET etags *.el RET
address@hidden smallexample
+
address@hidden
+to create a @file{TAGS} file for Emacs Lisp.
+
+For example, if you have a large number of files in your
address@hidden/emacs} directory, as I do---I have 137 @file{.el} files in it,
+of which I load 12---you can create a @file{TAGS} file for the Emacs
+Lisp files in that directory.
+
address@hidden 1250
+The @code{etags} program takes all the usual shell `wildcards'. For
+example, if you have two directories for which you want a single
address@hidden file, type @address@hidden *.el ../elisp/*.el}}, where
address@hidden/elisp/} is the second directory:
+
address@hidden
+M-x compile RET etags *.el ../elisp/*.el RET
address@hidden smallexample
+
address@hidden 1250
+Type
+
address@hidden
+M-x compile RET etags --help RET
address@hidden smallexample
+
address@hidden
+to see a list of the options accepted by @code{etags} as well as a
+list of supported languages.
+
+The @code{etags} program handles more than 20 languages, including
+Emacs Lisp, Common Lisp, Scheme, C, C++, Ada, Fortran, HTML, Java,
+LaTeX, Pascal, Perl, Postscript, Python, TeX, Texinfo, makefiles, and
+most assemblers. The program has no switches for specifying the
+language; it recognizes the language in an input file according to its
+file name and contents.
+
address@hidden is very helpful when you are writing code yourself and
+want to refer back to functions you have already written. Just run
address@hidden again at intervals as you write new functions, so they
+become part of the @file{TAGS} file.
+
+If you think an appropriate @file{TAGS} file already exists for what
+you want, but do not know where it is, you can use the @code{locate}
+program to attempt to find it.
+
+Type @address@hidden locate @key{RET} TAGS @key{RET}}} and Emacs will list
+for you the full path names of all your @file{TAGS} files. On my
+system, this command lists 34 @file{TAGS} files. On the other hand, a
+`plain vanilla' system I recently installed did not contain any
address@hidden files.
+
+If the tags table you want has been created, you can use the @code{M-x
+visit-tags-table} command to specify it. Otherwise, you will need to
+create the tag table yourself and then use @code{M-x
+visit-tags-table}.
+
address@hidden Building Tags in the Emacs sources
address@hidden Building Tags in the Emacs sources
address@hidden Tags in the Emacs sources
address@hidden make tags
+
+The GNU Emacs sources come with a @file{Makefile} that contains a
+sophisticated @code{etags} command that creates, collects, and merges
+tags tables from all over the Emacs sources and puts the information
+into one @file{TAGS} file in the @file{src/} directory. (The
address@hidden/} directory is below the top level of your Emacs directory.)
+
address@hidden 1250
+To build this @file{TAGS} file, go to the top level of your Emacs
+source directory and run the compile command @code{make tags}:
+
address@hidden
+M-x compile RET make tags RET
address@hidden smallexample
+
address@hidden
+(The @code{make tags} command works well with the GNU Emacs sources,
+as well as with some other source packages.)
+
+For more information, see @ref{Tags, , Tag Tables, emacs, The GNU Emacs
+Manual}.
+
address@hidden Regexp Review, re-search Exercises, etags, Regexp Search
address@hidden node-name, next, previous, up
address@hidden Review
+
+Here is a brief summary of some recently introduced functions.
+
address@hidden @code
address@hidden while
+Repeatedly evaluate the body of the expression so long as the first
+element of the body tests true. Then return @code{nil}. (The
+expression is evaluated only for its side effects.)
+
address@hidden 1250
+For example:
+
address@hidden
address@hidden
+(let ((foo 2))
+ (while (> foo 0)
+ (insert (format "foo is %d.\n" foo))
+ (setq foo (1- foo))))
+
+ @result{} foo is 2.
+ foo is 1.
+ nil
address@hidden group
address@hidden smallexample
+
address@hidden
+(The @code{insert} function inserts its arguments at point; the
address@hidden function returns a string formatted from its arguments
+the way @code{message} formats its arguments; @code{\n} produces a new
+line.)
+
address@hidden re-search-forward
+Search for a pattern, and if the pattern is found, move point to rest
+just after it.
+
address@hidden
+Takes four arguments, like @code{search-forward}:
+
address@hidden
address@hidden
+A regular expression that specifies the pattern to search for.
+(Remember to put quotation marks around this argument!)
+
address@hidden
+Optionally, the limit of the search.
+
address@hidden
+Optionally, what to do if the search fails, return @code{nil} or an
+error message.
+
address@hidden
+Optionally, how many times to repeat the search; if negative, the
+search goes backwards.
address@hidden enumerate
+
address@hidden let*
+Bind some variables locally to particular values,
+and then evaluate the remaining arguments, returning the value of the
+last one. While binding the local variables, use the local values of
+variables bound earlier, if any.
+
address@hidden 1250
+For example:
+
address@hidden
address@hidden
+(let* ((foo 7)
+ (bar (* 3 foo)))
+ (message "`bar' is %d." bar))
+ @result{} `bar' is 21.
address@hidden group
address@hidden smallexample
+
address@hidden match-beginning
+Return the position of the start of the text found by the last regular
+expression search.
+
address@hidden looking-at
+Return @code{t} for true if the text after point matches the argument,
+which should be a regular expression.
+
address@hidden eobp
+Return @code{t} for true if point is at the end of the accessible part
+of a buffer. The end of the accessible part is the end of the buffer
+if the buffer is not narrowed; it is the end of the narrowed part if
+the buffer is narrowed.
address@hidden table
+
address@hidden 1500
address@hidden re-search Exercises, , Regexp Review, Regexp Search
address@hidden Exercises with @code{re-search-forward}
+
address@hidden @bullet
address@hidden
+Write a function to search for a regular expression that matches two
+or more blank lines in sequence.
+
address@hidden
+Write a function to search for duplicated words, such as `the the'.
address@hidden, , Syntax of Regular Expressions, emacs, The GNU Emacs
+Manual}, for information on how to write a regexp (a regular
+expression) to match a string that is composed of two identical
+halves. You can devise several regexps; some are better than others.
+The function I use is described in an appendix, along with several
+regexps. @xref{the-the, , @code{the-the} Duplicated Words Function}.
address@hidden itemize
+
address@hidden Counting Words, Words in a defun, Regexp Search, Top
address@hidden Counting: Repetition and Regexps
address@hidden Repetition for word counting
address@hidden Regular expressions for word counting
+
+Repetition and regular expression searches are powerful tools that you
+often use when you write code in Emacs Lisp. This chapter illustrates
+the use of regular expression searches through the construction of
+word count commands using @code{while} loops and recursion.
+
address@hidden
+* Why Count Words::
+* count-words-region:: Use a regexp, but find a problem.
+* recursive-count-words:: Start with case of no words in region.
+* Counting Exercise::
address@hidden menu
+
address@hidden Why Count Words, count-words-region, Counting Words, Counting
Words
address@hidden
address@hidden Counting words
address@hidden ifnottex
+
+The standard Emacs distribution contains a function for counting the
+number of lines within a region. However, there is no corresponding
+function for counting words.
+
+Certain types of writing ask you to count words. Thus, if you write
+an essay, you may be limited to 800 words; if you write a novel, you
+may discipline yourself to write 1000 words a day. It seems odd to me
+that Emacs lacks a word count command. Perhaps people use Emacs
+mostly for code or types of documentation that do not require word
+counts; or perhaps they restrict themselves to the operating system
+word count command, @code{wc}. Alternatively, people may follow
+the publishers' convention and compute a word count by dividing the
+number of characters in a document by five. In any event, here are
+commands to count words.
+
address@hidden count-words-region, recursive-count-words, Why Count Words,
Counting Words
address@hidden node-name, next, previous, up
address@hidden The @code{count-words-region} Function
address@hidden count-words-region
+
+A word count command could count words in a line, paragraph, region,
+or buffer. What should the command cover? You could design the
+command to count the number of words in a complete buffer. However,
+the Emacs tradition encourages flexibility---you may want to count
+words in just a section, rather than all of a buffer. So it makes
+more sense to design the command to count the number of words in a
+region. Once you have a @code{count-words-region} command, you can,
+if you wish, count words in a whole buffer by marking it with
address@hidden@kbd{C-x h}} (@code{mark-whole-buffer}).
+
+Clearly, counting words is a repetitive act: starting from the
+beginning of the region, you count the first word, then the second
+word, then the third word, and so on, until you reach the end of the
+region. This means that word counting is ideally suited to recursion
+or to a @code{while} loop.
+
address@hidden
+* Design count-words-region:: The definition using a @code{while} loop.
+* Whitespace Bug:: The Whitespace Bug in
@code{count-words-region}.
address@hidden menu
+
address@hidden Design count-words-region, Whitespace Bug, count-words-region,
count-words-region
address@hidden
address@hidden Designing @code{count-words-region}
address@hidden ifnottex
+
+First, we will implement the word count command with a @code{while}
+loop, then with recursion. The command will, of course, be
+interactive.
+
address@hidden 800
+The template for an interactive function definition is, as always:
+
address@hidden
address@hidden
+(defun @var{name-of-function} (@var{argument-list})
+ "@address@hidden"
+ (@address@hidden)
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+What we need to do is fill in the slots.
+
+The name of the function should be self-explanatory and similar to the
+existing @code{count-lines-region} name. This makes the name easier
+to remember. @code{count-words-region} is a good choice.
+
+The function counts words within a region. This means that the
+argument list must contain symbols that are bound to the two
+positions, the beginning and end of the region. These two positions
+can be called @samp{beginning} and @samp{end} respectively. The first
+line of the documentation should be a single sentence, since that is
+all that is printed as documentation by a command such as
address@hidden The interactive expression will be of the form
address@hidden(interactive "r")}, since that will cause Emacs to pass the
+beginning and end of the region to the function's argument list. All
+this is routine.
+
+The body of the function needs to be written to do three tasks:
+first, to set up conditions under which the @code{while} loop can
+count words, second, to run the @code{while} loop, and third, to send
+a message to the user.
+
+When a user calls @code{count-words-region}, point may be at the
+beginning or the end of the region. However, the counting process
+must start at the beginning of the region. This means we will want
+to put point there if it is not already there. Executing
address@hidden(goto-char beginning)} ensures this. Of course, we will want to
+return point to its expected position when the function finishes its
+work. For this reason, the body must be enclosed in a
address@hidden expression.
+
+The central part of the body of the function consists of a
address@hidden loop in which one expression jumps point forward word by
+word, and another expression counts those jumps. The true-or-false-test
+of the @code{while} loop should test true so long as point should jump
+forward, and false when point is at the end of the region.
+
+We could use @code{(forward-word 1)} as the expression for moving point
+forward word by word, but it is easier to see what Emacs identifies as a
+`word' if we use a regular expression search.
+
+A regular expression search that finds the pattern for which it is
+searching leaves point after the last character matched. This means
+that a succession of successful word searches will move point forward
+word by word.
+
+As a practical matter, we want the regular expression search to jump
+over whitespace and punctuation between words as well as over the
+words themselves. A regexp that refuses to jump over interword
+whitespace would never jump more than one word! This means that
+the regexp should include the whitespace and punctuation that follows
+a word, if any, as well as the word itself. (A word may end a buffer
+and not have any following whitespace or punctuation, so that part of
+the regexp must be optional.)
+
+Thus, what we want for the regexp is a pattern defining one or more
+word constituent characters followed, optionally, by one or more
+characters that are not word constituents. The regular expression for
+this is:
+
address@hidden
+\w+\W*
address@hidden smallexample
+
address@hidden
+The buffer's syntax table determines which characters are and are not
+word constituents. (@xref{Syntax, , What Constitutes a Word or
+Symbol?}, for more about syntax. Also, see @ref{Syntax, Syntax, The
+Syntax Table, emacs, The GNU Emacs Manual}, and @ref{Syntax Tables, ,
+Syntax Tables, elisp, The GNU Emacs Lisp Reference Manual}.)
+
address@hidden 800
+The search expression looks like this:
+
address@hidden
+(re-search-forward "\\w+\\W*")
address@hidden smallexample
+
address@hidden
+(Note that paired backslashes precede the @samp{w} and @samp{W}. A
+single backslash has special meaning to the Emacs Lisp interpreter.
+It indicates that the following character is interpreted differently
+than usual. For example, the two characters, @samp{\n}, stand for
address@hidden, rather than for a backslash followed by @samp{n}. Two
+backslashes in a row stand for an ordinary, `unspecial' backslash, so
+Emacs Lisp interpreter ends of seeing a single backslash followed by a
+letter. So it discovers the letter is special.)
+
+We need a counter to count how many words there are; this variable
+must first be set to 0 and then incremented each time Emacs goes
+around the @code{while} loop. The incrementing expression is simply:
+
address@hidden
+(setq count (1+ count))
address@hidden smallexample
+
+Finally, we want to tell the user how many words there are in the
+region. The @code{message} function is intended for presenting this
+kind of information to the user. The message has to be phrased so
+that it reads properly regardless of how many words there are in the
+region: we don't want to say that ``there are 1 words in the region''.
+The conflict between singular and plural is ungrammatical. We can
+solve this problem by using a conditional expression that evaluates
+different messages depending on the number of words in the region.
+There are three possibilities: no words in the region, one word in the
+region, and more than one word. This means that the @code{cond}
+special form is appropriate.
+
address@hidden 1500
+All this leads to the following function definition:
+
address@hidden
address@hidden
+;;; @r{First version; has bugs!}
+(defun count-words-region (beginning end)
+ "Print number of words in the region.
+Words are defined as at least one word-constituent
+character followed by at least one character that
+is not a word-constituent. The buffer's syntax
+table determines which characters these are."
+ (interactive "r")
+ (message "Counting words in region ... ")
address@hidden group
+
address@hidden
+;;; @r{1. Set up appropriate conditions.}
+ (save-excursion
+ (goto-char beginning)
+ (let ((count 0))
address@hidden group
+
address@hidden
+;;; @r{2. Run the} while @r{loop.}
+ (while (< (point) end)
+ (re-search-forward "\\w+\\W*")
+ (setq count (1+ count)))
address@hidden group
+
address@hidden
+;;; @r{3. Send a message to the user.}
+ (cond ((zerop count)
+ (message
+ "The region does NOT have any words."))
+ ((= 1 count)
+ (message
+ "The region has 1 word."))
+ (t
+ (message
+ "The region has %d words." count))))))
address@hidden group
address@hidden smallexample
+
address@hidden
+As written, the function works, but not in all circumstances.
+
address@hidden Whitespace Bug, , Design count-words-region, count-words-region
address@hidden node-name, next, previous, up
address@hidden The Whitespace Bug in @code{count-words-region}
+
+The @code{count-words-region} command described in the preceding
+section has two bugs, or rather, one bug with two manifestations.
+First, if you mark a region containing only whitespace in the middle
+of some text, the @code{count-words-region} command tells you that the
+region contains one word! Second, if you mark a region containing
+only whitespace at the end of the buffer or the accessible portion of
+a narrowed buffer, the command displays an error message that looks
+like this:
+
address@hidden
+Search failed: "\\w+\\W*"
address@hidden smallexample
+
+If you are reading this in Info in GNU Emacs, you can test for these
+bugs yourself.
+
+First, evaluate the function in the usual manner to install it.
address@hidden
+Here is a copy of the definition. Place your cursor after the closing
+parenthesis and type @kbd{C-x C-e} to install it.
+
address@hidden
address@hidden
+;; @r{First version; has bugs!}
+(defun count-words-region (beginning end)
+ "Print number of words in the region.
+Words are defined as at least one word-constituent character followed
+by at least one character that is not a word-constituent. The buffer's
+syntax table determines which characters these are."
address@hidden group
address@hidden
+ (interactive "r")
+ (message "Counting words in region ... ")
address@hidden group
+
address@hidden
+;;; @r{1. Set up appropriate conditions.}
+ (save-excursion
+ (goto-char beginning)
+ (let ((count 0))
address@hidden group
+
address@hidden
+;;; @r{2. Run the} while @r{loop.}
+ (while (< (point) end)
+ (re-search-forward "\\w+\\W*")
+ (setq count (1+ count)))
address@hidden group
+
address@hidden
+;;; @r{3. Send a message to the user.}
+ (cond ((zerop count)
+ (message "The region does NOT have any words."))
+ ((= 1 count) (message "The region has 1 word."))
+ (t (message "The region has %d words." count))))))
address@hidden group
address@hidden smallexample
address@hidden ifinfo
+
address@hidden 1000
+If you wish, you can also install this keybinding by evaluating it:
+
address@hidden
+(global-set-key "\C-c=" 'count-words-region)
address@hidden smallexample
+
+To conduct the first test, set mark and point to the beginning and end
+of the following line and then type @kbd{C-c =} (or @kbd{M-x
+count-words-region} if you have not bound @kbd{C-c =}):
+
address@hidden
+ one two three
address@hidden smallexample
+
address@hidden
+Emacs will tell you, correctly, that the region has three words.
+
+Repeat the test, but place mark at the beginning of the line and place
+point just @emph{before} the word @samp{one}. Again type the command
address@hidden =} (or @kbd{M-x count-words-region}). Emacs should tell you
+that the region has no words, since it is composed only of the
+whitespace at the beginning of the line. But instead Emacs tells you
+that the region has one word!
+
+For the third test, copy the sample line to the end of the
address@hidden buffer and then type several spaces at the end of the
+line. Place mark right after the word @samp{three} and point at the
+end of line. (The end of the line will be the end of the buffer.)
+Type @kbd{C-c =} (or @kbd{M-x count-words-region}) as you did before.
+Again, Emacs should tell you that the region has no words, since it is
+composed only of the whitespace at the end of the line. Instead,
+Emacs displays an error message saying @samp{Search failed}.
+
+The two bugs stem from the same problem.
+
+Consider the first manifestation of the bug, in which the command
+tells you that the whitespace at the beginning of the line contains
+one word. What happens is this: The @code{M-x count-words-region}
+command moves point to the beginning of the region. The @code{while}
+tests whether the value of point is smaller than the value of
address@hidden, which it is. Consequently, the regular expression search
+looks for and finds the first word. It leaves point after the word.
address@hidden is set to one. The @code{while} loop repeats; but this
+time the value of point is larger than the value of @code{end}, the
+loop is exited; and the function displays a message saying the number
+of words in the region is one. In brief, the regular expression
+search looks for and finds the word even though it is outside
+the marked region.
+
+In the second manifestation of the bug, the region is whitespace at
+the end of the buffer. Emacs says @samp{Search failed}. What happens
+is that the true-or-false-test in the @code{while} loop tests true, so
+the search expression is executed. But since there are no more words
+in the buffer, the search fails.
+
+In both manifestations of the bug, the search extends or attempts to
+extend outside of the region.
+
+The solution is to limit the search to the region---this is a fairly
+simple action, but as you may have come to expect, it is not quite as
+simple as you might think.
+
+As we have seen, the @code{re-search-forward} function takes a search
+pattern as its first argument. But in addition to this first,
+mandatory argument, it accepts three optional arguments. The optional
+second argument bounds the search. The optional third argument, if
address@hidden, causes the function to return @code{nil} rather than signal
+an error if the search fails. The optional fourth argument is a
+repeat count. (In Emacs, you can see a function's documentation by
+typing @kbd{C-h f}, the name of the function, and then @key{RET}.)
+
+In the @code{count-words-region} definition, the value of the end of
+the region is held by the variable @code{end} which is passed as an
+argument to the function. Thus, we can add @code{end} as an argument
+to the regular expression search expression:
+
address@hidden
+(re-search-forward "\\w+\\W*" end)
address@hidden smallexample
+
+However, if you make only this change to the @code{count-words-region}
+definition and then test the new version of the definition on a
+stretch of whitespace, you will receive an error message saying
address@hidden failed}.
+
+What happens is this: the search is limited to the region, and fails
+as you expect because there are no word-constituent characters in the
+region. Since it fails, we receive an error message. But we do not
+want to receive an error message in this case; we want to receive the
+message that "The region does NOT have any words."
+
+The solution to this problem is to provide @code{re-search-forward}
+with a third argument of @code{t}, which causes the function to return
address@hidden rather than signal an error if the search fails.
+
+However, if you make this change and try it, you will see the message
+``Counting words in region ... '' and @dots{} you will keep on seeing
+that message @dots{}, until you type @kbd{C-g} (@code{keyboard-quit}).
+
+Here is what happens: the search is limited to the region, as before,
+and it fails because there are no word-constituent characters in the
+region, as expected. Consequently, the @code{re-search-forward}
+expression returns @code{nil}. It does nothing else. In particular,
+it does not move point, which it does as a side effect if it finds the
+search target. After the @code{re-search-forward} expression returns
address@hidden, the next expression in the @code{while} loop is evaluated.
+This expression increments the count. Then the loop repeats. The
+true-or-false-test tests true because the value of point is still less
+than the value of end, since the @code{re-search-forward} expression
+did not move point. @dots{} and the cycle repeats @dots{}
+
+The @code{count-words-region} definition requires yet another
+modification, to cause the true-or-false-test of the @code{while} loop
+to test false if the search fails. Put another way, there are two
+conditions that must be satisfied in the true-or-false-test before the
+word count variable is incremented: point must still be within the
+region and the search expression must have found a word to count.
+
+Since both the first condition and the second condition must be true
+together, the two expressions, the region test and the search
+expression, can be joined with an @code{and} special form and embedded in
+the @code{while} loop as the true-or-false-test, like this:
+
address@hidden
+(and (< (point) end) (re-search-forward "\\w+\\W*" end t))
address@hidden smallexample
+
address@hidden colon in printed section title causes problem in Info cross
reference
address@hidden also trouble with an overfull hbox
address@hidden
address@hidden
+(For information about @code{and}, see
address@hidden function, , The @code{kill-new} function}.)
address@hidden iftex
address@hidden
address@hidden
+(@xref{kill-new function, , The @code{kill-new} function}, for
+information about @code{and}.)
address@hidden ifinfo
+
+The @code{re-search-forward} expression returns @code{t} if the search
+succeeds and as a side effect moves point. Consequently, as words are
+found, point is moved through the region. When the search expression
+fails to find another word, or when point reaches the end of the
+region, the true-or-false-test tests false, the @code{while} loop
+exits, and the @code{count-words-region} function displays one or
+other of its messages.
+
+After incorporating these final changes, the @code{count-words-region}
+works without bugs (or at least, without bugs that I have found!).
+Here is what it looks like:
+
address@hidden
address@hidden
+;;; @r{Final version:} @code{while}
+(defun count-words-region (beginning end)
+ "Print number of words in the region."
+ (interactive "r")
+ (message "Counting words in region ... ")
address@hidden group
+
address@hidden
+;;; @r{1. Set up appropriate conditions.}
+ (save-excursion
+ (let ((count 0))
+ (goto-char beginning)
address@hidden group
+
address@hidden
+;;; @r{2. Run the} while @r{loop.}
+ (while (and (< (point) end)
+ (re-search-forward "\\w+\\W*" end t))
+ (setq count (1+ count)))
address@hidden group
+
address@hidden
+;;; @r{3. Send a message to the user.}
+ (cond ((zerop count)
+ (message
+ "The region does NOT have any words."))
+ ((= 1 count)
+ (message
+ "The region has 1 word."))
+ (t
+ (message
+ "The region has %d words." count))))))
address@hidden group
address@hidden smallexample
+
address@hidden recursive-count-words, Counting Exercise, count-words-region,
Counting Words
address@hidden node-name, next, previous, up
address@hidden Count Words Recursively
address@hidden Count words recursively
address@hidden Recursively counting words
address@hidden Words, counted recursively
+
+You can write the function for counting words recursively as well as
+with a @code{while} loop. Let's see how this is done.
+
+First, we need to recognize that the @code{count-words-region}
+function has three jobs: it sets up the appropriate conditions for
+counting to occur; it counts the words in the region; and it sends a
+message to the user telling how many words there are.
+
+If we write a single recursive function to do everything, we will
+receive a message for every recursive call. If the region contains 13
+words, we will receive thirteen messages, one right after the other.
+We don't want this! Instead, we must write two functions to do the
+job, one of which (the recursive function) will be used inside of the
+other. One function will set up the conditions and display the
+message; the other will return the word count.
+
+Let us start with the function that causes the message to be displayed.
+We can continue to call this @code{count-words-region}.
+
+This is the function that the user will call. It will be interactive.
+Indeed, it will be similar to our previous versions of this
+function, except that it will call @code{recursive-count-words} to
+determine how many words are in the region.
+
address@hidden 1250
+We can readily construct a template for this function, based on our
+previous versions:
+
address@hidden
address@hidden
+;; @r{Recursive version; uses regular expression search}
+(defun count-words-region (beginning end)
+ "@address@hidden"
+ (@address@hidden)
address@hidden group
address@hidden
+
+;;; @r{1. Set up appropriate conditions.}
+ (@var{explanatory message})
+ (@var{set-up address@hidden
address@hidden group
address@hidden
+
+;;; @r{2. Count the words.}
+ @var{recursive call}
address@hidden group
address@hidden
+
+;;; @r{3. Send a message to the user.}
+ @var{message providing word count}))
address@hidden group
address@hidden smallexample
+
+The definition looks straightforward, except that somehow the count
+returned by the recursive call must be passed to the message
+displaying the word count. A little thought suggests that this can be
+done by making use of a @code{let} expression: we can bind a variable
+in the varlist of a @code{let} expression to the number of words in
+the region, as returned by the recursive call; and then the
address@hidden expression, using binding, can display the value to the
+user.
+
+Often, one thinks of the binding within a @code{let} expression as
+somehow secondary to the `primary' work of a function. But in this
+case, what you might consider the `primary' job of the function,
+counting words, is done within the @code{let} expression.
+
address@hidden 1250
+Using @code{let}, the function definition looks like this:
+
address@hidden
address@hidden
+(defun count-words-region (beginning end)
+ "Print number of words in the region."
+ (interactive "r")
address@hidden group
+
address@hidden
+;;; @r{1. Set up appropriate conditions.}
+ (message "Counting words in region ... ")
+ (save-excursion
+ (goto-char beginning)
address@hidden group
+
address@hidden
+;;; @r{2. Count the words.}
+ (let ((count (recursive-count-words end)))
address@hidden group
+
address@hidden
+;;; @r{3. Send a message to the user.}
+ (cond ((zerop count)
+ (message
+ "The region does NOT have any words."))
+ ((= 1 count)
+ (message
+ "The region has 1 word."))
+ (t
+ (message
+ "The region has %d words." count))))))
address@hidden group
address@hidden smallexample
+
+Next, we need to write the recursive counting function.
+
+A recursive function has at least three parts: the `do-again-test', the
+`next-step-expression', and the recursive call.
+
+The do-again-test determines whether the function will or will not be
+called again. Since we are counting words in a region and can use a
+function that moves point forward for every word, the do-again-test
+can check whether point is still within the region. The do-again-test
+should find the value of point and determine whether point is before,
+at, or after the value of the end of the region. We can use the
address@hidden function to locate point. Clearly, we must pass the
+value of the end of the region to the recursive counting function as an
+argument.
+
+In addition, the do-again-test should also test whether the search finds a
+word. If it does not, the function should not call itself again.
+
+The next-step-expression changes a value so that when the recursive
+function is supposed to stop calling itself, it stops. More
+precisely, the next-step-expression changes a value so that at the
+right time, the do-again-test stops the recursive function from
+calling itself again. In this case, the next-step-expression can be
+the expression that moves point forward, word by word.
+
+The third part of a recursive function is the recursive call.
+
+Somewhere, also, we also need a part that does the `work' of the
+function, a part that does the counting. A vital part!
+
address@hidden 1250
+But already, we have an outline of the recursive counting function:
+
address@hidden
address@hidden
+(defun recursive-count-words (region-end)
+ "@address@hidden"
+ @var{do-again-test}
+ @var{next-step-expression}
+ @var{recursive call})
address@hidden group
address@hidden smallexample
+
+Now we need to fill in the slots. Let's start with the simplest cases
+first: if point is at or beyond the end of the region, there cannot
+be any words in the region, so the function should return zero.
+Likewise, if the search fails, there are no words to count, so the
+function should return zero.
+
+On the other hand, if point is within the region and the search
+succeeds, the function should call itself again.
+
address@hidden 800
+Thus, the do-again-test should look like this:
+
address@hidden
address@hidden
+(and (< (point) region-end)
+ (re-search-forward "\\w+\\W*" region-end t))
address@hidden group
address@hidden smallexample
+
+Note that the search expression is part of the do-again-test---the
+function returns @code{t} if its search succeeds and @code{nil} if it
+fails. (@xref{Whitespace Bug, , The Whitespace Bug in
address@hidden, for an explanation of how
address@hidden works.)
+
+The do-again-test is the true-or-false test of an @code{if} clause.
+Clearly, if the do-again-test succeeds, the then-part of the @code{if}
+clause should call the function again; but if it fails, the else-part
+should return zero since either point is outside the region or the
+search failed because there were no words to find.
+
+But before considering the recursive call, we need to consider the
+next-step-expression. What is it? Interestingly, it is the search
+part of the do-again-test.
+
+In addition to returning @code{t} or @code{nil} for the
+do-again-test, @code{re-search-forward} moves point forward as a side
+effect of a successful search. This is the action that changes the
+value of point so that the recursive function stops calling itself
+when point completes its movement through the region. Consequently,
+the @code{re-search-forward} expression is the next-step-expression.
+
address@hidden 1200
+In outline, then, the body of the @code{recursive-count-words}
+function looks like this:
+
address@hidden
address@hidden
+(if @var{do-again-test-and-next-step-combined}
+ ;; @r{then}
+ @var{recursive-call-returning-count}
+ ;; @r{else}
+ @var{return-zero})
address@hidden group
address@hidden smallexample
+
+How to incorporate the mechanism that counts?
+
+If you are not used to writing recursive functions, a question like
+this can be troublesome. But it can and should be approached
+systematically.
+
+We know that the counting mechanism should be associated in some way
+with the recursive call. Indeed, since the next-step-expression moves
+point forward by one word, and since a recursive call is made for
+each word, the counting mechanism must be an expression that adds one
+to the value returned by a call to @code{recursive-count-words}.
+
address@hidden 800
+Consider several cases:
+
address@hidden @bullet
address@hidden
+If there are two words in the region, the function should return
+a value resulting from adding one to the value returned when it counts
+the first word, plus the number returned when it counts the remaining
+words in the region, which in this case is one.
+
address@hidden
+If there is one word in the region, the function should return
+a value resulting from adding one to the value returned when it counts
+that word, plus the number returned when it counts the remaining
+words in the region, which in this case is zero.
+
address@hidden
+If there are no words in the region, the function should return zero.
address@hidden itemize
+
+From the sketch we can see that the else-part of the @code{if} returns
+zero for the case of no words. This means that the then-part of the
address@hidden must return a value resulting from adding one to the value
+returned from a count of the remaining words.
+
address@hidden 1200
+The expression will look like this, where @code{1+} is a function that
+adds one to its argument.
+
address@hidden
+(1+ (recursive-count-words region-end))
address@hidden smallexample
+
address@hidden 1200
+The whole @code{recursive-count-words} function will then look like
+this:
+
address@hidden
address@hidden
+(defun recursive-count-words (region-end)
+ "@address@hidden"
+
+;;; @r{1. do-again-test}
+ (if (and (< (point) region-end)
+ (re-search-forward "\\w+\\W*" region-end t))
address@hidden group
+
address@hidden
+;;; @r{2. then-part: the recursive call}
+ (1+ (recursive-count-words region-end))
+
+;;; @r{3. else-part}
+ 0))
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+Let's examine how this works:
+
+If there are no words in the region, the else part of the @code{if}
+expression is evaluated and consequently the function returns zero.
+
+If there is one word in the region, the value of point is less than
+the value of @code{region-end} and the search succeeds. In this case,
+the true-or-false-test of the @code{if} expression tests true, and the
+then-part of the @code{if} expression is evaluated. The counting
+expression is evaluated. This expression returns a value (which will
+be the value returned by the whole function) that is the sum of one
+added to the value returned by a recursive call.
+
+Meanwhile, the next-step-expression has caused point to jump over the
+first (and in this case only) word in the region. This means that
+when @code{(recursive-count-words region-end)} is evaluated a second
+time, as a result of the recursive call, the value of point will be
+equal to or greater than the value of region end. So this time,
address@hidden will return zero. The zero will be added
+to one, and the original evaluation of @code{recursive-count-words}
+will return one plus zero, which is one, which is the correct amount.
+
+Clearly, if there are two words in the region, the first call to
address@hidden returns one added to the value returned
+by calling @code{recursive-count-words} on a region containing the
+remaining word---that is, it adds one to one, producing two, which is
+the correct amount.
+
+Similarly, if there are three words in the region, the first call to
address@hidden returns one added to the value returned
+by calling @code{recursive-count-words} on a region containing the
+remaining two words---and so on and so on.
+
address@hidden 1250
address@hidden
+With full documentation the two functions look like this:
+
address@hidden 1250
address@hidden
+The recursive function:
+
address@hidden recursive-count-words
address@hidden
address@hidden
+(defun recursive-count-words (region-end)
+ "Number of words between point and REGION-END."
address@hidden group
+
address@hidden
+;;; @r{1. do-again-test}
+ (if (and (< (point) region-end)
+ (re-search-forward "\\w+\\W*" region-end t))
address@hidden group
+
address@hidden
+;;; @r{2. then-part: the recursive call}
+ (1+ (recursive-count-words region-end))
+
+;;; @r{3. else-part}
+ 0))
address@hidden group
address@hidden smallexample
+
address@hidden 800
address@hidden
+The wrapper:
+
address@hidden
address@hidden
+;;; @r{Recursive version}
+(defun count-words-region (beginning end)
+ "Print number of words in the region.
address@hidden group
+
address@hidden
+Words are defined as at least one word-constituent
+character followed by at least one character that is
+not a word-constituent. The buffer's syntax table
+determines which characters these are."
address@hidden group
address@hidden
+ (interactive "r")
+ (message "Counting words in region ... ")
+ (save-excursion
+ (goto-char beginning)
+ (let ((count (recursive-count-words end)))
address@hidden group
address@hidden
+ (cond ((zerop count)
+ (message
+ "The region does NOT have any words."))
address@hidden group
address@hidden
+ ((= 1 count)
+ (message "The region has 1 word."))
+ (t
+ (message
+ "The region has %d words." count))))))
address@hidden group
address@hidden smallexample
+
address@hidden Counting Exercise, , recursive-count-words, Counting Words
address@hidden Exercise: Counting Punctuation
+
+Using a @code{while} loop, write a function to count the number of
+punctuation marks in a region---period, comma, semicolon, colon,
+exclamation mark, and question mark. Do the same using recursion.
+
address@hidden Words in a defun, Readying a Graph, Counting Words, Top
address@hidden Counting Words in a @code{defun}
address@hidden Counting words in a @code{defun}
address@hidden Word counting in a @code{defun}
+
+Our next project is to count the number of words in a function
+definition. Clearly, this can be done using some variant of
address@hidden @xref{Counting Words, , Counting Words:
+Repetition and Regexps}. If we are just going to count the words in
+one definition, it is easy enough to mark the definition with the
address@hidden (@code{mark-defun}) command, and then call
address@hidden
+
+However, I am more ambitious: I want to count the words and symbols in
+every definition in the Emacs sources and then print a graph that
+shows how many functions there are of each length: how many contain 40
+to 49 words or symbols, how many contain 50 to 59 words or symbols,
+and so on. I have often been curious how long a typical function is,
+and this will tell.
+
address@hidden
+* Divide and Conquer::
+* Words and Symbols:: What to count?
+* Syntax:: What constitutes a word or symbol?
+* count-words-in-defun:: Very like @code{count-words}.
+* Several defuns:: Counting several defuns in a file.
+* Find a File:: Do you want to look at a file?
+* lengths-list-file:: A list of the lengths of many definitions.
+* Several files:: Counting in definitions in different files.
+* Several files recursively:: Recursively counting in different files.
+* Prepare the data:: Prepare the data for display in a graph.
address@hidden menu
+
address@hidden Divide and Conquer, Words and Symbols, Words in a defun, Words
in a defun
address@hidden
address@hidden Divide and Conquer
address@hidden ifnottex
+
+Described in one phrase, the histogram project is daunting; but
+divided into numerous small steps, each of which we can take one at a
+time, the project becomes less fearsome. Let us consider what the
+steps must be:
+
address@hidden @bullet
address@hidden
+First, write a function to count the words in one definition. This
+includes the problem of handling symbols as well as words.
+
address@hidden
+Second, write a function to list the numbers of words in each function
+in a file. This function can use the @code{count-words-in-defun}
+function.
+
address@hidden
+Third, write a function to list the numbers of words in each function
+in each of several files. This entails automatically finding the
+various files, switching to them, and counting the words in the
+definitions within them.
+
address@hidden
+Fourth, write a function to convert the list of numbers that we
+created in step three to a form that will be suitable for printing as
+a graph.
+
address@hidden
+Fifth, write a function to print the results as a graph.
address@hidden itemize
+
+This is quite a project! But if we take each step slowly, it will not
+be difficult.
+
address@hidden Words and Symbols, Syntax, Divide and Conquer, Words in a defun
address@hidden What to Count?
address@hidden Words and symbols in defun
+
+When we first start thinking about how to count the words in a
+function definition, the first question is (or ought to be) what are
+we going to count? When we speak of `words' with respect to a Lisp
+function definition, we are actually speaking, in large part, of
+`symbols'. For example, the following @code{multiply-by-seven}
+function contains the five symbols @code{defun},
address@hidden, @code{number}, @code{*}, and @code{7}. In
+addition, in the documentation string, it contains the four words
address@hidden, @samp{NUMBER}, @samp{by}, and @samp{seven}. The
+symbol @samp{number} is repeated, so the definition contains a total
+of ten words and symbols.
+
address@hidden
address@hidden
+(defun multiply-by-seven (number)
+ "Multiply NUMBER by seven."
+ (* 7 number))
address@hidden group
address@hidden smallexample
+
address@hidden
+However, if we mark the @code{multiply-by-seven} definition with
address@hidden (@code{mark-defun}), and then call
address@hidden on it, we will find that
address@hidden claims the definition has eleven words, not
+ten! Something is wrong!
+
+The problem is twofold: @code{count-words-region} does not count the
address@hidden as a word, and it counts the single symbol,
address@hidden, as containing three words. The hyphens are
+treated as if they were interword spaces rather than intraword
+connectors: @samp{multiply-by-seven} is counted as if it were written
address@hidden by seven}.
+
+The cause of this confusion is the regular expression search within
+the @code{count-words-region} definition that moves point forward word
+by word. In the canonical version of @code{count-words-region}, the
+regexp is:
+
address@hidden
+"\\w+\\W*"
address@hidden smallexample
+
address@hidden
+This regular expression is a pattern defining one or more word
+constituent characters possibly followed by one or more characters
+that are not word constituents. What is meant by `word constituent
+characters' brings us to the issue of syntax, which is worth a section
+of its own.
+
address@hidden Syntax, count-words-in-defun, Words and Symbols, Words in a defun
address@hidden What Constitutes a Word or Symbol?
address@hidden Syntax categories and tables
+
+Emacs treats different characters as belonging to different
address@hidden categories}. For example, the regular expression,
address@hidden, is a pattern specifying one or more @emph{word
+constituent} characters. Word constituent characters are members of
+one syntax category. Other syntax categories include the class of
+punctuation characters, such as the period and the comma, and the
+class of whitespace characters, such as the blank space and the tab
+character. (For more information, see @ref{Syntax, Syntax, The Syntax
+Table, emacs, The GNU Emacs Manual}, and @ref{Syntax Tables, , Syntax
+Tables, elisp, The GNU Emacs Lisp Reference Manual}.)
+
+Syntax tables specify which characters belong to which categories.
+Usually, a hyphen is not specified as a `word constituent character'.
+Instead, it is specified as being in the `class of characters that are
+part of symbol names but not words.' This means that the
address@hidden function treats it in the same way it treats
+an interword white space, which is why @code{count-words-region}
+counts @samp{multiply-by-seven} as three words.
+
+There are two ways to cause Emacs to count @samp{multiply-by-seven} as
+one symbol: modify the syntax table or modify the regular expression.
+
+We could redefine a hyphen as a word constituent character by
+modifying the syntax table that Emacs keeps for each mode. This
+action would serve our purpose, except that a hyphen is merely the
+most common character within symbols that is not typically a word
+constituent character; there are others, too.
+
+Alternatively, we can redefine the regular expression used in the
address@hidden definition so as to include symbols. This
+procedure has the merit of clarity, but the task is a little tricky.
+
address@hidden 1200
+The first part is simple enough: the pattern must match ``at least one
+character that is a word or symbol constituent''. Thus:
+
address@hidden
+"\\(\\w\\|\\s_\\)+"
address@hidden smallexample
+
address@hidden
+The @samp{\\(} is the first part of the grouping construct that
+includes the @samp{\\w} and the @samp{\\s_} as alternatives, separated
+by the @samp{\\|}. The @samp{\\w} matches any word-constituent
+character and the @samp{\\s_} matches any character that is part of a
+symbol name but not a word-constituent character. The @samp{+}
+following the group indicates that the word or symbol constituent
+characters must be matched at least once.
+
+However, the second part of the regexp is more difficult to design.
+What we want is to follow the first part with ``optionally one or more
+characters that are not constituents of a word or symbol''. At first,
+I thought I could define this with the following:
+
address@hidden
+"\\(\\W\\|\\S_\\)*"
address@hidden smallexample
+
address@hidden
+The upper case @samp{W} and @samp{S} match characters that are
address@hidden word or symbol constituents. Unfortunately, this
+expression matches any character that is either not a word constituent
+or not a symbol constituent. This matches any character!
+
+I then noticed that every word or symbol in my test region was
+followed by white space (blank space, tab, or newline). So I tried
+placing a pattern to match one or more blank spaces after the pattern
+for one or more word or symbol constituents. This failed, too. Words
+and symbols are often separated by whitespace, but in actual code
+parentheses may follow symbols and punctuation may follow words. So
+finally, I designed a pattern in which the word or symbol constituents
+are followed optionally by characters that are not white space and
+then followed optionally by white space.
+
address@hidden 800
+Here is the full regular expression:
+
address@hidden
+"\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"
address@hidden smallexample
+
address@hidden count-words-in-defun, Several defuns, Syntax, Words in a defun
address@hidden The @code{count-words-in-defun} Function
address@hidden Counting words in a @code{defun}
+
+We have seen that there are several ways to write a
address@hidden function. To write a
address@hidden, we need merely adapt one of these
+versions.
+
+The version that uses a @code{while} loop is easy to understand, so I
+am going to adapt that. Because @code{count-words-in-defun} will be
+part of a more complex program, it need not be interactive and it need
+not display a message but just return the count. These considerations
+simplify the definition a little.
+
+On the other hand, @code{count-words-in-defun} will be used within a
+buffer that contains function definitions. Consequently, it is
+reasonable to ask that the function determine whether it is called
+when point is within a function definition, and if it is, to return
+the count for that definition. This adds complexity to the
+definition, but saves us from needing to pass arguments to the
+function.
+
address@hidden 1250
+These considerations lead us to prepare the following template:
+
address@hidden
address@hidden
+(defun count-words-in-defun ()
+ "@address@hidden"
+ (@var{set address@hidden
+ (@var{while address@hidden)
+ @var{return count})
address@hidden group
address@hidden smallexample
+
address@hidden
+As usual, our job is to fill in the slots.
+
+First, the set up.
+
+We are presuming that this function will be called within a buffer
+containing function definitions. Point will either be within a
+function definition or not. For @code{count-words-in-defun} to work,
+point must move to the beginning of the definition, a counter must
+start at zero, and the counting loop must stop when point reaches the
+end of the definition.
+
+The @code{beginning-of-defun} function searches backwards for an
+opening delimiter such as a @samp{(} at the beginning of a line, and
+moves point to that position, or else to the limit of the search. In
+practice, this means that @code{beginning-of-defun} moves point to the
+beginning of an enclosing or preceding function definition, or else to
+the beginning of the buffer. We can use @code{beginning-of-defun} to
+place point where we wish to start.
+
+The @code{while} loop requires a counter to keep track of the words or
+symbols being counted. A @code{let} expression can be used to create
+a local variable for this purpose, and bind it to an initial value of zero.
+
+The @code{end-of-defun} function works like @code{beginning-of-defun}
+except that it moves point to the end of the definition.
address@hidden can be used as part of an expression that
+determines the position of the end of the definition.
+
+The set up for @code{count-words-in-defun} takes shape rapidly: first
+we move point to the beginning of the definition, then we create a
+local variable to hold the count, and finally, we record the position
+of the end of the definition so the @code{while} loop will know when to stop
+looping.
+
address@hidden 1250
+The code looks like this:
+
address@hidden
address@hidden
+(beginning-of-defun)
+(let ((count 0)
+ (end (save-excursion (end-of-defun) (point))))
address@hidden group
address@hidden smallexample
+
address@hidden
+The code is simple. The only slight complication is likely to concern
address@hidden: it is bound to the position of the end of the definition
+by a @code{save-excursion} expression that returns the value of point
+after @code{end-of-defun} temporarily moves it to the end of the
+definition.
+
+The second part of the @code{count-words-in-defun}, after the set up,
+is the @code{while} loop.
+
+The loop must contain an expression that jumps point forward word by
+word and symbol by symbol, and another expression that counts the
+jumps. The true-or-false-test for the @code{while} loop should test
+true so long as point should jump forward, and false when point is at
+the end of the definition. We have already redefined the regular
+expression for this (@pxref{Syntax}), so the loop is straightforward:
+
address@hidden
address@hidden
+(while (and (< (point) end)
+ (re-search-forward
+ "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" end t)
+ (setq count (1+ count)))
address@hidden group
address@hidden smallexample
+
+The third part of the function definition returns the count of words
+and symbols. This part is the last expression within the body of the
address@hidden expression, and can be, very simply, the local variable
address@hidden, which when evaluated returns the count.
+
address@hidden 1250
+Put together, the @code{count-words-in-defun} definition looks like this:
+
address@hidden count-words-in-defun
address@hidden
address@hidden
+(defun count-words-in-defun ()
+ "Return the number of words and symbols in a defun."
+ (beginning-of-defun)
+ (let ((count 0)
+ (end (save-excursion (end-of-defun) (point))))
address@hidden group
address@hidden
+ (while
+ (and (< (point) end)
+ (re-search-forward
+ "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"
+ end t))
+ (setq count (1+ count)))
+ count))
address@hidden group
address@hidden smallexample
+
+How to test this? The function is not interactive, but it is easy to
+put a wrapper around the function to make it interactive; we can use
+almost the same code as for the recursive version of
address@hidden:
+
address@hidden
address@hidden
+;;; @r{Interactive version.}
+(defun count-words-defun ()
+ "Number of words and symbols in a function definition."
+ (interactive)
+ (message
+ "Counting words and symbols in function definition ... ")
address@hidden group
address@hidden
+ (let ((count (count-words-in-defun)))
+ (cond
+ ((zerop count)
+ (message
+ "The definition does NOT have any words or symbols."))
address@hidden group
address@hidden
+ ((= 1 count)
+ (message
+ "The definition has 1 word or symbol."))
+ (t
+ (message
+ "The definition has %d words or symbols." count)))))
address@hidden group
address@hidden smallexample
+
address@hidden 800
address@hidden
+Let's re-use @kbd{C-c =} as a convenient keybinding:
+
address@hidden
+(global-set-key "\C-c=" 'count-words-defun)
address@hidden smallexample
+
+Now we can try out @code{count-words-defun}: install both
address@hidden and @code{count-words-defun}, and set the
+keybinding, and then place the cursor within the following definition:
+
address@hidden
address@hidden
+(defun multiply-by-seven (number)
+ "Multiply NUMBER by seven."
+ (* 7 number))
+ @result{} 10
address@hidden group
address@hidden smallexample
+
address@hidden
+Success! The definition has 10 words and symbols.
+
+The next problem is to count the numbers of words and symbols in
+several definitions within a single file.
+
address@hidden Several defuns, Find a File, count-words-in-defun, Words in a
defun
address@hidden Count Several @code{defuns} Within a File
+
+A file such as @file{simple.el} may have a hundred or more function
+definitions within it. Our long term goal is to collect statistics on
+many files, but as a first step, our immediate goal is to collect
+statistics on one file.
+
+The information will be a series of numbers, each number being the
+length of a function definition. We can store the numbers in a list.
+
+We know that we will want to incorporate the information regarding one
+file with information about many other files; this means that the
+function for counting definition lengths within one file need only
+return the list of lengths. It need not and should not display any
+messages.
+
+The word count commands contain one expression to jump point forward
+word by word and another expression to count the jumps. The function
+to return the lengths of definitions can be designed to work the same
+way, with one expression to jump point forward definition by
+definition and another expression to construct the lengths' list.
+
+This statement of the problem makes it elementary to write the
+function definition. Clearly, we will start the count at the
+beginning of the file, so the first command will be @code{(goto-char
+(point-min))}. Next, we start the @code{while} loop; and the
+true-or-false test of the loop can be a regular expression search for
+the next function definition---so long as the search succeeds, point
+is moved forward and then the body of the loop is evaluated. The body
+needs an expression that constructs the lengths' list. @code{cons},
+the list construction command, can be used to create the list. That
+is almost all there is to it.
+
address@hidden 800
+Here is what this fragment of code looks like:
+
address@hidden
address@hidden
+(goto-char (point-min))
+(while (re-search-forward "^(defun" nil t)
+ (setq lengths-list
+ (cons (count-words-in-defun) lengths-list)))
address@hidden group
address@hidden smallexample
+
+What we have left out is the mechanism for finding the file that
+contains the function definitions.
+
+In previous examples, we either used this, the Info file, or we
+switched back and forth to some other buffer, such as the
address@hidden buffer.
+
+Finding a file is a new process that we have not yet discussed.
+
address@hidden Find a File, lengths-list-file, Several defuns, Words in a defun
address@hidden node-name, next, previous, up
address@hidden Find a File
address@hidden Find a File
+
+To find a file in Emacs, you use the @kbd{C-x C-f} (@code{find-file})
+command. This command is almost, but not quite right for the lengths
+problem.
+
address@hidden 1200
+Let's look at the source for @code{find-file}:
+
address@hidden
address@hidden
+(defun find-file (filename)
+ "Edit file FILENAME.
+Switch to a buffer visiting file FILENAME,
+creating one if none already exists."
+ (interactive "FFind file: ")
+ (switch-to-buffer (find-file-noselect filename)))
address@hidden group
address@hidden smallexample
+
address@hidden
+(The most recent version of the @code{find-file} function definition
+permits you to specify optional wildcards to visit multiple files; that
+makes the definition more complex and we will not discuss it here,
+since it is not relevant. You can see its source using either
address@hidden (@code{find-tag}) or @kbd{C-h f} (@code{describe-function}).)
+
address@hidden
+In Emacs 22
+(defun find-file (filename &optional wildcards)
+ "Edit file FILENAME.
+Switch to a buffer visiting file FILENAME,
+creating one if none already exists.
+Interactively, the default if you just type RET is the current directory,
+but the visited file name is available through the minibuffer history:
+type M-n to pull it into the minibuffer.
+
+Interactively, or if WILDCARDS is non-nil in a call from Lisp,
+expand wildcards (if any) and visit multiple files. You can
+suppress wildcard expansion by setting `find-file-wildcards' to nil.
+
+To visit a file without any kind of conversion and without
+automatically choosing a major mode, use \\[find-file-literally]."
+ (interactive (find-file-read-args "Find file: " nil))
+ (let ((value (find-file-noselect filename nil nil wildcards)))
+ (if (listp value)
+ (mapcar 'switch-to-buffer (nreverse value))
+ (switch-to-buffer value))))
address@hidden ignore
+
+The definition I am showing possesses short but complete documentation
+and an interactive specification that prompts you for a file name when
+you use the command interactively. The body of the definition
+contains two functions, @code{find-file-noselect} and
address@hidden
+
+According to its documentation as shown by @kbd{C-h f} (the
address@hidden command), the @code{find-file-noselect}
+function reads the named file into a buffer and returns the buffer.
+(Its most recent version includes an optional wildcards argument,
+too, as well as another to read a file literally and an other you
+suppress warning messages. These optional arguments are irrelevant.)
+
+However, the @code{find-file-noselect} function does not select the
+buffer in which it puts the file. Emacs does not switch its attention
+(or yours if you are using @code{find-file-noselect}) to the selected
+buffer. That is what @code{switch-to-buffer} does: it switches the
+buffer to which Emacs attention is directed; and it switches the
+buffer displayed in the window to the new buffer. We have discussed
+buffer switching elsewhere. (@xref{Switching Buffers}.)
+
+In this histogram project, we do not need to display each file on the
+screen as the program determines the length of each definition within
+it. Instead of employing @code{switch-to-buffer}, we can work with
address@hidden, which redirects the attention of the computer
+program to a different buffer but does not redisplay it on the screen.
+So instead of calling on @code{find-file} to do the job, we must write
+our own expression.
+
+The task is easy: use @code{find-file-noselect} and @code{set-buffer}.
+
address@hidden lengths-list-file, Several files, Find a File, Words in a defun
address@hidden @code{lengths-list-file} in Detail
+
+The core of the @code{lengths-list-file} function is a @code{while}
+loop containing a function to move point forward `defun by defun' and
+a function to count the number of words and symbols in each defun.
+This core must be surrounded by functions that do various other tasks,
+including finding the file, and ensuring that point starts out at the
+beginning of the file. The function definition looks like this:
address@hidden lengths-list-file
+
address@hidden
address@hidden
+(defun lengths-list-file (filename)
+ "Return list of definitions' lengths within FILE.
+The returned list is a list of numbers.
+Each number is the number of words or
+symbols in one function definition."
address@hidden group
address@hidden
+ (message "Working on `%s' ... " filename)
+ (save-excursion
+ (let ((buffer (find-file-noselect filename))
+ (lengths-list))
+ (set-buffer buffer)
+ (setq buffer-read-only t)
+ (widen)
+ (goto-char (point-min))
+ (while (re-search-forward "^(defun" nil t)
+ (setq lengths-list
+ (cons (count-words-in-defun) lengths-list)))
+ (kill-buffer buffer)
+ lengths-list)))
address@hidden group
address@hidden smallexample
+
address@hidden
+The function is passed one argument, the name of the file on which it
+will work. It has four lines of documentation, but no interactive
+specification. Since people worry that a computer is broken if they
+don't see anything going on, the first line of the body is a
+message.
+
+The next line contains a @code{save-excursion} that returns Emacs'
+attention to the current buffer when the function completes. This is
+useful in case you embed this function in another function that
+presumes point is restored to the original buffer.
+
+In the varlist of the @code{let} expression, Emacs finds the file and
+binds the local variable @code{buffer} to the buffer containing the
+file. At the same time, Emacs creates @code{lengths-list} as a local
+variable.
+
+Next, Emacs switches its attention to the buffer.
+
+In the following line, Emacs makes the buffer read-only. Ideally,
+this line is not necessary. None of the functions for counting words
+and symbols in a function definition should change the buffer.
+Besides, the buffer is not going to be saved, even if it were changed.
+This line is entirely the consequence of great, perhaps excessive,
+caution. The reason for the caution is that this function and those
+it calls work on the sources for Emacs and it is inconvenient if they
+are inadvertently modified. It goes without saying that I did not
+realize a need for this line until an experiment went awry and started
+to modify my Emacs source files @dots{}
+
+Next comes a call to widen the buffer if it is narrowed. This
+function is usually not needed---Emacs creates a fresh buffer if none
+already exists; but if a buffer visiting the file already exists Emacs
+returns that one. In this case, the buffer may be narrowed and must
+be widened. If we wanted to be fully `user-friendly', we would
+arrange to save the restriction and the location of point, but we
+won't.
+
+The @code{(goto-char (point-min))} expression moves point to the
+beginning of the buffer.
+
+Then comes a @code{while} loop in which the `work' of the function is
+carried out. In the loop, Emacs determines the length of each
+definition and constructs a lengths' list containing the information.
+
+Emacs kills the buffer after working through it. This is to save
+space inside of Emacs. My version of GNU Emacs 19 contained over 300
+source files of interest; GNU Emacs 22 contains over a thousand source
+files. Another function will apply @code{lengths-list-file} to each
+of the files.
+
+Finally, the last expression within the @code{let} expression is the
address@hidden variable; its value is returned as the value of
+the whole function.
+
+You can try this function by installing it in the usual fashion. Then
+place your cursor after the following expression and type @kbd{C-x
+C-e} (@code{eval-last-sexp}).
+
address@hidden !!! 22.1.1 lisp sources location here
address@hidden
+(lengths-list-file
+ "/usr/local/share/emacs/22.1.1/lisp/emacs-lisp/debug.el")
address@hidden smallexample
+
address@hidden
+(You may need to change the pathname of the file; the one here is for
+GNU Emacs version 22.1.1. To change the expression, copy it to
+the @file{*scratch*} buffer and edit it.
+
address@hidden 1200
address@hidden
+(Also, to see the full length of the list, rather than a truncated
+version, you may have to evaluate the following:
+
address@hidden
+(custom-set-variables '(eval-expression-print-length nil))
address@hidden smallexample
+
address@hidden
+(@xref{defcustom, , Specifying Variables using @code{defcustom}}.
+Then evaluate the @code{lengths-list-file} expression.)
+
address@hidden 1200
+The lengths' list for @file{debug.el} takes less than a second to
+produce and looks like this in GNU Emacs 22:
+
address@hidden
+(83 113 105 144 289 22 30 97 48 89 25 52 52 88 28 29 77 49 43 290 232 587)
address@hidden smallexample
+
address@hidden 1500
+(Using my old machine, the version 19 lengths' list for @file{debug.el}
+took seven seconds to produce and looked like this:
+
address@hidden
+(75 41 80 62 20 45 44 68 45 12 34 235)
address@hidden smallexample
+
+(The newer version of @file{debug.el} contains more defuns than the
+earlier one; and my new machine is much faster than the old one.)
+
+Note that the length of the last definition in the file is first in
+the list.
+
address@hidden Several files, Several files recursively, lengths-list-file,
Words in a defun
address@hidden Count Words in @code{defuns} in Different Files
+
+In the previous section, we created a function that returns a list of
+the lengths of each definition in a file. Now, we want to define a
+function to return a master list of the lengths of the definitions in
+a list of files.
+
+Working on each of a list of files is a repetitious act, so we can use
+either a @code{while} loop or recursion.
+
address@hidden
+* lengths-list-many-files:: Return a list of the lengths of defuns.
+* append:: Attach one list to another.
address@hidden menu
+
address@hidden lengths-list-many-files, append, Several files, Several files
address@hidden
address@hidden Determine the lengths of @code{defuns}
address@hidden ifnottex
+
+The design using a @code{while} loop is routine. The argument passed
+the function is a list of files. As we saw earlier (@pxref{Loop
+Example}), you can write a @code{while} loop so that the body of the
+loop is evaluated if such a list contains elements, but to exit the
+loop if the list is empty. For this design to work, the body of the
+loop must contain an expression that shortens the list each time the
+body is evaluated, so that eventually the list is empty. The usual
+technique is to set the value of the list to the value of the @sc{cdr}
+of the list each time the body is evaluated.
+
address@hidden 800
+The template looks like this:
+
address@hidden
address@hidden
+(while @var{test-whether-list-is-empty}
+ @address@hidden
+ @var{set-list-to-cdr-of-list})
address@hidden group
address@hidden smallexample
+
+Also, we remember that a @code{while} loop returns @code{nil} (the
+result of evaluating the true-or-false-test), not the result of any
+evaluation within its body. (The evaluations within the body of the
+loop are done for their side effects.) However, the expression that
+sets the lengths' list is part of the body---and that is the value
+that we want returned by the function as a whole. To do this, we
+enclose the @code{while} loop within a @code{let} expression, and
+arrange that the last element of the @code{let} expression contains
+the value of the lengths' list. (@xref{Incrementing Example, , Loop
+Example with an Incrementing Counter}.)
+
address@hidden lengths-list-many-files
address@hidden 1250
+These considerations lead us directly to the function itself:
+
address@hidden
address@hidden
+;;; @r{Use @code{while} loop.}
+(defun lengths-list-many-files (list-of-files)
+ "Return list of lengths of defuns in LIST-OF-FILES."
address@hidden group
address@hidden
+ (let (lengths-list)
+
+;;; @r{true-or-false-test}
+ (while list-of-files
+ (setq lengths-list
+ (append
+ lengths-list
+
+;;; @r{Generate a lengths' list.}
+ (lengths-list-file
+ (expand-file-name (car list-of-files)))))
address@hidden group
+
address@hidden
+;;; @r{Make files' list shorter.}
+ (setq list-of-files (cdr list-of-files)))
+
+;;; @r{Return final value of lengths' list.}
+ lengths-list))
address@hidden group
address@hidden smallexample
+
address@hidden is a built-in function that converts a file
+name to the absolute, long, path name form. The function employs the
+name of the directory in which the function is called.
+
address@hidden !!! 22.1.1 lisp sources location here
address@hidden 1500
+Thus, if @code{expand-file-name} is called on @code{debug.el} when
+Emacs is visiting the
address@hidden/usr/local/share/emacs/22.1.1/lisp/emacs-lisp/} directory,
+
address@hidden
+debug.el
address@hidden smallexample
+
address@hidden 800
address@hidden
+becomes
+
address@hidden !!! 22.1.1 lisp sources location here
address@hidden
+/usr/local/share/emacs/22.1.1/lisp/emacs-lisp/debug.el
address@hidden smallexample
+
+The only other new element of this function definition is the as yet
+unstudied function @code{append}, which merits a short section for
+itself.
+
address@hidden append, , lengths-list-many-files, Several files
address@hidden The @code{append} Function
+
address@hidden 800
+The @code{append} function attaches one list to another. Thus,
+
address@hidden
+(append '(1 2 3 4) '(5 6 7 8))
address@hidden smallexample
+
address@hidden 800
address@hidden
+produces the list
+
address@hidden
+(1 2 3 4 5 6 7 8)
address@hidden smallexample
+
+This is exactly how we want to attach two lengths' lists produced by
address@hidden to each other. The results contrast with
address@hidden,
+
address@hidden
+(cons '(1 2 3 4) '(5 6 7 8))
address@hidden smallexample
+
address@hidden 1250
address@hidden
+which constructs a new list in which the first argument to @code{cons}
+becomes the first element of the new list:
+
address@hidden
+((1 2 3 4) 5 6 7 8)
address@hidden smallexample
+
address@hidden Several files recursively, Prepare the data, Several files,
Words in a defun
address@hidden Recursively Count Words in Different Files
+
+Besides a @code{while} loop, you can work on each of a list of files
+with recursion. A recursive version of @code{lengths-list-many-files}
+is short and simple.
+
+The recursive function has the usual parts: the `do-again-test', the
+`next-step-expression', and the recursive call. The `do-again-test'
+determines whether the function should call itself again, which it
+will do if the @code{list-of-files} contains any remaining elements;
+the `next-step-expression' resets the @code{list-of-files} to the
address@hidden of itself, so eventually the list will be empty; and the
+recursive call calls itself on the shorter list. The complete
+function is shorter than this description!
address@hidden recursive-lengths-list-many-files
+
address@hidden
address@hidden
+(defun recursive-lengths-list-many-files (list-of-files)
+ "Return list of lengths of each defun in LIST-OF-FILES."
+ (if list-of-files ; @r{do-again-test}
+ (append
+ (lengths-list-file
+ (expand-file-name (car list-of-files)))
+ (recursive-lengths-list-many-files
+ (cdr list-of-files)))))
address@hidden group
address@hidden smallexample
+
address@hidden
+In a sentence, the function returns the lengths' list for the first of
+the @code{list-of-files} appended to the result of calling itself on
+the rest of the @code{list-of-files}.
+
+Here is a test of @code{recursive-lengths-list-many-files}, along with
+the results of running @code{lengths-list-file} on each of the files
+individually.
+
+Install @code{recursive-lengths-list-many-files} and
address@hidden, if necessary, and then evaluate the
+following expressions. You may need to change the files' pathnames;
+those here work when this Info file and the Emacs sources are located
+in their customary places. To change the expressions, copy them to
+the @file{*scratch*} buffer, edit them, and then evaluate them.
+
+The results are shown after the @address@hidden (These results are
+for files from Emacs version 22.1.1; files from other versions of
+Emacs may produce different results.)
+
address@hidden !!! 22.1.1 lisp sources location here
address@hidden
address@hidden
+(cd "/usr/local/share/emacs/22.1.1/")
+
+(lengths-list-file "./lisp/macros.el")
+ @result{} (283 263 480 90)
address@hidden group
+
address@hidden
+(lengths-list-file "./lisp/mail/mailalias.el")
+ @result{} (38 32 29 95 178 180 321 218 324)
address@hidden group
+
address@hidden
+(lengths-list-file "./lisp/makesum.el")
+ @result{} (85 181)
address@hidden group
+
address@hidden
+ (recursive-lengths-list-many-files
+ '("./lisp/macros.el"
+ "./lisp/mail/mailalias.el"
+ "./lisp/makesum.el"))
+ @result{} (283 263 480 90 38 32 29 95 178 180 321 218 324 85 181)
address@hidden group
address@hidden smallexample
+
+The @code{recursive-lengths-list-many-files} function produces the
+output we want.
+
+The next step is to prepare the data in the list for display in a graph.
+
address@hidden Prepare the data, , Several files recursively, Words in a defun
address@hidden Prepare the Data for Display in a Graph
+
+The @code{recursive-lengths-list-many-files} function returns a list
+of numbers. Each number records the length of a function definition.
+What we need to do now is transform this data into a list of numbers
+suitable for generating a graph. The new list will tell how many
+functions definitions contain less than 10 words and
+symbols, how many contain between 10 and 19 words and symbols, how
+many contain between 20 and 29 words and symbols, and so on.
+
+In brief, we need to go through the lengths' list produced by the
address@hidden function and count the number
+of defuns within each range of lengths, and produce a list of those
+numbers.
+
address@hidden
+* Data for Display in Detail::
+* Sorting:: Sorting lists.
+* Files List:: Making a list of files.
+* Counting function definitions::
address@hidden menu
+
address@hidden Data for Display in Detail, Sorting, Prepare the data, Prepare
the data
address@hidden
address@hidden The Data for Display in Detail
address@hidden ifnottex
+
+Based on what we have done before, we can readily foresee that it
+should not be too hard to write a function that address@hidden' down the
+lengths' list, looks at each element, determines which length range it
+is in, and increments a counter for that range.
+
+However, before beginning to write such a function, we should consider
+the advantages of sorting the lengths' list first, so the numbers are
+ordered from smallest to largest. First, sorting will make it easier
+to count the numbers in each range, since two adjacent numbers will
+either be in the same length range or in adjacent ranges. Second, by
+inspecting a sorted list, we can discover the highest and lowest
+number, and thereby determine the largest and smallest length range
+that we will need.
+
address@hidden Sorting, Files List, Data for Display in Detail, Prepare the data
address@hidden Sorting Lists
address@hidden sort
+
+Emacs contains a function to sort lists, called (as you might guess)
address@hidden The @code{sort} function takes two arguments, the list
+to be sorted, and a predicate that determines whether the first of
+two list elements is ``less'' than the second.
+
+As we saw earlier (@pxref{Wrong Type of Argument, , Using the Wrong
+Type Object as an Argument}), a predicate is a function that
+determines whether some property is true or false. The @code{sort}
+function will reorder a list according to whatever property the
+predicate uses; this means that @code{sort} can be used to sort
+non-numeric lists by non-numeric criteria---it can, for example,
+alphabetize a list.
+
address@hidden 1250
+The @code{<} function is used when sorting a numeric list. For example,
+
address@hidden
+(sort '(4 8 21 17 33 7 21 7) '<)
address@hidden smallexample
+
address@hidden 800
address@hidden
+produces this:
+
address@hidden
+(4 7 7 8 17 21 21 33)
address@hidden smallexample
+
address@hidden
+(Note that in this example, both the arguments are quoted so that the
+symbols are not evaluated before being passed to @code{sort} as
+arguments.)
+
+Sorting the list returned by the
address@hidden function is straightforward;
+it uses the @code{<} function:
+
address@hidden
+2006 Oct 29
+In GNU Emacs 22, eval
+(progn
+ (cd "/usr/local/share/emacs/22.0.50/")
+ (sort
+ (recursive-lengths-list-many-files
+ '("./lisp/macros.el"
+ "./lisp/mail/mailalias.el"
+ "./lisp/makesum.el"))
+ '<))
+
address@hidden ignore
+
address@hidden
address@hidden
+(sort
+ (recursive-lengths-list-many-files
+ '("./lisp/macros.el"
+ "./lisp/mailalias.el"
+ "./lisp/makesum.el"))
+ '<)
address@hidden group
address@hidden smallexample
+
address@hidden 800
address@hidden
+which produces:
+
address@hidden
+(29 32 38 85 90 95 178 180 181 218 263 283 321 324 480)
address@hidden smallexample
+
address@hidden
+(Note that in this example, the first argument to @code{sort} is not
+quoted, since the expression must be evaluated so as to produce the
+list that is passed to @code{sort}.)
+
address@hidden Files List, Counting function definitions, Sorting, Prepare the
data
address@hidden Making a List of Files
+
+The @code{recursive-lengths-list-many-files} function requires a list
+of files as its argument. For our test examples, we constructed such
+a list by hand; but the Emacs Lisp source directory is too large for
+us to do for that. Instead, we will write a function to do the job
+for us. In this function, we will use both a @code{while} loop and a
+recursive call.
+
address@hidden directory-files
+We did not have to write a function like this for older versions of
+GNU Emacs, since they placed all the @samp{.el} files in one
+directory. Instead, we were able to use the @code{directory-files}
+function, which lists the names of files that match a specified
+pattern within a single directory.
+
+However, recent versions of Emacs place Emacs Lisp files in
+sub-directories of the top level @file{lisp} directory. This
+re-arrangement eases navigation. For example, all the mail related
+files are in a @file{lisp} sub-directory called @file{mail}. But at
+the same time, this arrangement forces us to create a file listing
+function that descends into the sub-directories.
+
address@hidden files-in-below-directory
+We can create this function, called @code{files-in-below-directory},
+using familiar functions such as @code{car}, @code{nthcdr}, and
address@hidden in conjunction with an existing function called
address@hidden This latter function not only
+lists all the filenames in a directory, including the names
+of sub-directories, but also their attributes.
+
+To restate our goal: to create a function that will enable us
+to feed filenames to @code{recursive-lengths-list-many-files}
+as a list that looks like this (but with more elements):
+
address@hidden
address@hidden
+("./lisp/macros.el"
+ "./lisp/mail/rmail.el"
+ "./lisp/makesum.el")
address@hidden group
address@hidden smallexample
+
+The @code{directory-files-and-attributes} function returns a list of
+lists. Each of the lists within the main list consists of 13
+elements. The first element is a string that contains the name of the
+file -- which, in GNU/Linux, may be a `directory file', that is to
+say, a file with the special attributes of a directory. The second
+element of the list is @code{t} for a directory, a string
+for symbolic link (the string is the name linked to), or @code{nil}.
+
+For example, the first @samp{.el} file in the @file{lisp/} directory
+is @file{abbrev.el}. Its name is
address@hidden/usr/local/share/emacs/22.1.1/lisp/abbrev.el} and it is not a
+directory or a symbolic link.
+
address@hidden 1000
+This is how @code{directory-files-and-attributes} lists that file and
+its attributes:
+
address@hidden
address@hidden
+("abbrev.el"
+nil
+1
+1000
+100
address@hidden group
address@hidden
+(17733 259)
+(17491 28834)
+(17596 62124)
+13157
+"-rw-rw-r--"
address@hidden group
address@hidden
+nil
+2971624
+773)
address@hidden group
address@hidden smallexample
+
address@hidden 1200
+On the other hand, @file{mail/} is a directory within the @file{lisp/}
+directory. The beginning of its listing looks like this:
+
address@hidden
address@hidden
+("mail"
+t
address@hidden
+)
address@hidden group
address@hidden smallexample
+
+(To learn about the different attributes, look at the documentation of
address@hidden Bear in mind that the @code{file-attributes}
+function does not list the filename, so its first element is
address@hidden's second element.)
+
+We will want our new function, @code{files-in-below-directory}, to
+list the @samp{.el} files in the directory it is told to check, and in
+any directories below that directory.
+
+This gives us a hint on how to construct
address@hidden: within a directory, the function
+should add @samp{.el} filenames to a list; and if, within a directory,
+the function comes upon a sub-directory, it should go into that
+sub-directory and repeat its actions.
+
+However, we should note that every directory contains a name that
+refers to itself, called @file{.}, (``dot'') and a name that refers to
+its parent directory, called @file{..} (``double dot''). (In
address@hidden/}, the root directory, @file{..} refers to itself, since
address@hidden/} has no parent.) Clearly, we do not want our
address@hidden function to enter those directories,
+since they always lead us, directly or indirectly, to the current
+directory.
+
+Consequently, our @code{files-in-below-directory} function must do
+several tasks:
+
address@hidden @bullet
address@hidden
+Check to see whether it is looking at a filename that ends in
address@hidden; and if so, add its name to a list.
+
address@hidden
+Check to see whether it is looking at a filename that is the name of a
+directory; and if so,
+
address@hidden @minus
address@hidden
+Check to see whether it is looking at @file{.} or @file{..}; and if
+so skip it.
+
address@hidden
+Or else, go into that directory and repeat the process.
address@hidden itemize
address@hidden itemize
+
+Let's write a function definition to do these tasks. We will use a
address@hidden loop to move from one filename to another within a
+directory, checking what needs to be done; and we will use a recursive
+call to repeat the actions on each sub-directory. The recursive
+pattern is `accumulate'
+(@pxref{Accumulate, , Recursive Pattern: @emph{accumulate}}),
+using @code{append} as the combiner.
+
address@hidden
+(directory-files "/usr/local/src/emacs/lisp/" t "\\.el$")
+(shell-command "find /usr/local/src/emacs/lisp/ -name '*.el'")
+
+(directory-files "/usr/local/share/emacs/22.1.1/lisp/" t "\\.el$")
+(shell-command "find /usr/local/share/emacs/22.1.1/lisp/ -name '*.el'")
address@hidden ignore
+
address@hidden /usr/local/share/emacs/22.1.1/lisp/
+
address@hidden 800
+Here is the function:
+
address@hidden
address@hidden
+(defun files-in-below-directory (directory)
+ "List the .el files in DIRECTORY and in its sub-directories."
+ ;; Although the function will be used non-interactively,
+ ;; it will be easier to test if we make it interactive.
+ ;; The directory will have a name such as
+ ;; "/usr/local/share/emacs/22.1.1/lisp/"
+ (interactive "DDirectory name: ")
address@hidden group
address@hidden
+ (let (el-files-list
+ (current-directory-list
+ (directory-files-and-attributes directory t)))
+ ;; while we are in the current directory
+ (while current-directory-list
address@hidden group
address@hidden
+ (cond
+ ;; check to see whether filename ends in `.el'
+ ;; and if so, append its name to a list.
+ ((equal ".el" (substring (car (car current-directory-list)) -3))
+ (setq el-files-list
+ (cons (car (car current-directory-list)) el-files-list)))
address@hidden group
address@hidden
+ ;; check whether filename is that of a directory
+ ((eq t (car (cdr (car current-directory-list))))
+ ;; decide whether to skip or recurse
+ (if
+ (equal "."
+ (substring (car (car current-directory-list)) -1))
+ ;; then do nothing since filename is that of
+ ;; current directory or parent, "." or ".."
+ ()
address@hidden group
address@hidden
+ ;; else descend into the directory and repeat the process
+ (setq el-files-list
+ (append
+ (files-in-below-directory
+ (car (car current-directory-list)))
+ el-files-list)))))
+ ;; move to the next filename in the list; this also
+ ;; shortens the list so the while loop eventually comes to an end
+ (setq current-directory-list (cdr current-directory-list)))
+ ;; return the filenames
+ el-files-list))
address@hidden group
address@hidden smallexample
+
address@hidden (files-in-below-directory "/usr/local/src/emacs/lisp/")
address@hidden (files-in-below-directory "/usr/local/share/emacs/22.1.1/lisp/")
+
+The @code{files-in-below-directory} @code{directory-files} function
+takes one argument, the name of a directory.
+
address@hidden 1250
+Thus, on my system,
+
address@hidden (length (files-in-below-directory "/usr/local/src/emacs/lisp/"))
+
address@hidden !!! 22.1.1 lisp sources location here
address@hidden
address@hidden
+(length
+ (files-in-below-directory "/usr/local/share/emacs/22.1.1/lisp/"))
address@hidden group
address@hidden smallexample
+
address@hidden
+tells me that in and below my Lisp sources directory are 1031
address@hidden files.
+
address@hidden returns a list in reverse alphabetical
+order. An expression to sort the list in alphabetical order looks
+like this:
+
address@hidden
address@hidden
+(sort
+ (files-in-below-directory "/usr/local/share/emacs/22.1.1/lisp/")
+ 'string-lessp)
address@hidden group
address@hidden smallexample
+
address@hidden
+(defun test ()
+ "Test how long it takes to find lengths of all sorted elisp defuns."
+ (insert "\n" (current-time-string) "\n")
+ (sit-for 0)
+ (sort
+ (recursive-lengths-list-many-files
+ (files-in-below-directory "/usr/local/src/emacs/lisp/"))
+ '<)
+ (insert (format "%s" (current-time-string))))
address@hidden ignore
+
address@hidden Counting function definitions, , Files List, Prepare the data
address@hidden Counting function definitions
+
+Our immediate goal is to generate a list that tells us how many
+function definitions contain fewer than 10 words and symbols, how many
+contain between 10 and 19 words and symbols, how many contain between
+20 and 29 words and symbols, and so on.
+
+With a sorted list of numbers, this is easy: count how many elements
+of the list are smaller than 10, then, after moving past the numbers
+just counted, count how many are smaller than 20, then, after moving
+past the numbers just counted, count how many are smaller than 30, and
+so on. Each of the numbers, 10, 20, 30, 40, and the like, is one
+larger than the top of that range. We can call the list of such
+numbers the @code{top-of-ranges} list.
+
address@hidden 1200
+If we wished, we could generate this list automatically, but it is
+simpler to write a list manually. Here it is:
address@hidden top-of-ranges
+
address@hidden
address@hidden
+(defvar top-of-ranges
+ '(10 20 30 40 50
+ 60 70 80 90 100
+ 110 120 130 140 150
+ 160 170 180 190 200
+ 210 220 230 240 250
+ 260 270 280 290 300)
+ "List specifying ranges for `defuns-per-range'.")
address@hidden group
address@hidden smallexample
+
+To change the ranges, we edit this list.
+
+Next, we need to write the function that creates the list of the
+number of definitions within each range. Clearly, this function must
+take the @code{sorted-lengths} and the @code{top-of-ranges} lists
+as arguments.
+
+The @code{defuns-per-range} function must do two things again and
+again: it must count the number of definitions within a range
+specified by the current top-of-range value; and it must shift to the
+next higher value in the @code{top-of-ranges} list after counting the
+number of definitions in the current range. Since each of these
+actions is repetitive, we can use @code{while} loops for the job.
+One loop counts the number of definitions in the range defined by the
+current top-of-range value, and the other loop selects each of the
+top-of-range values in turn.
+
+Several entries of the @code{sorted-lengths} list are counted for each
+range; this means that the loop for the @code{sorted-lengths} list
+will be inside the loop for the @code{top-of-ranges} list, like a
+small gear inside a big gear.
+
+The inner loop counts the number of definitions within the range. It
+is a simple counting loop of the type we have seen before.
+(@xref{Incrementing Loop, , A loop with an incrementing counter}.)
+The true-or-false test of the loop tests whether the value from the
address@hidden list is smaller than the current value of the
+top of the range. If it is, the function increments the counter and
+tests the next value from the @code{sorted-lengths} list.
+
address@hidden 1250
+The inner loop looks like this:
+
address@hidden
address@hidden
+(while @var{length-element-smaller-than-top-of-range}
+ (setq number-within-range (1+ number-within-range))
+ (setq sorted-lengths (cdr sorted-lengths)))
address@hidden group
address@hidden smallexample
+
+The outer loop must start with the lowest value of the
address@hidden list, and then be set to each of the succeeding
+higher values in turn. This can be done with a loop like this:
+
address@hidden
address@hidden
+(while top-of-ranges
+ @address@hidden
+ (setq top-of-ranges (cdr top-of-ranges)))
address@hidden group
address@hidden smallexample
+
address@hidden 1200
+Put together, the two loops look like this:
+
address@hidden
address@hidden
+(while top-of-ranges
+
+ ;; @r{Count the number of elements within the current range.}
+ (while @var{length-element-smaller-than-top-of-range}
+ (setq number-within-range (1+ number-within-range))
+ (setq sorted-lengths (cdr sorted-lengths)))
+
+ ;; @r{Move to next range.}
+ (setq top-of-ranges (cdr top-of-ranges)))
address@hidden group
address@hidden smallexample
+
+In addition, in each circuit of the outer loop, Emacs should record
+the number of definitions within that range (the value of
address@hidden) in a list. We can use @code{cons} for
+this purpose. (@xref{cons, , @code{cons}}.)
+
+The @code{cons} function works fine, except that the list it
+constructs will contain the number of definitions for the highest
+range at its beginning and the number of definitions for the lowest
+range at its end. This is because @code{cons} attaches new elements
+of the list to the beginning of the list, and since the two loops are
+working their way through the lengths' list from the lower end first,
+the @code{defuns-per-range-list} will end up largest number first.
+But we will want to print our graph with smallest values first and the
+larger later. The solution is to reverse the order of the
address@hidden We can do this using the
address@hidden function, which reverses the order of a list.
address@hidden nreverse
+
address@hidden 800
+For example,
+
address@hidden
+(nreverse '(1 2 3 4))
address@hidden smallexample
+
address@hidden 800
address@hidden
+produces:
+
address@hidden
+(4 3 2 1)
address@hidden smallexample
+
+Note that the @code{nreverse} function is ``destructive''---that is,
+it changes the list to which it is applied; this contrasts with the
address@hidden and @code{cdr} functions, which are non-destructive. In
+this case, we do not want the original @code{defuns-per-range-list},
+so it does not matter that it is destroyed. (The @code{reverse}
+function provides a reversed copy of a list, leaving the original list
+as is.)
address@hidden reverse
+
address@hidden 1250
+Put all together, the @code{defuns-per-range} looks like this:
+
address@hidden
address@hidden
+(defun defuns-per-range (sorted-lengths top-of-ranges)
+ "SORTED-LENGTHS defuns in each TOP-OF-RANGES range."
+ (let ((top-of-range (car top-of-ranges))
+ (number-within-range 0)
+ defuns-per-range-list)
address@hidden group
+
address@hidden
+ ;; @r{Outer loop.}
+ (while top-of-ranges
address@hidden group
+
address@hidden
+ ;; @r{Inner loop.}
+ (while (and
+ ;; @r{Need number for numeric test.}
+ (car sorted-lengths)
+ (< (car sorted-lengths) top-of-range))
address@hidden group
+
address@hidden
+ ;; @r{Count number of definitions within current range.}
+ (setq number-within-range (1+ number-within-range))
+ (setq sorted-lengths (cdr sorted-lengths)))
+
+ ;; @r{Exit inner loop but remain within outer loop.}
address@hidden group
+
address@hidden
+ (setq defuns-per-range-list
+ (cons number-within-range defuns-per-range-list))
+ (setq number-within-range 0) ; @r{Reset count to zero.}
address@hidden group
+
address@hidden
+ ;; @r{Move to next range.}
+ (setq top-of-ranges (cdr top-of-ranges))
+ ;; @r{Specify next top of range value.}
+ (setq top-of-range (car top-of-ranges)))
address@hidden group
+
address@hidden
+ ;; @r{Exit outer loop and count the number of defuns larger than}
+ ;; @r{ the largest top-of-range value.}
+ (setq defuns-per-range-list
+ (cons
+ (length sorted-lengths)
+ defuns-per-range-list))
address@hidden group
+
address@hidden
+ ;; @r{Return a list of the number of definitions within each range,}
+ ;; @r{ smallest to largest.}
+ (nreverse defuns-per-range-list)))
address@hidden group
address@hidden smallexample
+
address@hidden 1200
address@hidden
+The function is straightforward except for one subtle feature. The
+true-or-false test of the inner loop looks like this:
+
address@hidden
address@hidden
+(and (car sorted-lengths)
+ (< (car sorted-lengths) top-of-range))
address@hidden group
address@hidden smallexample
+
address@hidden 800
address@hidden
+instead of like this:
+
address@hidden
+(< (car sorted-lengths) top-of-range)
address@hidden smallexample
+
+The purpose of the test is to determine whether the first item in the
address@hidden list is less than the value of the top of the
+range.
+
+The simple version of the test works fine unless the
address@hidden list has a @code{nil} value. In that case, the
address@hidden(car sorted-lengths)} expression function returns
address@hidden The @code{<} function cannot compare a number to
address@hidden, which is an empty list, so Emacs signals an error and
+stops the function from attempting to continue to execute.
+
+The @code{sorted-lengths} list always becomes @code{nil} when the
+counter reaches the end of the list. This means that any attempt to
+use the @code{defuns-per-range} function with the simple version of
+the test will fail.
+
+We solve the problem by using the @code{(car sorted-lengths)}
+expression in conjunction with the @code{and} expression. The
address@hidden(car sorted-lengths)} expression returns a address@hidden
+value so long as the list has at least one number within it, but
+returns @code{nil} if the list is empty. The @code{and} expression
+first evaluates the @code{(car sorted-lengths)} expression, and
+if it is @code{nil}, returns false @emph{without} evaluating the
address@hidden<} expression. But if the @code{(car sorted-lengths)}
+expression returns a address@hidden value, the @code{and} expression
+evaluates the @code{<} expression, and returns that value as the value
+of the @code{and} expression.
+
address@hidden colon in printed section title causes problem in Info cross
reference
+This way, we avoid an error.
address@hidden
address@hidden
+(For information about @code{and}, see
address@hidden function, , The @code{kill-new} function}.)
address@hidden iftex
address@hidden
address@hidden
+(@xref{kill-new function, , The @code{kill-new} function}, for
+information about @code{and}.)
address@hidden ifinfo
+
+Here is a short test of the @code{defuns-per-range} function. First,
+evaluate the expression that binds (a shortened)
address@hidden list to the list of values, then evaluate the
+expression for binding the @code{sorted-lengths} list, and then
+evaluate the @code{defuns-per-range} function.
+
address@hidden
address@hidden
+;; @r{(Shorter list than we will use later.)}
+(setq top-of-ranges
+ '(110 120 130 140 150
+ 160 170 180 190 200))
+
+(setq sorted-lengths
+ '(85 86 110 116 122 129 154 176 179 200 265 300 300))
+
+(defuns-per-range sorted-lengths top-of-ranges)
address@hidden group
address@hidden smallexample
+
address@hidden 800
address@hidden
+The list returned looks like this:
+
address@hidden
+(2 2 2 0 0 1 0 2 0 0 4)
address@hidden smallexample
+
address@hidden
+Indeed, there are two elements of the @code{sorted-lengths} list
+smaller than 110, two elements between 110 and 119, two elements
+between 120 and 129, and so on. There are four elements with a value
+of 200 or larger.
+
address@hidden The next step is to turn this numbers' list into a graph.
address@hidden Readying a Graph, Emacs Initialization, Words in a defun, Top
address@hidden Readying a Graph
address@hidden Readying a graph
address@hidden Graph prototype
address@hidden Prototype graph
address@hidden Body of graph
+
+Our goal is to construct a graph showing the numbers of function
+definitions of various lengths in the Emacs lisp sources.
+
+As a practical matter, if you were creating a graph, you would
+probably use a program such as @code{gnuplot} to do the job.
+(@code{gnuplot} is nicely integrated into GNU Emacs.) In this case,
+however, we create one from scratch, and in the process we will
+re-acquaint ourselves with some of what we learned before and learn
+more.
+
+In this chapter, we will first write a simple graph printing function.
+This first definition will be a @dfn{prototype}, a rapidly written
+function that enables us to reconnoiter this unknown graph-making
+territory. We will discover dragons, or find that they are myth.
+After scouting the terrain, we will feel more confident and enhance
+the function to label the axes automatically.
+
address@hidden
+* Columns of a graph::
+* graph-body-print:: How to print the body of a graph.
+* recursive-graph-body-print::
+* Printed Axes::
+* Line Graph Exercise::
address@hidden menu
+
address@hidden Columns of a graph, graph-body-print, Readying a Graph, Readying
a Graph
address@hidden
address@hidden Printing the Columns of a Graph
address@hidden ifnottex
+
+Since Emacs is designed to be flexible and work with all kinds of
+terminals, including character-only terminals, the graph will need to
+be made from one of the `typewriter' symbols. An asterisk will do; as
+we enhance the graph-printing function, we can make the choice of
+symbol a user option.
+
+We can call this function @code{graph-body-print}; it will take a
address@hidden as its only argument. At this stage, we will not
+label the graph, but only print its body.
+
+The @code{graph-body-print} function inserts a vertical column of
+asterisks for each element in the @code{numbers-list}. The height of
+each line is determined by the value of that element of the
address@hidden
+
+Inserting columns is a repetitive act; that means that this function can
+be written either with a @code{while} loop or recursively.
+
+Our first challenge is to discover how to print a column of asterisks.
+Usually, in Emacs, we print characters onto a screen horizontally,
+line by line, by typing. We have two routes we can follow: write our
+own column-insertion function or discover whether one exists in Emacs.
+
+To see whether there is one in Emacs, we can use the @kbd{M-x apropos}
+command. This command is like the @kbd{C-h a} (@code{command-apropos})
+command, except that the latter finds only those functions that are
+commands. The @kbd{M-x apropos} command lists all symbols that match
+a regular expression, including functions that are not interactive.
address@hidden apropos
+
+What we want to look for is some command that prints or inserts
+columns. Very likely, the name of the function will contain either
+the word `print' or the word `insert' or the word `column'.
+Therefore, we can simply type @kbd{M-x apropos RET
+print\|insert\|column RET} and look at the result. On my system, this
+command once too takes quite some time, and then produced a list of 79
+functions and variables. Now it does not take much time at all and
+produces a list of 211 functions and variables. Scanning down the
+list, the only function that looks as if it might do the job is
address@hidden
+
address@hidden 1200
+Indeed, this is the function we want; its documentation says:
+
address@hidden
address@hidden
+insert-rectangle:
+Insert text of RECTANGLE with upper left corner at point.
+RECTANGLE's first line is inserted at point,
+its second line is inserted at a point vertically under point, etc.
+RECTANGLE should be a list of strings.
+After this command, the mark is at the upper left corner
+and point is at the lower right corner.
address@hidden group
address@hidden smallexample
+
+We can run a quick test, to make sure it does what we expect of it.
+
+Here is the result of placing the cursor after the
address@hidden expression and typing @kbd{C-u C-x C-e}
+(@code{eval-last-sexp}). The function inserts the strings
address@hidden"first"}, @samp{"second"}, and @samp{"third"} at and below
+point. Also the function returns @code{nil}.
+
address@hidden
address@hidden
+(insert-rectangle '("first" "second" "third"))first
+ second
+ thirdnil
address@hidden group
address@hidden smallexample
+
address@hidden
+Of course, we won't be inserting the text of the
address@hidden expression itself into the buffer in which we
+are making the graph, but will call the function from our program. We
+shall, however, have to make sure that point is in the buffer at the
+place where the @code{insert-rectangle} function will insert its
+column of strings.
+
+If you are reading this in Info, you can see how this works by
+switching to another buffer, such as the @file{*scratch*} buffer,
+placing point somewhere in the buffer, typing @kbd{M-:}, typing the
address@hidden expression into the minibuffer at the prompt,
+and then typing @key{RET}. This causes Emacs to evaluate the
+expression in the minibuffer, but to use as the value of point the
+position of point in the @file{*scratch*} buffer. (@kbd{M-:} is the
+keybinding for @code{eval-expression}. Also, @code{nil} does not
+appear in the @file{*scratch*} buffer since the expression is
+evaluated in the minibuffer.)
+
+We find when we do this that point ends up at the end of the last
+inserted line---that is to say, this function moves point as a
+side-effect. If we were to repeat the command, with point at this
+position, the next insertion would be below and to the right of the
+previous insertion. We don't want this! If we are going to make a
+bar graph, the columns need to be beside each other.
+
+So we discover that each cycle of the column-inserting @code{while}
+loop must reposition point to the place we want it, and that place
+will be at the top, not the bottom, of the column. Moreover, we
+remember that when we print a graph, we do not expect all the columns
+to be the same height. This means that the top of each column may be
+at a different height from the previous one. We cannot simply
+reposition point to the same line each time, but moved over to the
+right---or perhaps we address@hidden
+
+We are planning to make the columns of the bar graph out of asterisks.
+The number of asterisks in the column is the number specified by the
+current element of the @code{numbers-list}. We need to construct a
+list of asterisks of the right length for each call to
address@hidden If this list consists solely of the requisite
+number of asterisks, then we will have position point the right number
+of lines above the base for the graph to print correctly. This could
+be difficult.
+
+Alternatively, if we can figure out some way to pass
address@hidden a list of the same length each time, then we
+can place point on the same line each time, but move it over one
+column to the right for each new column. If we do this, however, some
+of the entries in the list passed to @code{insert-rectangle} must be
+blanks rather than asterisks. For example, if the maximum height of
+the graph is 5, but the height of the column is 3, then
address@hidden requires an argument that looks like this:
+
address@hidden
+(" " " " "*" "*" "*")
address@hidden smallexample
+
+This last proposal is not so difficult, so long as we can determine
+the column height. There are two ways for us to specify the column
+height: we can arbitrarily state what it will be, which would work
+fine for graphs of that height; or we can search through the list of
+numbers and use the maximum height of the list as the maximum height
+of the graph. If the latter operation were difficult, then the former
+procedure would be easiest, but there is a function built into Emacs
+that determines the maximum of its arguments. We can use that
+function. The function is called @code{max} and it returns the
+largest of all its arguments, which must be numbers. Thus, for
+example,
+
address@hidden
+(max 3 4 6 5 7 3)
address@hidden smallexample
+
address@hidden
+returns 7. (A corresponding function called @code{min} returns the
+smallest of all its arguments.)
address@hidden max
address@hidden min
+
+However, we cannot simply call @code{max} on the @code{numbers-list};
+the @code{max} function expects numbers as its argument, not a list of
+numbers. Thus, the following expression,
+
address@hidden
+(max '(3 4 6 5 7 3))
address@hidden smallexample
+
address@hidden 800
address@hidden
+produces the following error message;
+
address@hidden
+Wrong type of argument: number-or-marker-p, (3 4 6 5 7 3)
address@hidden smallexample
+
address@hidden apply
+We need a function that passes a list of arguments to a function.
+This function is @code{apply}. This function `applies' its first
+argument (a function) to its remaining arguments, the last of which
+may be a list.
+
address@hidden 1250
+For example,
+
address@hidden
+(apply 'max 3 4 7 3 '(4 8 5))
address@hidden smallexample
+
address@hidden
+returns 8.
+
+(Incidentally, I don't know how you would learn of this function
+without a book such as this. It is possible to discover other
+functions, like @code{search-forward} or @code{insert-rectangle}, by
+guessing at a part of their names and then using @code{apropos}. Even
+though its base in metaphor is clear---`apply' its first argument to
+the rest---I doubt a novice would come up with that particular word
+when using @code{apropos} or other aid. Of course, I could be wrong;
+after all, the function was first named by someone who had to invent
+it.)
+
+The second and subsequent arguments to @code{apply} are optional, so
+we can use @code{apply} to call a function and pass the elements of a
+list to it, like this, which also returns 8:
+
address@hidden
+(apply 'max '(4 8 5))
address@hidden smallexample
+
+This latter way is how we will use @code{apply}. The
address@hidden function returns a numbers'
+list to which we can apply @code{max} (we could also apply @code{max} to
+the sorted numbers' list; it does not matter whether the list is
+sorted or not.)
+
address@hidden 800
+Hence, the operation for finding the maximum height of the graph is this:
+
address@hidden
+(setq max-graph-height (apply 'max numbers-list))
address@hidden smallexample
+
+Now we can return to the question of how to create a list of strings
+for a column of the graph. Told the maximum height of the graph
+and the number of asterisks that should appear in the column, the
+function should return a list of strings for the
address@hidden command to insert.
+
+Each column is made up of asterisks or blanks. Since the function is
+passed the value of the height of the column and the number of
+asterisks in the column, the number of blanks can be found by
+subtracting the number of asterisks from the height of the column.
+Given the number of blanks and the number of asterisks, two
address@hidden loops can be used to construct the list:
+
address@hidden
address@hidden
+;;; @r{First version.}
+(defun column-of-graph (max-graph-height actual-height)
+ "Return list of strings that is one column of a graph."
+ (let ((insert-list nil)
+ (number-of-top-blanks
+ (- max-graph-height actual-height)))
address@hidden group
+
address@hidden
+ ;; @r{Fill in asterisks.}
+ (while (> actual-height 0)
+ (setq insert-list (cons "*" insert-list))
+ (setq actual-height (1- actual-height)))
address@hidden group
+
address@hidden
+ ;; @r{Fill in blanks.}
+ (while (> number-of-top-blanks 0)
+ (setq insert-list (cons " " insert-list))
+ (setq number-of-top-blanks
+ (1- number-of-top-blanks)))
address@hidden group
+
address@hidden
+ ;; @r{Return whole list.}
+ insert-list))
address@hidden group
address@hidden smallexample
+
+If you install this function and then evaluate the following
+expression you will see that it returns the list as desired:
+
address@hidden
+(column-of-graph 5 3)
address@hidden smallexample
+
address@hidden 800
address@hidden
+returns
+
address@hidden
+(" " " " "*" "*" "*")
address@hidden smallexample
+
+As written, @code{column-of-graph} contains a major flaw: the symbols
+used for the blank and for the marked entries in the column are
+`hard-coded' as a space and asterisk. This is fine for a prototype,
+but you, or another user, may wish to use other symbols. For example,
+in testing the graph function, you many want to use a period in place
+of the space, to make sure the point is being repositioned properly
+each time the @code{insert-rectangle} function is called; or you might
+want to substitute a @samp{+} sign or other symbol for the asterisk.
+You might even want to make a graph-column that is more than one
+display column wide. The program should be more flexible. The way to
+do that is to replace the blank and the asterisk with two variables
+that we can call @code{graph-blank} and @code{graph-symbol} and define
+those variables separately.
+
+Also, the documentation is not well written. These considerations
+lead us to the second version of the function:
+
address@hidden
address@hidden
+(defvar graph-symbol "*"
+ "String used as symbol in graph, usually an asterisk.")
address@hidden group
+
address@hidden
+(defvar graph-blank " "
+ "String used as blank in graph, usually a blank space.
+graph-blank must be the same number of columns wide
+as graph-symbol.")
address@hidden group
address@hidden smallexample
+
address@hidden
+(For an explanation of @code{defvar}, see
address@hidden, , Initializing a Variable with @code{defvar}}.)
+
address@hidden
address@hidden
+;;; @r{Second version.}
+(defun column-of-graph (max-graph-height actual-height)
+ "Return MAX-GRAPH-HEIGHT strings; ACTUAL-HEIGHT are graph-symbols.
+
address@hidden group
address@hidden
+The graph-symbols are contiguous entries at the end
+of the list.
+The list will be inserted as one column of a graph.
+The strings are either graph-blank or graph-symbol."
address@hidden group
+
address@hidden
+ (let ((insert-list nil)
+ (number-of-top-blanks
+ (- max-graph-height actual-height)))
address@hidden group
+
address@hidden
+ ;; @r{Fill in @code{graph-symbols}.}
+ (while (> actual-height 0)
+ (setq insert-list (cons graph-symbol insert-list))
+ (setq actual-height (1- actual-height)))
address@hidden group
+
address@hidden
+ ;; @r{Fill in @code{graph-blanks}.}
+ (while (> number-of-top-blanks 0)
+ (setq insert-list (cons graph-blank insert-list))
+ (setq number-of-top-blanks
+ (1- number-of-top-blanks)))
+
+ ;; @r{Return whole list.}
+ insert-list))
address@hidden group
address@hidden smallexample
+
+If we wished, we could rewrite @code{column-of-graph} a third time to
+provide optionally for a line graph as well as for a bar graph. This
+would not be hard to do. One way to think of a line graph is that it
+is no more than a bar graph in which the part of each bar that is
+below the top is blank. To construct a column for a line graph, the
+function first constructs a list of blanks that is one shorter than
+the value, then it uses @code{cons} to attach a graph symbol to the
+list; then it uses @code{cons} again to attach the `top blanks' to
+the list.
+
+It is easy to see how to write such a function, but since we don't
+need it, we will not do it. But the job could be done, and if it were
+done, it would be done with @code{column-of-graph}. Even more
+important, it is worth noting that few changes would have to be made
+anywhere else. The enhancement, if we ever wish to make it, is
+simple.
+
+Now, finally, we come to our first actual graph printing function.
+This prints the body of a graph, not the labels for the vertical and
+horizontal axes, so we can call this @code{graph-body-print}.
+
address@hidden graph-body-print, recursive-graph-body-print, Columns of a
graph, Readying a Graph
address@hidden The @code{graph-body-print} Function
address@hidden graph-body-print
+
+After our preparation in the preceding section, the
address@hidden function is straightforward. The function
+will print column after column of asterisks and blanks, using the
+elements of a numbers' list to specify the number of asterisks in each
+column. This is a repetitive act, which means we can use a
+decrementing @code{while} loop or recursive function for the job. In
+this section, we will write the definition using a @code{while} loop.
+
+The @code{column-of-graph} function requires the height of the graph
+as an argument, so we should determine and record that as a local variable.
+
+This leads us to the following template for the @code{while} loop
+version of this function:
+
address@hidden
address@hidden
+(defun graph-body-print (numbers-list)
+ "@address@hidden"
+ (let ((height @dots{}
+ @dots{}))
address@hidden group
+
address@hidden
+ (while numbers-list
+ @var{insert-columns-and-reposition-point}
+ (setq numbers-list (cdr numbers-list)))))
address@hidden group
address@hidden smallexample
+
address@hidden
+We need to fill in the slots of the template.
+
+Clearly, we can use the @code{(apply 'max numbers-list)} expression to
+determine the height of the graph.
+
+The @code{while} loop will cycle through the @code{numbers-list} one
+element at a time. As it is shortened by the @code{(setq numbers-list
+(cdr numbers-list))} expression, the @sc{car} of each instance of the
+list is the value of the argument for @code{column-of-graph}.
+
+At each cycle of the @code{while} loop, the @code{insert-rectangle}
+function inserts the list returned by @code{column-of-graph}. Since
+the @code{insert-rectangle} function moves point to the lower right of
+the inserted rectangle, we need to save the location of point at the
+time the rectangle is inserted, move back to that position after the
+rectangle is inserted, and then move horizontally to the next place
+from which @code{insert-rectangle} is called.
+
+If the inserted columns are one character wide, as they will be if
+single blanks and asterisks are used, the repositioning command is
+simply @code{(forward-char 1)}; however, the width of a column may be
+greater than one. This means that the repositioning command should be
+written @code{(forward-char symbol-width)}. The @code{symbol-width}
+itself is the length of a @code{graph-blank} and can be found using
+the expression @code{(length graph-blank)}. The best place to bind
+the @code{symbol-width} variable to the value of the width of graph
+column is in the varlist of the @code{let} expression.
+
address@hidden 1250
+These considerations lead to the following function definition:
+
address@hidden
address@hidden
+(defun graph-body-print (numbers-list)
+ "Print a bar graph of the NUMBERS-LIST.
+The numbers-list consists of the Y-axis values."
+
+ (let ((height (apply 'max numbers-list))
+ (symbol-width (length graph-blank))
+ from-position)
address@hidden group
+
address@hidden
+ (while numbers-list
+ (setq from-position (point))
+ (insert-rectangle
+ (column-of-graph height (car numbers-list)))
+ (goto-char from-position)
+ (forward-char symbol-width)
address@hidden group
address@hidden
+ ;; @r{Draw graph column by column.}
+ (sit-for 0)
+ (setq numbers-list (cdr numbers-list)))
address@hidden group
address@hidden
+ ;; @r{Place point for X axis labels.}
+ (forward-line height)
+ (insert "\n")
+))
address@hidden group
address@hidden smallexample
+
address@hidden
+The one unexpected expression in this function is the
address@hidden@code{(sit-for 0)}} expression in the @code{while} loop. This
+expression makes the graph printing operation more interesting to
+watch than it would be otherwise. The expression causes Emacs to
+`sit' or do nothing for a zero length of time and then redraw the
+screen. Placed here, it causes Emacs to redraw the screen column by
+column. Without it, Emacs would not redraw the screen until the
+function exits.
+
+We can test @code{graph-body-print} with a short list of numbers.
+
address@hidden
address@hidden
+Install @code{graph-symbol}, @code{graph-blank},
address@hidden, which are in
address@hidden
address@hidden a Graph, , Readying a Graph},
address@hidden iftex
address@hidden
address@hidden of a graph},
address@hidden ifinfo
+and @code{graph-body-print}.
+
address@hidden 800
address@hidden
+Copy the following expression:
+
address@hidden
+(graph-body-print '(1 2 3 4 6 4 3 5 7 6 5 2 3))
address@hidden smallexample
+
address@hidden
+Switch to the @file{*scratch*} buffer and place the cursor where you
+want the graph to start.
+
address@hidden
+Type @kbd{M-:} (@code{eval-expression}).
+
address@hidden
+Yank the @code{graph-body-print} expression into the minibuffer
+with @kbd{C-y} (@code{yank)}.
+
address@hidden
+Press @key{RET} to evaluate the @code{graph-body-print} expression.
address@hidden enumerate
+
address@hidden 800
+Emacs will print a graph like this:
+
address@hidden
address@hidden
+ *
+ * **
+ * ****
+ *** ****
+ ********* *
+ ************
+ *************
address@hidden group
address@hidden smallexample
+
address@hidden recursive-graph-body-print, Printed Axes, graph-body-print,
Readying a Graph
address@hidden The @code{recursive-graph-body-print} Function
address@hidden recursive-graph-body-print
+
+The @code{graph-body-print} function may also be written recursively.
+The recursive solution is divided into two parts: an outside `wrapper'
+that uses a @code{let} expression to determine the values of several
+variables that need only be found once, such as the maximum height of
+the graph, and an inside function that is called recursively to print
+the graph.
+
address@hidden 1250
+The `wrapper' is uncomplicated:
+
address@hidden
address@hidden
+(defun recursive-graph-body-print (numbers-list)
+ "Print a bar graph of the NUMBERS-LIST.
+The numbers-list consists of the Y-axis values."
+ (let ((height (apply 'max numbers-list))
+ (symbol-width (length graph-blank))
+ from-position)
+ (recursive-graph-body-print-internal
+ numbers-list
+ height
+ symbol-width)))
address@hidden group
address@hidden smallexample
+
+The recursive function is a little more difficult. It has four parts:
+the `do-again-test', the printing code, the recursive call, and the
+`next-step-expression'. The `do-again-test' is a @code{when}
+expression that determines whether the @code{numbers-list} contains
+any remaining elements; if it does, the function prints one column of
+the graph using the printing code and calls itself again. The
+function calls itself again according to the value produced by the
+`next-step-expression' which causes the call to act on a shorter
+version of the @code{numbers-list}.
+
address@hidden
address@hidden
+(defun recursive-graph-body-print-internal
+ (numbers-list height symbol-width)
+ "Print a bar graph.
+Used within recursive-graph-body-print function."
address@hidden group
+
address@hidden
+ (when numbers-list
+ (setq from-position (point))
+ (insert-rectangle
+ (column-of-graph height (car numbers-list)))
address@hidden group
address@hidden
+ (goto-char from-position)
+ (forward-char symbol-width)
+ (sit-for 0) ; @r{Draw graph column by column.}
+ (recursive-graph-body-print-internal
+ (cdr numbers-list) height symbol-width)))
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+After installation, this expression can be tested; here is a sample:
+
address@hidden
+(recursive-graph-body-print '(3 2 5 6 7 5 3 4 6 4 3 2 1))
address@hidden smallexample
+
address@hidden 800
+Here is what @code{recursive-graph-body-print} produces:
+
address@hidden
address@hidden
+ *
+ ** *
+ **** *
+ **** ***
+ * *********
+ ************
+ *************
address@hidden group
address@hidden smallexample
+
+Either of these two functions, @code{graph-body-print} or
address@hidden, create the body of a graph.
+
address@hidden Printed Axes, Line Graph Exercise, recursive-graph-body-print,
Readying a Graph
address@hidden Need for Printed Axes
+
+A graph needs printed axes, so you can orient yourself. For a do-once
+project, it may be reasonable to draw the axes by hand using Emacs'
+Picture mode; but a graph drawing function may be used more than once.
+
+For this reason, I have written enhancements to the basic
address@hidden function that automatically print labels for
+the horizontal and vertical axes. Since the label printing functions
+do not contain much new material, I have placed their description in
+an appendix. @xref{Full Graph, , A Graph with Labelled Axes}.
+
address@hidden Line Graph Exercise, , Printed Axes, Readying a Graph
address@hidden Exercise
+
+Write a line graph version of the graph printing functions.
+
address@hidden Emacs Initialization, Debugging, Readying a Graph, Top
address@hidden Your @file{.emacs} File
address@hidden @file{.emacs} file
address@hidden Customizing your @file{.emacs} file
address@hidden Initialization file
+
+``You don't have to like Emacs to like it'' -- this seemingly
+paradoxical statement is the secret of GNU Emacs. The plain, `out of
+the box' Emacs is a generic tool. Most people who use it, customize
+it to suit themselves.
+
+GNU Emacs is mostly written in Emacs Lisp; this means that by writing
+expressions in Emacs Lisp you can change or extend Emacs.
+
address@hidden
+* Default Configuration::
+* Site-wide Init:: You can write site-wide init files.
+* defcustom:: Emacs will write code for you.
+* Beginning a .emacs File:: How to write a @code{.emacs file}.
+* Text and Auto-fill:: Automatically wrap lines.
+* Mail Aliases:: Use abbreviations for email addresses.
+* Indent Tabs Mode:: Don't use tabs with @TeX{}
+* Keybindings:: Create some personal keybindings.
+* Keymaps:: More about key binding.
+* Loading Files:: Load (i.e., evaluate) files automatically.
+* Autoload:: Make functions available.
+* Simple Extension:: Define a function; bind it to a key.
+* X11 Colors:: Colors in X.
+* Miscellaneous::
+* Mode Line:: How to customize your mode line.
address@hidden menu
+
address@hidden Default Configuration, Site-wide Init, Emacs Initialization,
Emacs Initialization
address@hidden
address@hidden Emacs' Default Configuration
address@hidden ifnottex
+
+There are those who appreciate Emacs' default configuration. After
+all, Emacs starts you in C mode when you edit a C file, starts you in
+Fortran mode when you edit a Fortran file, and starts you in
+Fundamental mode when you edit an unadorned file. This all makes
+sense, if you do not know who is going to use Emacs. Who knows what a
+person hopes to do with an unadorned file? Fundamental mode is the
+right default for such a file, just as C mode is the right default for
+editing C code. (Enough programming languages have syntaxes
+that enable them to share or nearly share features, so C mode is
+now provided by by CC mode, the `C Collection'.)
+
+But when you do know who is going to use Emacs---you,
+yourself---then it makes sense to customize Emacs.
+
+For example, I seldom want Fundamental mode when I edit an
+otherwise undistinguished file; I want Text mode. This is why I
+customize Emacs: so it suits me.
+
+You can customize and extend Emacs by writing or adapting a
address@hidden/.emacs} file. This is your personal initialization file; its
+contents, written in Emacs Lisp, tell Emacs what to address@hidden
+may also add @file{.el} to @file{~/.emacs} and call it a
address@hidden/.emacs.el} file. In the past, you were forbidden to type the
+extra keystrokes that the name @file{~/.emacs.el} requires, but now
+you may. The new format is consistent with the Emacs Lisp file
+naming conventions; the old format saves typing.}
+
+A @file{~/.emacs} file contains Emacs Lisp code. You can write this
+code yourself; or you can use Emacs' @code{customize} feature to write
+the code for you. You can combine your own expressions and
+auto-written Customize expressions in your @file{.emacs} file.
+
+(I myself prefer to write my own expressions, except for those,
+particularly fonts, that I find easier to manipulate using the
address@hidden command. I combine the two methods.)
+
+Most of this chapter is about writing expressions yourself. It
+describes a simple @file{.emacs} file; for more information, see
address@hidden File, , The Init File, emacs, The GNU Emacs Manual}, and
address@hidden File, , The Init File, elisp, The GNU Emacs Lisp Reference
+Manual}.
+
address@hidden Site-wide Init, defcustom, Default Configuration, Emacs
Initialization
address@hidden Site-wide Initialization Files
+
address@hidden @file{default.el} init file
address@hidden @file{site-init.el} init file
address@hidden @file{site-load.el} init file
+In addition to your personal initialization file, Emacs automatically
+loads various site-wide initialization files, if they exist. These
+have the same form as your @file{.emacs} file, but are loaded by
+everyone.
+
+Two site-wide initialization files, @file{site-load.el} and
address@hidden, are loaded into Emacs and then `dumped' if a
+`dumped' version of Emacs is created, as is most common. (Dumped
+copies of Emacs load more quickly. However, once a file is loaded and
+dumped, a change to it does not lead to a change in Emacs unless you
+load it yourself or re-dump Emacs. @xref{Building Emacs, , Building
+Emacs, elisp, The GNU Emacs Lisp Reference Manual}, and the
address@hidden file.)
+
+Three other site-wide initialization files are loaded automatically
+each time you start Emacs, if they exist. These are
address@hidden, which is loaded @emph{before} your @file{.emacs}
+file, and @file{default.el}, and the terminal type file, which are both
+loaded @emph{after} your @file{.emacs} file.
+
+Settings and definitions in your @file{.emacs} file will overwrite
+conflicting settings and definitions in a @file{site-start.el} file,
+if it exists; but the settings and definitions in a @file{default.el}
+or terminal type file will overwrite those in your @file{.emacs} file.
+(You can prevent interference from a terminal type file by setting
address@hidden to @code{nil}. @xref{Simple Extension, , A
+Simple Extension}.)
+
address@hidden Rewritten to avoid overfull hbox.
+The @file{INSTALL} file that comes in the distribution contains
+descriptions of the @file{site-init.el} and @file{site-load.el} files.
+
+The @file{loadup.el}, @file{startup.el}, and @file{loaddefs.el} files
+control loading. These files are in the @file{lisp} directory of the
+Emacs distribution and are worth perusing.
+
+The @file{loaddefs.el} file contains a good many suggestions as to
+what to put into your own @file{.emacs} file, or into a site-wide
+initialization file.
+
address@hidden defcustom, Beginning a .emacs File, Site-wide Init, Emacs
Initialization
address@hidden Specifying Variables using @code{defcustom}
address@hidden defcustom
+
+You can specify variables using @code{defcustom} so that you and
+others can then use Emacs' @code{customize} feature to set their
+values. (You cannot use @code{customize} to write function
+definitions; but you can write @code{defuns} in your @file{.emacs}
+file. Indeed, you can write any Lisp expression in your @file{.emacs}
+file.)
+
+The @code{customize} feature depends on the @code{defcustom} special
+form. Although you can use @code{defvar} or @code{setq} for variables
+that users set, the @code{defcustom} special form is designed for the
+job.
+
+You can use your knowledge of @code{defvar} for writing the
+first three arguments for @code{defcustom}. The first argument to
address@hidden is the name of the variable. The second argument is
+the variable's initial value, if any; and this value is set only if
+the value has not already been set. The third argument is the
+documentation.
+
+The fourth and subsequent arguments to @code{defcustom} specify types
+and options; these are not featured in @code{defvar}. (These
+arguments are optional.)
+
+Each of these arguments consists of a keyword followed by a value.
+Each keyword starts with the colon character @samp{:}.
+
address@hidden 1250
+For example, the customizable user option variable
address@hidden looks like this:
+
address@hidden
address@hidden
+(defcustom text-mode-hook nil
+ "Normal hook run when entering Text mode and many related modes."
+ :type 'hook
+ :options '(turn-on-auto-fill flyspell-mode)
+ :group 'data)
address@hidden group
address@hidden smallexample
+
address@hidden
+The name of the variable is @code{text-mode-hook}; it has no default
+value; and its documentation string tells you what it does.
+
+The @code{:type} keyword tells Emacs the kind of data to which
address@hidden should be set and how to display the value in a
+Customization buffer.
+
+The @code{:options} keyword specifies a suggested list of values for
+the variable. Usually, @code{:options} applies to a hook.
+The list is only a suggestion; it is not exclusive; a person who sets
+the variable may set it to other values; the list shown following the
address@hidden:options} keyword is intended to offer convenient choices to a
+user.
+
+Finally, the @code{:group} keyword tells the Emacs Customization
+command in which group the variable is located. This tells where to
+find it.
+
+The @code{defcustom} function recognizes more than a dozen keywords.
+For more information, see @ref{Customization, , Writing Customization
+Definitions, elisp, The GNU Emacs Lisp Reference Manual}.
+
+Consider @code{text-mode-hook} as an example.
+
+There are two ways to customize this variable. You can use the
+customization command or write the appropriate expressions yourself.
+
address@hidden 800
+Using the customization command, you can type:
+
address@hidden
+M-x customize
address@hidden smallexample
+
address@hidden
+and find that the group for editing files of data is called `data'.
+Enter that group. Text Mode Hook is the first member. You can click
+on its various options, such as @code{turn-on-auto-fill}, to set the
+values. After you click on the button to
+
address@hidden
+Save for Future Sessions
address@hidden smallexample
+
address@hidden
+Emacs will write an expression into your @file{.emacs} file.
+It will look like this:
+
address@hidden
address@hidden
+(custom-set-variables
+ ;; custom-set-variables was added by Custom.
+ ;; If you edit it by hand, you could mess it up, so be careful.
+ ;; Your init file should contain only one such instance.
+ ;; If there is more than one, they won't work right.
+ '(text-mode-hook (quote (turn-on-auto-fill text-mode-hook-identify))))
address@hidden group
address@hidden smallexample
+
address@hidden
+(The @code{text-mode-hook-identify} function tells
address@hidden which buffers are in Text mode.
+It comes on automatically.)
+
+The @code{custom-set-variables} function works somewhat differently
+than a @code{setq}. While I have never learned the differences, I
+modify the @code{custom-set-variables} expressions in my @file{.emacs}
+file by hand: I make the changes in what appears to me to be a
+reasonable manner and have not had any problems. Others prefer to use
+the Customization command and let Emacs do the work for them.
+
+Another @address@hidden function is @code{custom-set-faces}.
+This function sets the various font faces. Over time, I have set a
+considerable number of faces. Some of the time, I re-set them using
address@hidden; other times, I simply edit the
address@hidden expression in my @file{.emacs} file itself.
+
+The second way to customize your @code{text-mode-hook} is to set it
+yourself in your @file{.emacs} file using code that has nothing to do
+with the @address@hidden functions.
+
address@hidden 800
+When you do this, and later use @code{customize}, you will see a
+message that says
+
address@hidden
+CHANGED outside Customize; operating on it here may be unreliable.
address@hidden smallexample
+
address@hidden 800
+This message is only a warning. If you click on the button to
+
address@hidden
+Save for Future Sessions
address@hidden smallexample
+
address@hidden
+Emacs will write a @address@hidden expression near the end
+of your @file{.emacs} file that will be evaluated after your
+hand-written expression. It will, therefore, overrule your
+hand-written expression. No harm will be done. When you do this,
+however, be careful to remember which expression is active; if you
+forget, you may confuse yourself.
+
+So long as you remember where the values are set, you will have no
+trouble. In any event, the values are always set in your
+initialization file, which is usually called @file{.emacs}.
+
+I myself use @code{customize} for hardly anything. Mostly, I write
+expressions myself.
+
address@hidden defsubst
address@hidden defconst
+Incidentally, to be more complete concerning defines: @code{defsubst}
+defines an inline function. The syntax is just like that of
address@hidden @code{defconst} defines a symbol as a constant. The
+intent is that neither programs nor users should ever change a value
+set by @code{defconst}. (You can change it; the value set is a
+variable; but please do not.)
+
address@hidden Beginning a .emacs File, Text and Auto-fill, defcustom, Emacs
Initialization
address@hidden Beginning a @file{.emacs} File
address@hidden @file{.emacs} file, beginning of
+
+When you start Emacs, it loads your @file{.emacs} file unless you tell
+it not to by specifying @samp{-q} on the command line. (The
address@hidden -q} command gives you a plain, out-of-the-box Emacs.)
+
+A @file{.emacs} file contains Lisp expressions. Often, these are no
+more than expressions to set values; sometimes they are function
+definitions.
+
address@hidden File, , The Init File @file{~/.emacs}, emacs, The GNU Emacs
+Manual}, for a short description of initialization files.
+
+This chapter goes over some of the same ground, but is a walk among
+extracts from a complete, long-used @file{.emacs} file---my own.
+
+The first part of the file consists of comments: reminders to myself.
+By now, of course, I remember these things, but when I started, I did
+not.
+
address@hidden 1200
address@hidden
address@hidden
+;;;; Bob's .emacs file
+; Robert J. Chassell
+; 26 September 1985
address@hidden group
address@hidden smallexample
+
address@hidden
+Look at that date! I started this file a long time ago. I have been
+adding to it ever since.
+
address@hidden
address@hidden
+; Each section in this file is introduced by a
+; line beginning with four semicolons; and each
+; entry is introduced by a line beginning with
+; three semicolons.
address@hidden group
address@hidden smallexample
+
address@hidden
+This describes the usual conventions for comments in Emacs Lisp.
+Everything on a line that follows a semicolon is a comment. Two,
+three, and four semicolons are used as subsection and section markers.
+(@xref{Comments, ,, elisp, The GNU Emacs Lisp Reference Manual}, for
+more about comments.)
+
address@hidden
address@hidden
+;;;; The Help Key
+; Control-h is the help key;
+; after typing control-h, type a letter to
+; indicate the subject about which you want help.
+; For an explanation of the help facility,
+; type control-h two times in a row.
address@hidden group
address@hidden smallexample
+
address@hidden
+Just remember: type @kbd{C-h} two times for help.
+
address@hidden
address@hidden
+; To find out about any mode, type control-h m
+; while in that mode. For example, to find out
+; about mail mode, enter mail mode and then type
+; control-h m.
address@hidden group
address@hidden smallexample
+
address@hidden
+`Mode help', as I call this, is very helpful. Usually, it tells you
+all you need to know.
+
+Of course, you don't need to include comments like these in your
address@hidden file. I included them in mine because I kept forgetting
+about Mode help or the conventions for comments---but I was able to
+remember to look here to remind myself.
+
address@hidden Text and Auto-fill, Mail Aliases, Beginning a .emacs File, Emacs
Initialization
address@hidden Text and Auto Fill Mode
+
+Now we come to the part that `turns on' Text mode and
+Auto Fill mode.
+
address@hidden
address@hidden
+;;; Text mode and Auto Fill mode
+; The next two lines put Emacs into Text mode
+; and Auto Fill mode, and are for writers who
+; want to start writing prose rather than code.
+(setq default-major-mode 'text-mode)
+(add-hook 'text-mode-hook 'turn-on-auto-fill)
address@hidden group
address@hidden smallexample
+
+Here is the first part of this @file{.emacs} file that does something
+besides remind a forgetful human!
+
+The first of the two lines in parentheses tells Emacs to turn on Text
+mode when you find a file, @emph{unless} that file should go into some
+other mode, such as C mode.
+
address@hidden Per-buffer, local variables list
address@hidden Local variables list, per-buffer,
address@hidden Automatic mode selection
address@hidden Mode selection, automatic
+When Emacs reads a file, it looks at the extension to the file name,
+if any. (The extension is the part that comes after a @samp{.}.) If
+the file ends with a @samp{.c} or @samp{.h} extension then Emacs turns
+on C mode. Also, Emacs looks at first nonblank line of the file; if
+the line says @address@hidden C -*-}}, Emacs turns on C mode. Emacs
+possesses a list of extensions and specifications that it uses
+automatically. In addition, Emacs looks near the last page for a
+per-buffer, ``local variables list'', if any.
+
address@hidden
address@hidden Modes, , How Major Modes are Chosen, emacs, The GNU
+Emacs Manual}.
+
address@hidden Variables, , Local Variables in Files, emacs, The GNU Emacs
+Manual}.
address@hidden ifinfo
address@hidden
+See sections ``How Major Modes are Chosen'' and ``Local Variables in
+Files'' in @cite{The GNU Emacs Manual}.
address@hidden iftex
+
+Now, back to the @file{.emacs} file.
+
address@hidden 800
+Here is the line again; how does it work?
+
address@hidden Text Mode turned on
address@hidden
+(setq default-major-mode 'text-mode)
address@hidden smallexample
+
address@hidden
+This line is a short, but complete Emacs Lisp expression.
+
+We are already familiar with @code{setq}. It sets the following variable,
address@hidden, to the subsequent value, which is
address@hidden The single quote mark before @code{text-mode} tells
+Emacs to deal directly with the @code{text-mode} variable, not with
+whatever it might stand for. @xref{set & setq, , Setting the Value of
+a Variable}, for a reminder of how @code{setq} works. The main point
+is that there is no difference between the procedure you use to set
+a value in your @file{.emacs} file and the procedure you use anywhere
+else in Emacs.
+
address@hidden 800
+Here is the next line:
+
address@hidden Auto Fill mode turned on
address@hidden add-hook
address@hidden
+(add-hook 'text-mode-hook 'turn-on-auto-fill)
address@hidden smallexample
+
address@hidden
+In this line, the @code{add-hook} command adds
address@hidden to the variable.
+
address@hidden is the name of a program, that, you guessed
+it!, turns on Auto Fill mode.
+
+Every time Emacs turns on Text mode, Emacs runs the commands `hooked'
+onto Text mode. So every time Emacs turns on Text mode, Emacs also
+turns on Auto Fill mode.
+
+In brief, the first line causes Emacs to enter Text mode when you edit a
+file, unless the file name extension, a first non-blank line, or local
+variables to tell Emacs otherwise.
+
+Text mode among other actions, sets the syntax table to work
+conveniently for writers. In Text mode, Emacs considers an apostrophe
+as part of a word like a letter; but Emacs does not consider a period
+or a space as part of a word. Thus, @kbd{M-f} moves you over
address@hidden's}. On the other hand, in C mode, @kbd{M-f} stops just after
+the @samp{t} of @samp{it's}.
+
+The second line causes Emacs to turn on Auto Fill mode when it turns
+on Text mode. In Auto Fill mode, Emacs automatically breaks a line
+that is too wide and brings the excessively wide part of the line down
+to the next line. Emacs breaks lines between words, not within them.
+
+When Auto Fill mode is turned off, lines continue to the right as you
+type them. Depending on how you set the value of
address@hidden, the words you type either disappear off the
+right side of the screen, or else are shown, in a rather ugly and
+unreadable manner, as a continuation line on the screen.
+
address@hidden 1250
+In addition, in this part of my @file{.emacs} file, I tell the Emacs
+fill commands to insert two spaces after a colon:
+
address@hidden
+(setq colon-double-space t)
address@hidden smallexample
+
address@hidden Mail Aliases, Indent Tabs Mode, Text and Auto-fill, Emacs
Initialization
address@hidden Mail Aliases
+
+Here is a @code{setq} that `turns on' mail aliases, along with more
+reminders.
+
address@hidden
address@hidden
+;;; Mail mode
+; To enter mail mode, type `C-x m'
+; To enter RMAIL (for reading mail),
+; type `M-x rmail'
+(setq mail-aliases t)
address@hidden group
address@hidden smallexample
+
address@hidden Mail aliases
address@hidden
+This @code{setq} command sets the value of the variable
address@hidden to @code{t}. Since @code{t} means true, the line
+says, in effect, ``Yes, use mail aliases.''
+
+Mail aliases are convenient short names for long email addresses or
+for lists of email addresses. The file where you keep your `aliases'
+is @file{~/.mailrc}. You write an alias like this:
+
address@hidden
+alias geo george@@foobar.wiz.edu
address@hidden smallexample
+
address@hidden
+When you write a message to George, address it to @samp{geo}; the
+mailer will automatically expand @samp{geo} to the full address.
+
address@hidden Indent Tabs Mode, Keybindings, Mail Aliases, Emacs Initialization
address@hidden Indent Tabs Mode
address@hidden Tabs, preventing
address@hidden indent-tabs-mode
+
+By default, Emacs inserts tabs in place of multiple spaces when it
+formats a region. (For example, you might indent many lines of text
+all at once with the @code{indent-region} command.) Tabs look fine on
+a terminal or with ordinary printing, but they produce badly indented
+output when you use @TeX{} or Texinfo since @TeX{} ignores tabs.
+
address@hidden 1250
+The following turns off Indent Tabs mode:
+
address@hidden
address@hidden
+;;; Prevent Extraneous Tabs
+(setq-default indent-tabs-mode nil)
address@hidden group
address@hidden smallexample
+
+Note that this line uses @code{setq-default} rather than the
address@hidden command that we have seen before. The @code{setq-default}
+command sets values only in buffers that do not have their own local
+values for the variable.
+
address@hidden
address@hidden Spaces, , Tabs vs. Spaces, emacs, The GNU Emacs Manual}.
+
address@hidden Variables, , Local Variables in Files, emacs, The GNU Emacs
+Manual}.
address@hidden ifinfo
address@hidden
+See sections ``Tabs vs.@: Spaces'' and ``Local Variables in
+Files'' in @cite{The GNU Emacs Manual}.
address@hidden iftex
+
address@hidden 1700
address@hidden Keybindings, Keymaps, Indent Tabs Mode, Emacs Initialization
address@hidden Some Keybindings
+
+Now for some personal keybindings:
+
address@hidden
address@hidden
+;;; Compare windows
+(global-set-key "\C-cw" 'compare-windows)
address@hidden group
address@hidden smallexample
+
address@hidden compare-windows
address@hidden is a nifty command that compares the text in
+your current window with text in the next window. It makes the
+comparison by starting at point in each window, moving over text in
+each window as far as they match. I use this command all the time.
+
+This also shows how to set a key globally, for all modes.
+
address@hidden Setting a key globally
address@hidden Global set key
address@hidden Key setting globally
address@hidden global-set-key
+The command is @code{global-set-key}. It is followed by the
+keybinding. In a @file{.emacs} file, the keybinding is written as
+shown: @code{\C-c} stands for `control-c', which means `press the
+control key and the @key{c} key at the same time'. The @code{w} means
+`press the @key{w} key'. The keybinding is surrounded by double
+quotation marks. In documentation, you would write this as
address@hidden@kbd{C-c w}}. (If you were binding a @key{META} key, such as
address@hidden, rather than a @key{CTRL} key, you would write
address@hidden@code{\M-c}} in your @file{.emacs} file. @xref{Init Rebinding, ,
+Rebinding Keys in Your Init File, emacs, The GNU Emacs Manual}, for
+details.)
+
+The command invoked by the keys is @code{compare-windows}. Note that
address@hidden is preceded by a single quote; otherwise, Emacs
+would first try to evaluate the symbol to determine its value.
+
+These three things, the double quotation marks, the backslash before
+the @samp{C}, and the single quote mark are necessary parts of
+keybinding that I tend to forget. Fortunately, I have come to
+remember that I should look at my existing @file{.emacs} file, and
+adapt what is there.
+
+As for the keybinding itself: @kbd{C-c w}. This combines the prefix
+key, @kbd{C-c}, with a single character, in this case, @kbd{w}. This
+set of keys, @kbd{C-c} followed by a single character, is strictly
+reserved for individuals' own use. (I call these `own' keys, since
+these are for my own use.) You should always be able to create such a
+keybinding for your own use without stomping on someone else's
+keybinding. If you ever write an extension to Emacs, please avoid
+taking any of these keys for public use. Create a key like @kbd{C-c
+C-w} instead. Otherwise, we will run out of `own' keys.
+
address@hidden 1250
+Here is another keybinding, with a comment:
+
address@hidden
address@hidden
+;;; Keybinding for `occur'
+; I use occur a lot, so let's bind it to a key:
+(global-set-key "\C-co" 'occur)
address@hidden group
address@hidden smallexample
+
address@hidden occur
+The @code{occur} command shows all the lines in the current buffer
+that contain a match for a regular expression. Matching lines are
+shown in a buffer called @file{*Occur*}. That buffer serves as a menu
+to jump to occurrences.
+
address@hidden global-unset-key
address@hidden Unbinding key
address@hidden Key unbinding
address@hidden 1250
+Here is how to unbind a key, so it does not
+work:
+
address@hidden
address@hidden
+;;; Unbind `C-x f'
+(global-unset-key "\C-xf")
address@hidden group
address@hidden smallexample
+
+There is a reason for this unbinding: I found I inadvertently typed
address@hidden@kbd{C-x f}} when I meant to type @kbd{C-x C-f}. Rather than
find a
+file, as I intended, I accidentally set the width for filled text,
+almost always to a width I did not want. Since I hardly ever reset my
+default width, I simply unbound the key.
+
address@hidden list-buffers, @r{rebound}
address@hidden buffer-menu, @r{bound to key}
address@hidden 1250
+The following rebinds an existing key:
+
address@hidden
address@hidden
+;;; Rebind `C-x C-b' for `buffer-menu'
+(global-set-key "\C-x\C-b" 'buffer-menu)
address@hidden group
address@hidden smallexample
+
+By default, @kbd{C-x C-b} runs the
address@hidden command. This command lists
+your buffers in @emph{another} window. Since I
+almost always want to do something in that
+window, I prefer the @code{buffer-menu}
+command, which not only lists the buffers,
+but moves point into that window.
+
address@hidden Keymaps, Loading Files, Keybindings, Emacs Initialization
address@hidden Keymaps
address@hidden Keymaps
address@hidden Rebinding keys
+
+Emacs uses @dfn{keymaps} to record which keys call which commands.
+When you use @code{global-set-key} to set the keybinding for a single
+command in all parts of Emacs, you are specifying the keybinding in
address@hidden
+
+Specific modes, such as C mode or Text mode, have their own keymaps;
+the mode-specific keymaps override the global map that is shared by
+all buffers.
+
+The @code{global-set-key} function binds, or rebinds, the global
+keymap. For example, the following binds the key @kbd{C-x C-b} to the
+function @code{buffer-menu}:
+
address@hidden
+(global-set-key "\C-x\C-b" 'buffer-menu)
address@hidden smallexample
+
+Mode-specific keymaps are bound using the @code{define-key} function,
+which takes a specific keymap as an argument, as well as the key and
+the command. For example, my @file{.emacs} file contains the
+following expression to bind the @code{texinfo-insert-@@group} command
+to @kbd{C-c C-c g}:
+
address@hidden
address@hidden
+(define-key texinfo-mode-map "\C-c\C-cg" 'texinfo-insert-@@group)
address@hidden group
address@hidden smallexample
+
address@hidden
+The @code{texinfo-insert-@@group} function itself is a little extension
+to Texinfo mode that inserts @samp{@@group} into a Texinfo file. I
+use this command all the time and prefer to type the three strokes
address@hidden C-c g} rather than the six strokes @kbd{@@ g r o u p}.
+(@samp{@@group} and its matching @samp{@@end group} are commands that
+keep all enclosed text together on one page; many multi-line examples
+in this book are surrounded by @samp{@@group @dots{} @@end group}.)
+
address@hidden 1250
+Here is the @code{texinfo-insert-@@group} function definition:
+
address@hidden
address@hidden
+(defun texinfo-insert-@@group ()
+ "Insert the string @@group in a Texinfo buffer."
+ (interactive)
+ (beginning-of-line)
+ (insert "@@group\n"))
address@hidden group
address@hidden smallexample
+
+(Of course, I could have used Abbrev mode to save typing, rather than
+write a function to insert a word; but I prefer key strokes consistent
+with other Texinfo mode key bindings.)
+
+You will see numerous @code{define-key} expressions in
address@hidden as well as in the various mode libraries, such as
address@hidden and @file{lisp-mode.el}.
+
address@hidden Bindings, , Customizing Key Bindings, emacs, The GNU Emacs
+Manual}, and @ref{Keymaps, , Keymaps, elisp, The GNU Emacs Lisp
+Reference Manual}, for more information about keymaps.
+
address@hidden Loading Files, Autoload, Keymaps, Emacs Initialization
address@hidden Loading Files
address@hidden Loading files
address@hidden findex load
+
+Many people in the GNU Emacs community have written extensions to
+Emacs. As time goes by, these extensions are often included in new
+releases. For example, the Calendar and Diary packages are now part
+of the standard GNU Emacs, as is Calc.
+
+You can use a @code{load} command to evaluate a complete file and
+thereby install all the functions and variables in the file into Emacs.
+For example:
+
address@hidden (auto-compression-mode t)
+
address@hidden
+(load "~/emacs/slowsplit")
address@hidden smallexample
+
+This evaluates, i.e.@: loads, the @file{slowsplit.el} file or if it
+exists, the faster, byte compiled @file{slowsplit.elc} file from the
address@hidden sub-directory of your home directory. The file contains
+the function @code{split-window-quietly}, which John Robinson wrote in
+1989.
+
+The @code{split-window-quietly} function splits a window with the
+minimum of redisplay. I installed it in 1989 because it worked well
+with the slow 1200 baud terminals I was then using. Nowadays, I only
+occasionally come across such a slow connection, but I continue to use
+the function because I like the way it leaves the bottom half of a
+buffer in the lower of the new windows and the top half in the upper
+window.
+
address@hidden 1250
+To replace the key binding for the default
address@hidden, you must also unset that key and bind
+the keys to @code{split-window-quietly}, like this:
+
address@hidden
address@hidden
+(global-unset-key "\C-x2")
+(global-set-key "\C-x2" 'split-window-quietly)
address@hidden group
address@hidden smallexample
+
address@hidden load-path
+If you load many extensions, as I do, then instead of specifying the
+exact location of the extension file, as shown above, you can specify
+that directory as part of Emacs' @code{load-path}. Then, when Emacs
+loads a file, it will search that directory as well as its default
+list of directories. (The default list is specified in @file{paths.h}
+when Emacs is built.)
+
address@hidden 1250
+The following command adds your @file{~/emacs} directory to the
+existing load path:
+
address@hidden
address@hidden
+;;; Emacs Load Path
+(setq load-path (cons "~/emacs" load-path))
address@hidden group
address@hidden smallexample
+
+Incidentally, @code{load-library} is an interactive interface to the
address@hidden function. The complete function looks like this:
+
address@hidden load-library
address@hidden
address@hidden
+(defun load-library (library)
+ "Load the library named LIBRARY.
+This is an interface to the function `load'."
+ (interactive
+ (list (completing-read "Load library: "
+ 'locate-file-completion
+ (cons load-path (get-load-suffixes)))))
+ (load library))
address@hidden group
address@hidden smallexample
+
+The name of the function, @code{load-library}, comes from the use of
+`library' as a conventional synonym for `file'. The source for the
address@hidden command is in the @file{files.el} library.
+
+Another interactive command that does a slightly different job is
address@hidden @xref{Lisp Libraries, , Libraries of Lisp Code for
+Emacs, emacs, The GNU Emacs Manual}, for information on the
+distinction between @code{load-library} and this command.
+
address@hidden Autoload, Simple Extension, Loading Files, Emacs Initialization
address@hidden Autoloading
address@hidden autoload
+
+Instead of installing a function by loading the file that contains it,
+or by evaluating the function definition, you can make the function
+available but not actually install it until it is first called. This
+is called @dfn{autoloading}.
+
+When you execute an autoloaded function, Emacs automatically evaluates
+the file that contains the definition, and then calls the function.
+
+Emacs starts quicker with autoloaded functions, since their libraries
+are not loaded right away; but you need to wait a moment when you
+first use such a function, while its containing file is evaluated.
+
+Rarely used functions are frequently autoloaded. The
address@hidden library contains hundreds of autoloaded functions,
+from @code{bookmark-set} to @code{wordstar-mode}. Of course, you may
+come to use a `rare' function frequently. When you do, you should
+load that function's file with a @code{load} expression in your
address@hidden file.
+
+In my @file{.emacs} file, I load 14 libraries that contain functions
+that would otherwise be autoloaded. (Actually, it would have been
+better to include these files in my `dumped' Emacs, but I forgot.
address@hidden Emacs, , Building Emacs, elisp, The GNU Emacs Lisp
+Reference Manual}, and the @file{INSTALL} file for more about
+dumping.)
+
+You may also want to include autoloaded expressions in your @file{.emacs}
+file. @code{autoload} is a built-in function that takes up to five
+arguments, the final three of which are optional. The first argument
+is the name of the function to be autoloaded; the second is the name
+of the file to be loaded. The third argument is documentation for the
+function, and the fourth tells whether the function can be called
+interactively. The fifth argument tells what type of
address@hidden can handle a keymap or macro as well as a
+function (the default is a function).
+
address@hidden 800
+Here is a typical example:
+
address@hidden
address@hidden
+(autoload 'html-helper-mode
+ "html-helper-mode" "Edit HTML documents" t)
address@hidden group
address@hidden smallexample
+
address@hidden
+(@code{html-helper-mode} is an older alternative to @code{html-mode},
+which is a standard part of the distribution.)
+
address@hidden
+This expression autoloads the @code{html-helper-mode} function. It
+takes it from the @file{html-helper-mode.el} file (or from the byte
+compiled file @file{html-helper-mode.elc}, if it exists.) The file
+must be located in a directory specified by @code{load-path}. The
+documentation says that this is a mode to help you edit documents
+written in the HyperText Markup Language. You can call this mode
+interactively by typing @kbd{M-x html-helper-mode}. (You need to
+duplicate the function's regular documentation in the autoload
+expression because the regular function is not yet loaded, so its
+documentation is not available.)
+
address@hidden, , Autoload, elisp, The GNU Emacs Lisp Reference
+Manual}, for more information.
+
address@hidden Simple Extension, X11 Colors, Autoload, Emacs Initialization
address@hidden A Simple Extension: @code{line-to-top-of-window}
address@hidden line-to-top-of-window
address@hidden Simple extension in @file{.emacs} file
+
+Here is a simple extension to Emacs that moves the line point is on to
+the top of the window. I use this all the time, to make text easier
+to read.
+
+You can put the following code into a separate file and then load it
+from your @file{.emacs} file, or you can include it within your
address@hidden file.
+
address@hidden 1250
+Here is the definition:
+
address@hidden
address@hidden
+;;; Line to top of window;
+;;; replace three keystroke sequence C-u 0 C-l
+(defun line-to-top-of-window ()
+ "Move the line point is on to top of window."
+ (interactive)
+ (recenter 0))
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+Now for the keybinding.
+
+Nowadays, function keys as well as mouse button events and
address@hidden characters are written within square brackets, without
+quotation marks. (In Emacs version 18 and before, you had to write
+different function key bindings for each different make of terminal.)
+
+I bind @code{line-to-top-of-window} to my @key{F6} function key like
+this:
+
address@hidden
+(global-set-key [f6] 'line-to-top-of-window)
address@hidden smallexample
+
+For more information, see @ref{Init Rebinding, , Rebinding Keys in
+Your Init File, emacs, The GNU Emacs Manual}.
+
address@hidden Conditional 'twixt two versions of Emacs
address@hidden Version of Emacs, choosing
address@hidden Emacs version, choosing
+If you run two versions of GNU Emacs, such as versions 21 and 22, and
+use one @file{.emacs} file, you can select which code to evaluate with
+the following conditional:
+
address@hidden
address@hidden
+(cond
+ (= 21 emacs-major-version)
+ ;; evaluate version 21 code
+ ( @dots{} ))
+ (= 22 emacs-major-version)
+ ;; evaluate version 22 code
+ ( @dots{} )))
address@hidden group
address@hidden smallexample
+
+For example, in contrast to version 20, more recent versions blink
+their cursors by default. I hate such blinking, as well as other
+features, so I placed the following in my @file{.emacs}
address@hidden I start instances of Emacs that do not load my
address@hidden file or any site file, I also turn off blinking:
+
address@hidden
+emacs -q --no-site-file -eval '(blink-cursor-mode nil)'
+
address@hidden Or nowadays, using an even more sophisticated set of options,
+
+emacs -Q - D
address@hidden smallexample
+}:
+
address@hidden
address@hidden
+(when (or (= 21 emacs-major-version)
+ (= 22 emacs-major-version))
+ (blink-cursor-mode 0)
+ ;; Insert newline when you press `C-n' (next-line)
+ ;; at the end of the buffer
+ (setq next-line-add-newlines t)
address@hidden group
address@hidden
+ ;; Turn on image viewing
+ (auto-image-file-mode t)
address@hidden group
address@hidden
+ ;; Turn on menu bar (this bar has text)
+ ;; (Use numeric argument to turn on)
+ (menu-bar-mode 1)
address@hidden group
address@hidden
+ ;; Turn off tool bar (this bar has icons)
+ ;; (Use numeric argument to turn on)
+ (tool-bar-mode nil)
address@hidden group
address@hidden
+ ;; Turn off tooltip mode for tool bar
+ ;; (This mode causes icon explanations to pop up)
+ ;; (Use numeric argument to turn on)
+ (tooltip-mode nil)
+ ;; If tooltips turned on, make tips appear promptly
+ (setq tooltip-delay 0.1) ; default is 0.7 second
+ )
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+Alternatively, since @code{blink-cursor-mode} has existed since Emacs
+version 21 and is likely to continue, you could write
+
address@hidden
address@hidden
+(when (>= emacs-major-version 21)
+ (blink-cursor-mode 0)
address@hidden group
address@hidden smallexample
+
address@hidden
+and add other expressions, too.
+
+
address@hidden X11 Colors, Miscellaneous, Simple Extension, Emacs Initialization
address@hidden X11 Colors
+
+You can specify colors when you use Emacs with the MIT X Windowing
+system.
+
+I dislike the default colors and specify my own.
+
address@hidden 1250
+Here are the expressions in my @file{.emacs}
+file that set values:
+
address@hidden
address@hidden
+;; Set cursor color
+(set-cursor-color "white")
+
+;; Set mouse color
+(set-mouse-color "white")
+
+;; Set foreground and background
+(set-foreground-color "white")
+(set-background-color "darkblue")
address@hidden group
+
address@hidden
+;;; Set highlighting colors for isearch and drag
+(set-face-foreground 'highlight "white")
+(set-face-background 'highlight "blue")
address@hidden group
+
address@hidden
+(set-face-foreground 'region "cyan")
+(set-face-background 'region "blue")
address@hidden group
+
address@hidden
+(set-face-foreground 'secondary-selection "skyblue")
+(set-face-background 'secondary-selection "darkblue")
address@hidden group
+
address@hidden
+;; Set calendar highlighting colors
+(setq calendar-load-hook
+ '(lambda ()
+ (set-face-foreground 'diary-face "skyblue")
+ (set-face-background 'holiday-face "slate blue")
+ (set-face-foreground 'holiday-face "white")))
address@hidden group
address@hidden smallexample
+
+The various shades of blue soothe my eye and prevent me from seeing
+the screen flicker.
+
+Alternatively, I could have set my specifications in various X
+initialization files. For example, I could set the foreground,
+background, cursor, and pointer (i.e., mouse) colors in my
address@hidden/.Xresources} file like this:
+
address@hidden
address@hidden
+Emacs*foreground: white
+Emacs*background: darkblue
+Emacs*cursorColor: white
+Emacs*pointerColor: white
address@hidden group
address@hidden smallexample
+
+In any event, since it is not part of Emacs, I set the root color of
+my X window in my @file{~/.xinitrc} file, like address@hidden also
+run more modern window managers, such as Enlightenment, Gnome, or KDE;
+in those cases, I often specify an image rather than a plain color.}:
+
address@hidden
+xsetroot -solid Navy -fg white &
address@hidden smallexample
+
address@hidden 1700
address@hidden Miscellaneous, Mode Line, X11 Colors, Emacs Initialization
address@hidden Miscellaneous Settings for a @file{.emacs} File
+
address@hidden 1250
+Here are a few miscellaneous settings:
address@hidden 1
+
address@hidden @minus
address@hidden
+Set the shape and color of the mouse cursor:
+
address@hidden
address@hidden
+; Cursor shapes are defined in
+; `/usr/include/X11/cursorfont.h';
+; for example, the `target' cursor is number 128;
+; the `top_left_arrow' cursor is number 132.
address@hidden group
+
address@hidden
+(let ((mpointer (x-get-resource "*mpointer"
+ "*emacs*mpointer")))
+ ;; If you have not set your mouse pointer
+ ;; then set it, otherwise leave as is:
+ (if (eq mpointer nil)
+ (setq mpointer "132")) ; top_left_arrow
address@hidden group
address@hidden
+ (setq x-pointer-shape (string-to-int mpointer))
+ (set-mouse-color "white"))
address@hidden group
address@hidden smallexample
+
address@hidden
+Or you can set the values of a variety of features in an alist, like
+this:
+
address@hidden
address@hidden
+(setq-default
+ default-frame-alist
+ '((cursor-color . "white")
+ (mouse-color . "white")
+ (foreground-color . "white")
+ (background-color . "DodgerBlue4")
+ ;; (cursor-type . bar)
+ (cursor-type . box)
address@hidden group
address@hidden
+ (tool-bar-lines . 0)
+ (menu-bar-lines . 1)
+ (width . 80)
+ (height . 58)
+ (font .
+ "-Misc-Fixed-Medium-R-Normal--20-200-75-75-C-100-ISO8859-1")
+ ))
address@hidden group
address@hidden smallexample
+
address@hidden
+Convert @address@hidden into @key{DEL} and @key{DEL}
+into @address@hidden@*
+(Some older keyboards needed this, although I have not seen the
+problem recently.)
+
address@hidden
address@hidden
+;; Translate `C-h' to <DEL>.
+; (keyboard-translate ?\C-h ?\C-?)
+
+;; Translate <DEL> to `C-h'.
+(keyboard-translate ?\C-? ?\C-h)
address@hidden group
address@hidden smallexample
+
address@hidden Turn off a blinking cursor!
+
address@hidden
address@hidden
+(if (fboundp 'blink-cursor-mode)
+ (blink-cursor-mode -1))
address@hidden group
address@hidden smallexample
+
address@hidden
+or start GNU Emacs with the command @code{emacs -nbc}.
+
address@hidden 1250
address@hidden When using `grep'@*
address@hidden@w{ } Ignore case address@hidden
address@hidden@w{ } Prefix each line of output with line address@hidden
address@hidden@w{ } Print the filename for each address@hidden
address@hidden@w{ } Protect patterns beginning with a hyphen character,
@samp{-}
+
address@hidden
+(setq grep-command "grep -i -nH -e ")
address@hidden smallexample
+
address@hidden
address@hidden Evidently, no longer needed in GNU Emacs 22
+
+item Automatically uncompress compressed files when visiting them
+
+smallexample
+(load "uncompress")
+end smallexample
+
address@hidden ignore
+
address@hidden Find an existing buffer, even if it has a different
address@hidden
+This avoids problems with symbolic links.
+
address@hidden
+(setq find-file-existing-other-name t)
address@hidden smallexample
+
address@hidden Set your language environment and default input method
+
address@hidden
address@hidden
+(set-language-environment "latin-1")
+;; Remember you can enable or disable multilingual text input
+;; with the @code{toggle-input-method'} (@kbd{C-\}) command
+(setq default-input-method "latin-1-prefix")
address@hidden group
address@hidden smallexample
+
+If you want to write with Chinese `GB' characters, set this instead:
+
address@hidden
address@hidden
+(set-language-environment "Chinese-GB")
+(setq default-input-method "chinese-tonepy")
address@hidden group
address@hidden smallexample
address@hidden itemize
+
address@hidden Fixing Unpleasant Key Bindings
address@hidden Key bindings, fixing
address@hidden Bindings, key, fixing unpleasant
+
+Some systems bind keys unpleasantly. Sometimes, for example, the
address@hidden key appears in an awkward spot rather than at the far left
+of the home row.
+
+Usually, when people fix these sorts of keybindings, they do not
+change their @file{~/.emacs} file. Instead, they bind the proper keys
+on their consoles with the @code{loadkeys} or @code{install-keymap}
+commands in their boot script and then include @code{xmodmap} commands
+in their @file{.xinitrc} or @file{.Xsession} file for X Windows.
+
address@hidden 1250
address@hidden
+For a boot script:
+
address@hidden
address@hidden
+loadkeys /usr/share/keymaps/i386/qwerty/emacs2.kmap.gz
address@hidden or
+install-keymap emacs2
address@hidden group
address@hidden smallexample
+
address@hidden 1250
address@hidden
+For a @file{.xinitrc} or @file{.Xsession} file when the @key{Caps
+Lock} key is at the far left of the home row:
+
address@hidden
address@hidden
+# Bind the key labeled `Caps Lock' to `Control'
+# (Such a broken user interface suggests that keyboard manufacturers
+# think that computers are typewriters from 1885.)
+
+xmodmap -e "clear Lock"
+xmodmap -e "add Control = Caps_Lock"
address@hidden group
address@hidden smallexample
+
address@hidden 1250
address@hidden
+In a @file{.xinitrc} or @file{.Xsession} file, to convert an @key{ALT}
+key to a @key{META} key:
+
address@hidden
address@hidden
+# Some ill designed keyboards have a key labeled ALT and no Meta
+xmodmap -e "keysym Alt_L = Meta_L Alt_L"
address@hidden group
address@hidden smallexample
+
address@hidden 1700
address@hidden Mode Line, , Miscellaneous, Emacs Initialization
address@hidden A Modified Mode Line
address@hidden default-mode-line-format
address@hidden Mode line format
+
+Finally, a feature I really like: a modified mode line.
+
+When I work over a network, I forget which machine I am using. Also,
+I tend to I lose track of where I am, and which line point is on.
+
+So I reset my mode line to look like this:
+
address@hidden
+-:-- foo.texi rattlesnake:/home/bob/ Line 1 (Texinfo Fill) Top
address@hidden smallexample
+
+I am visiting a file called @file{foo.texi}, on my machine
address@hidden in my @file{/home/bob} buffer. I am on line 1, in
+Texinfo mode, and am at the top of the buffer.
+
address@hidden 1200
+My @file{.emacs} file has a section that looks like this:
+
address@hidden
address@hidden
+;; Set a Mode Line that tells me which machine, which directory,
+;; and which line I am on, plus the other customary information.
+(setq default-mode-line-format
+ (quote
+ (#("-" 0 1
+ (help-echo
+ "mouse-1: select window, mouse-2: delete others ..."))
+ mode-line-mule-info
+ mode-line-modified
+ mode-line-frame-identification
+ " "
address@hidden group
address@hidden
+ mode-line-buffer-identification
+ " "
+ (:eval (substring
+ (system-name) 0 (string-match "\\..+" (system-name))))
+ ":"
+ default-directory
+ #(" " 0 1
+ (help-echo
+ "mouse-1: select window, mouse-2: delete others ..."))
+ (line-number-mode " Line %l ")
+ global-mode-string
address@hidden group
address@hidden
+ #(" %[(" 0 6
+ (help-echo
+ "mouse-1: select window, mouse-2: delete others ..."))
+ (:eval (mode-line-mode-name))
+ mode-line-process
+ minor-mode-alist
+ #("%n" 0 2 (help-echo "mouse-2: widen" local-map (keymap ...)))
+ ")%] "
+ (-3 . "%P")
+ ;; "-%-"
+ )))
address@hidden group
address@hidden smallexample
+
address@hidden
+Here, I redefine the default mode line. Most of the parts are from
+the original; but I make a few changes. I set the @emph{default} mode
+line format so as to permit various modes, such as Info, to override
+it.
+
+Many elements in the list are self-explanatory:
address@hidden is a variable that tells whether the buffer
+has been modified, @code{mode-name} tells the name of the mode, and so
+on. However, the format looks complicated because of two features we
+have not discussed.
+
address@hidden Properties, in mode line example
+The first string in the mode line is a dash or hyphen, @samp{-}. In
+the old days, it would have been specified simply as @code{"-"}. But
+nowadays, Emacs can add properties to a string, such as highlighting
+or, as in this case, a help feature. If you place your mouse cursor
+over the hyphen, some help information appears (By default, you must
+wait seven-tenths of a second before the information appears. You can
+change that timing by changing the value of @code{tooltip-delay}.)
+
address@hidden 1000
+The new string format has a special syntax:
+
address@hidden
+#("-" 0 1 (help-echo "mouse-1: select window, ..."))
address@hidden smallexample
+
address@hidden
+The @code{#(} begins a list. The first element of the list is the
+string itself, just one @samp{-}. The second and third
+elements specify the range over which the fourth element applies. A
+range starts @emph{after} a character, so a zero means the range
+starts just before the first character; a 1 means that the range ends
+just after the first character. The third element is the property for
+the range. It consists of a property list, a
+property name, in this case, @samp{help-echo}, followed by a value, in this
+case, a string. The second, third, and fourth elements of this new
+string format can be repeated.
+
address@hidden Properties, , Text Properties, elisp, The GNU Emacs Lisp
+Reference Manual}, and see @ref{Mode Line Format, , Mode Line Format,
+elisp, The GNU Emacs Lisp Reference Manual}, for more information.
+
address@hidden
+displays the current buffer name. It is a list
+beginning @code{(#("%12b" 0 4 @dots{}}.
+The @code{#(} begins the list.
+
+The @samp{"%12b"} displays the current buffer name, using the
address@hidden function with which we are familiar; the `12'
+specifies the maximum number of characters that will be displayed.
+When a name has fewer characters, whitespace is added to fill out to
+this number. (Buffer names can and often should be longer than 12
+characters; this length works well in a typical 80 column wide
+window.)
+
address@hidden:eval} says to evaluate the following form and use the result as
+a string to display. In this case, the expression displays the first
+component of the full system name. The end of the first component is
+a @samp{.} (`period'), so I use the @code{string-match} function to
+tell me the length of the first component. The substring from the
+zeroth character to that length is the name of the machine.
+
address@hidden 1250
+This is the expression:
+
address@hidden
address@hidden
+(:eval (substring
+ (system-name) 0 (string-match "\\..+" (system-name))))
address@hidden group
address@hidden smallexample
+
address@hidden and @samp{%]} cause a pair of square brackets
+to appear for each recursive editing level. @samp{%n} says `Narrow'
+when narrowing is in effect. @samp{%P} tells you the percentage of
+the buffer that is above the bottom of the window, or `Top', `Bottom',
+or `All'. (A lower case @samp{p} tell you the percentage above the
address@hidden of the window.) @samp{%-} inserts enough dashes to fill
+out the line.
+
+Remember, ``You don't have to like Emacs to like it'' --- your own
+Emacs can have different colors, different commands, and different
+keys than a default Emacs.
+
+On the other hand, if you want to bring up a plain `out of the box'
+Emacs, with no customization, type:
+
address@hidden
+emacs -q
address@hidden smallexample
+
address@hidden
+This will start an Emacs that does @emph{not} load your
address@hidden/.emacs} initialization file. A plain, default Emacs. Nothing
+more.
+
address@hidden Debugging, Conclusion, Emacs Initialization, Top
address@hidden Debugging
address@hidden debugging
+
+GNU Emacs has two debuggers, @code{debug} and @code{edebug}. The
+first is built into the internals of Emacs and is always with you;
+the second requires that you instrument a function before you can use it.
+
+Both debuggers are described extensively in @ref{Debugging, ,
+Debugging Lisp Programs, elisp, The GNU Emacs Lisp Reference Manual}.
+In this chapter, I will walk through a short example of each.
+
address@hidden
+* debug:: How to use the built-in debugger.
+* debug-on-entry:: Start debugging when you call a function.
+* debug-on-quit:: Start debugging when you quit with @kbd{C-g}.
+* edebug:: How to use Edebug, a source level debugger.
+* Debugging Exercises::
address@hidden menu
+
address@hidden debug, debug-on-entry, Debugging, Debugging
address@hidden @code{debug}
address@hidden debug
+
+Suppose you have written a function definition that is intended to
+return the sum of the numbers 1 through a given number. (This is the
address@hidden function discussed earlier. @xref{Decrementing
+Example, , Example with Decrementing Counter}, for a discussion.)
address@hidden xref{Decrementing Loop,, Loop with a Decrementing Counter}, for
a discussion.)
+
+However, your function definition has a bug. You have mistyped
address@hidden for @samp{1-}. Here is the broken definition:
+
address@hidden triangle-bugged
address@hidden
address@hidden
+(defun triangle-bugged (number)
+ "Return sum of numbers 1 through NUMBER inclusive."
+ (let ((total 0))
+ (while (> number 0)
+ (setq total (+ total number))
+ (setq number (1= number))) ; @r{Error here.}
+ total))
address@hidden group
address@hidden smallexample
+
+If you are reading this in Info, you can evaluate this definition in
+the normal fashion. You will see @code{triangle-bugged} appear in the
+echo area.
+
address@hidden 1250
+Now evaluate the @code{triangle-bugged} function with an
+argument of 4:
+
address@hidden
+(triangle-bugged 4)
address@hidden smallexample
+
address@hidden
+In a recent GNU Emacs, you will create and enter a @file{*Backtrace*}
+buffer that says:
+
address@hidden
address@hidden
address@hidden
+---------- Buffer: *Backtrace* ----------
+Debugger entered--Lisp error: (void-function 1=)
+ (1= number)
+ (setq number (1= number))
+ (while (> number 0) (setq total (+ total number))
+ (setq number (1= number)))
+ (let ((total 0)) (while (> number 0) (setq total ...)
+ (setq number ...)) total)
+ triangle-bugged(4)
address@hidden group
address@hidden
+ eval((triangle-bugged 4))
+ eval-last-sexp-1(nil)
+ eval-last-sexp(nil)
+ call-interactively(eval-last-sexp)
+---------- Buffer: *Backtrace* ----------
address@hidden group
address@hidden smallexample
+
address@hidden
+(I have reformatted this example slightly; the debugger does not fold
+long lines. As usual, you can quit the debugger by typing @kbd{q} in
+the @file{*Backtrace*} buffer.)
+
+In practice, for a bug as simple as this, the `Lisp error' line will
+tell you what you need to know to correct the definition. The
+function @code{1=} is `void'.
+
address@hidden
address@hidden 800
+In GNU Emacs 20 and before, you will see:
+
address@hidden
+Symbol's function definition is void:@: 1=
address@hidden smallexample
+
address@hidden
+which has the same meaning as the @file{*Backtrace*} buffer line in
+version 21.
address@hidden ignore
+
+However, suppose you are not quite certain what is going on?
+You can read the complete backtrace.
+
+In this case, you need to run a recent GNU Emacs, which automatically
+starts the debugger that puts you in the @file{*Backtrace*} buffer; or
+else, you need to start the debugger manually as described below.
+
+Read the @file{*Backtrace*} buffer from the bottom up; it tells you
+what Emacs did that led to the error. Emacs made an interactive call
+to @kbd{C-x C-e} (@code{eval-last-sexp}), which led to the evaluation
+of the @code{triangle-bugged} expression. Each line above tells you
+what the Lisp interpreter evaluated next.
+
address@hidden 1250
+The third line from the top of the buffer is
+
address@hidden
+(setq number (1= number))
address@hidden smallexample
+
address@hidden
+Emacs tried to evaluate this expression; in order to do so, it tried
+to evaluate the inner expression shown on the second line from the
+top:
+
address@hidden
+(1= number)
address@hidden smallexample
+
address@hidden 1250
address@hidden
+This is where the error occurred; as the top line says:
+
address@hidden
+Debugger entered--Lisp error: (void-function 1=)
address@hidden smallexample
+
address@hidden
+You can correct the mistake, re-evaluate the function definition, and
+then run your test again.
+
address@hidden debug-on-entry, debug-on-quit, debug, Debugging
address@hidden @code{debug-on-entry}
address@hidden debug-on-entry
+
+A recent GNU Emacs starts the debugger automatically when your
+function has an error.
+
address@hidden
+GNU Emacs version 20 and before did not; it simply
+presented you with an error message. You had to start the debugger
+manually.
address@hidden ignore
+
+Incidentally, you can start the debugger manually for all versions of
+Emacs; the advantage is that the debugger runs even if you do not have
+a bug in your code. Sometimes your code will be free of bugs!
+
+You can enter the debugger when you call the function by calling
address@hidden
+
address@hidden 1250
address@hidden
+Type:
+
address@hidden
+M-x debug-on-entry RET triangle-bugged RET
address@hidden smallexample
+
address@hidden 1250
address@hidden
+Now, evaluate the following:
+
address@hidden
+(triangle-bugged 5)
address@hidden smallexample
+
address@hidden
+All versions of Emacs will create a @file{*Backtrace*} buffer and tell
+you that it is beginning to evaluate the @code{triangle-bugged}
+function:
+
address@hidden
address@hidden
+---------- Buffer: *Backtrace* ----------
+Debugger entered--entering a function:
+* triangle-bugged(5)
+ eval((triangle-bugged 5))
address@hidden group
address@hidden
+ eval-last-sexp-1(nil)
+ eval-last-sexp(nil)
+ call-interactively(eval-last-sexp)
+---------- Buffer: *Backtrace* ----------
address@hidden group
address@hidden smallexample
+
+In the @file{*Backtrace*} buffer, type @kbd{d}. Emacs will evaluate
+the first expression in @code{triangle-bugged}; the buffer will look
+like this:
+
address@hidden
address@hidden
+---------- Buffer: *Backtrace* ----------
+Debugger entered--beginning evaluation of function call form:
+* (let ((total 0)) (while (> number 0) (setq total ...)
+ (setq number ...)) total)
+* triangle-bugged(5)
+ eval((triangle-bugged 5))
address@hidden group
address@hidden
+ eval-last-sexp-1(nil)
+ eval-last-sexp(nil)
+ call-interactively(eval-last-sexp)
+---------- Buffer: *Backtrace* ----------
address@hidden group
address@hidden smallexample
+
address@hidden
+Now, type @kbd{d} again, eight times, slowly. Each time you type
address@hidden, Emacs will evaluate another expression in the function
+definition.
+
address@hidden 1750
+Eventually, the buffer will look like this:
+
address@hidden
address@hidden
+---------- Buffer: *Backtrace* ----------
+Debugger entered--beginning evaluation of function call form:
+* (setq number (1= number))
+* (while (> number 0) (setq total (+ total number))
+ (setq number (1= number)))
address@hidden
address@hidden group
+* (let ((total 0)) (while (> number 0) (setq total ...)
+ (setq number ...)) total)
+* triangle-bugged(5)
+ eval((triangle-bugged 5))
address@hidden
address@hidden group
+ eval-last-sexp-1(nil)
+ eval-last-sexp(nil)
+ call-interactively(eval-last-sexp)
+---------- Buffer: *Backtrace* ----------
address@hidden group
address@hidden smallexample
+
address@hidden 1500
address@hidden
+Finally, after you type @kbd{d} two more times, Emacs will reach the
+error, and the top two lines of the @file{*Backtrace*} buffer will look
+like this:
+
address@hidden
address@hidden
+---------- Buffer: *Backtrace* ----------
+Debugger entered--Lisp error: (void-function 1=)
+* (1= number)
address@hidden
+---------- Buffer: *Backtrace* ----------
address@hidden group
address@hidden smallexample
+
+By typing @kbd{d}, you were able to step through the function.
+
+You can quit a @file{*Backtrace*} buffer by typing @kbd{q} in it; this
+quits the trace, but does not cancel @code{debug-on-entry}.
+
address@hidden cancel-debug-on-entry
+To cancel the effect of @code{debug-on-entry}, call
address@hidden and the name of the function, like this:
+
address@hidden
+M-x cancel-debug-on-entry RET triangle-bugged RET
address@hidden smallexample
+
address@hidden
+(If you are reading this in Info, cancel @code{debug-on-entry} now.)
+
address@hidden debug-on-quit, edebug, debug-on-entry, Debugging
address@hidden @code{debug-on-quit} and @code{(debug)}
+
+In addition to setting @code{debug-on-error} or calling @code{debug-on-entry},
+there are two other ways to start @code{debug}.
+
address@hidden debug-on-quit
+You can start @code{debug} whenever you type @kbd{C-g}
+(@code{keyboard-quit}) by setting the variable @code{debug-on-quit} to
address@hidden This is useful for debugging infinite loops.
+
address@hidden 1500
address@hidden @code{(debug)} in code
+Or, you can insert a line that says @code{(debug)} into your code
+where you want the debugger to start, like this:
+
address@hidden
address@hidden
+(defun triangle-bugged (number)
+ "Return sum of numbers 1 through NUMBER inclusive."
+ (let ((total 0))
+ (while (> number 0)
+ (setq total (+ total number))
+ (debug) ; @r{Start debugger.}
+ (setq number (1= number))) ; @r{Error here.}
+ total))
address@hidden group
address@hidden smallexample
+
+The @code{debug} function is described in detail in @ref{Debugger, ,
+The Lisp Debugger, elisp, The GNU Emacs Lisp Reference Manual}.
+
address@hidden edebug, Debugging Exercises, debug-on-quit, Debugging
address@hidden The @code{edebug} Source Level Debugger
address@hidden Source level debugger
address@hidden edebug
+
+Edebug is a source level debugger. Edebug normally displays the
+source of the code you are debugging, with an arrow at the left that
+shows which line you are currently executing.
+
+You can walk through the execution of a function, line by line, or run
+quickly until reaching a @dfn{breakpoint} where execution stops.
+
+Edebug is described in @ref{edebug, , Edebug, elisp, The GNU Emacs
+Lisp Reference Manual}.
+
address@hidden 1250
+Here is a bugged function definition for @code{triangle-recursively}.
address@hidden triangle function, , Recursion in place of a counter},
+for a review of it.
+
address@hidden
address@hidden
+(defun triangle-recursively-bugged (number)
+ "Return sum of numbers 1 through NUMBER inclusive.
+Uses recursion."
+ (if (= number 1)
+ 1
+ (+ number
+ (triangle-recursively-bugged
+ (1= number))))) ; @r{Error here.}
address@hidden group
address@hidden smallexample
+
address@hidden
+Normally, you would install this definition by positioning your cursor
+after the function's closing parenthesis and typing @kbd{C-x C-e}
+(@code{eval-last-sexp}) or else by positioning your cursor within the
+definition and typing @kbd{C-M-x} (@code{eval-defun}). (By default,
+the @code{eval-defun} command works only in Emacs Lisp mode or in Lisp
+Interactive mode.)
+
address@hidden 1500
+However, to prepare this function definition for Edebug, you must
+first @dfn{instrument} the code using a different command. You can do
+this by positioning your cursor within or just after the definition
+and typing
+
address@hidden
+M-x edebug-defun RET
address@hidden smallexample
+
address@hidden
+This will cause Emacs to load Edebug automatically if it is not
+already loaded, and properly instrument the function.
+
+After instrumenting the function, place your cursor after the
+following expression and type @kbd{C-x C-e} (@code{eval-last-sexp}):
+
address@hidden
+(triangle-recursively-bugged 3)
address@hidden smallexample
+
address@hidden
+You will be jumped back to the source for
address@hidden and the cursor positioned at the
+beginning of the @code{if} line of the function. Also, you will see
+an arrowhead at the left hand side of that line. The arrowhead marks
+the line where the function is executing. (In the following examples,
+we show the arrowhead with @samp{=>}; in a windowing system, you may
+see the arrowhead as a solid triangle in the window `fringe'.)
+
address@hidden
+=>@point{}(if (= number 1)
address@hidden smallexample
+
address@hidden
address@hidden
+In the example, the location of point is displayed with a star,
address@hidden@point{}} (in Info, it is displayed as @samp{-!-}).
address@hidden iftex
address@hidden
+In the example, the location of point is displayed as @address@hidden
+(in a printed book, it is displayed with a five pointed star).
address@hidden ifnottex
+
+If you now press @key{SPC}, point will move to the next expression to
+be executed; the line will look like this:
+
address@hidden
+=>(if @point{}(= number 1)
address@hidden smallexample
+
address@hidden
+As you continue to press @key{SPC}, point will move from expression to
+expression. At the same time, whenever an expression returns a value,
+that value will be displayed in the echo area. For example, after you
+move point past @code{number}, you will see the following:
+
address@hidden
+Result: 3 (#o3, #x3, ?\C-c)
address@hidden smallexample
+
address@hidden
+This means the value of @code{number} is 3, which is octal three,
+hexadecimal three, and @sc{ascii} `control-c' (the third letter of the
+alphabet, in case you need to know this information).
+
+You can continue moving through the code until you reach the line with
+the error. Before evaluation, that line looks like this:
+
address@hidden
+=> @point{}(1= number))))) ; @r{Error here.}
address@hidden smallexample
+
address@hidden 1250
address@hidden
+When you press @key{SPC} once again, you will produce an error message
+that says:
+
address@hidden
+Symbol's function definition is void:@: 1=
address@hidden smallexample
+
address@hidden
+This is the bug.
+
+Press @kbd{q} to quit Edebug.
+
+To remove instrumentation from a function definition, simply
+re-evaluate it with a command that does not instrument it.
+For example, you could place your cursor after the definition's
+closing parenthesis and type @kbd{C-x C-e}.
+
+Edebug does a great deal more than walk with you through a function.
+You can set it so it races through on its own, stopping only at an
+error or at specified stopping points; you can cause it to display the
+changing values of various expressions; you can find out how many
+times a function is called, and more.
+
+Edebug is described in @ref{edebug, , Edebug, elisp, The GNU Emacs
+Lisp Reference Manual}.
+
address@hidden 1500
address@hidden Debugging Exercises, , edebug, Debugging
address@hidden Debugging Exercises
+
address@hidden @bullet
address@hidden
+Install the @code{count-words-region} function and then cause it to
+enter the built-in debugger when you call it. Run the command on a
+region containing two words. You will need to press @kbd{d} a
+remarkable number of times. On your system, is a `hook' called after
+the command finishes? (For information on hooks, see @ref{Command
+Overview, , Command Loop Overview, elisp, The GNU Emacs Lisp Reference
+Manual}.)
+
address@hidden
+Copy @code{count-words-region} into the @file{*scratch*} buffer,
+instrument the function for Edebug, and walk through its execution.
+The function does not need to have a bug, although you can introduce
+one if you wish. If the function lacks a bug, the walk-through
+completes without problems.
+
address@hidden
+While running Edebug, type @kbd{?} to see a list of all the Edebug commands.
+(The @code{global-edebug-prefix} is usually @kbd{C-x X}, i.e.@:
address@hidden@key{CTRL}-x} followed by an upper case @kbd{X}; use this prefix
+for commands made outside of the Edebug debugging buffer.)
+
address@hidden
+In the Edebug debugging buffer, use the @kbd{p}
+(@code{edebug-bounce-point}) command to see where in the region the
address@hidden is working.
+
address@hidden
+Move point to some spot further down the function and then type the
address@hidden (@code{edebug-goto-here}) command to jump to that location.
+
address@hidden
+Use the @kbd{t} (@code{edebug-trace-mode}) command to cause Edebug to
+walk through the function on its own; use an upper case @kbd{T} for
address@hidden
+
address@hidden
+Set a breakpoint, then run Edebug in Trace mode until it reaches the
+stopping point.
address@hidden itemize
+
address@hidden Conclusion, the-the, Debugging, Top
address@hidden Conclusion
+
+We have now reached the end of this Introduction. You have now
+learned enough about programming in Emacs Lisp to set values, to write
+simple @file{.emacs} files for yourself and your friends, and write
+simple customizations and extensions to Emacs.
+
+This is a place to stop. Or, if you wish, you can now go onward, and
+teach yourself.
+
+You have learned some of the basic nuts and bolts of programming. But
+only some. There are a great many more brackets and hinges that are
+easy to use that we have not touched.
+
+A path you can follow right now lies among the sources to GNU Emacs
+and in
address@hidden
address@hidden GNU Emacs Lisp Reference Manual}.
address@hidden ifnotinfo
address@hidden
address@hidden, , The GNU Emacs Lisp Reference Manual, elisp, The GNU
+Emacs Lisp Reference Manual}.
address@hidden ifinfo
+
+The Emacs Lisp sources are an adventure. When you read the sources and
+come across a function or expression that is unfamiliar, you need to
+figure out or find out what it does.
+
+Go to the Reference Manual. It is a thorough, complete, and fairly
+easy-to-read description of Emacs Lisp. It is written not only for
+experts, but for people who know what you know. (The @cite{Reference
+Manual} comes with the standard GNU Emacs distribution. Like this
+introduction, it comes as a Texinfo source file, so you can read it
+on-line and as a typeset, printed book.)
+
+Go to the other on-line help that is part of GNU Emacs: the on-line
+documentation for all functions and variables, and @code{find-tags},
+the program that takes you to sources.
+
+Here is an example of how I explore the sources. Because of its name,
address@hidden is the file I looked at first, a long time ago. As
+it happens some of the functions in @file{simple.el} are complicated,
+or at least look complicated at first sight. The @code{open-line}
+function, for example, looks complicated.
+
+You may want to walk through this function slowly, as we did with the
address@hidden function. (@xref{forward-sentence, The
address@hidden function}.) Or you may want to skip that
+function and look at another, such as @code{split-line}. You don't
+need to read all the functions. According to
address@hidden, the @code{split-line} function contains
+102 words and symbols.
+
+Even though it is short, @code{split-line} contains expressions
+we have not studied: @code{skip-chars-forward}, @code{indent-to},
address@hidden and @code{insert-and-inherit}.
+
+Consider the @code{skip-chars-forward} function. (It is part of the
+function definition for @code{back-to-indentation}, which is shown in
address@hidden, , Review}.)
+
+In GNU Emacs, you can find out more about @code{skip-chars-forward} by
+typing @kbd{C-h f} (@code{describe-function}) and the name of the
+function. This gives you the function documentation.
+
+You may be able to guess what is done by a well named function such as
address@hidden; or you can look it up, too. Incidentally, the
address@hidden function itself is in @file{help.el}; it is
+one of those long, but decipherable functions. You can look up
address@hidden using the @kbd{C-h f} command!
+
+In this instance, since the code is Lisp, the @file{*Help*} buffer
+contains the name of the library containing the function's source.
+You can put point over the name of the library and press the RET key,
+which in this situation is bound to @code{help-follow}, and be taken
+directly to the source, in the same way as @kbd{M-.}
+(@code{find-tag}).
+
+The definition for @code{describe-function} illustrates how to
+customize the @code{interactive} expression without using the standard
+character codes; and it shows how to create a temporary buffer.
+
+(The @code{indent-to} function is written in C rather than Emacs Lisp;
+it is a `built-in' function. @code{help-follow} takes you to its
+source as does @code{find-tag}, when properly set up.)
+
+You can look at a function's source using @code{find-tag}, which is
+bound to @kbd{M-.} Finally, you can find out what the Reference
+Manual has to say by visiting the manual in Info, and typing @kbd{i}
+(@code{Info-index}) and the name of the function, or by looking up the
+function in the index to a printed copy of the manual.
+
+Similarly, you can find out what is meant by
address@hidden
+
+Other interesting source files include @file{paragraphs.el},
address@hidden, and @file{loadup.el}. The @file{paragraphs.el}
+file includes short, easily understood functions as well as longer
+ones. The @file{loaddefs.el} file contains the many standard
+autoloads and many keymaps. I have never looked at it all; only at
+parts. @file{loadup.el} is the file that loads the standard parts of
+Emacs; it tells you a great deal about how Emacs is built.
+(@xref{Building Emacs, , Building Emacs, elisp, The GNU Emacs Lisp
+Reference Manual}, for more about building.)
+
+As I said, you have learned some nuts and bolts; however, and very
+importantly, we have hardly touched major aspects of programming; I
+have said nothing about how to sort information, except to use the
+predefined @code{sort} function; I have said nothing about how to store
+information, except to use variables and lists; I have said nothing
+about how to write programs that write programs. These are topics for
+another, and different kind of book, a different kind of learning.
+
+What you have done is learn enough for much practical work with GNU
+Emacs. What you have done is get started. This is the end of a
+beginning.
+
address@hidden ================ Appendix ================
+
address@hidden the-the, Kill Ring, Conclusion, Top
address@hidden The @code{the-the} Function
address@hidden the-the
address@hidden Duplicated words function
address@hidden Words, duplicated
+
+Sometimes when you you write text, you duplicate words---as with ``you
+you'' near the beginning of this sentence. I find that most
+frequently, I duplicate ``the''; hence, I call the function for
+detecting duplicated words, @code{the-the}.
+
address@hidden 1250
+As a first step, you could use the following regular expression to
+search for duplicates:
+
address@hidden
+\\(\\w+[ \t\n]+\\)\\1
address@hidden smallexample
+
address@hidden
+This regexp matches one or more word-constituent characters followed
+by one or more spaces, tabs, or newlines. However, it does not detect
+duplicated words on different lines, since the ending of the first
+word, the end of the line, is different from the ending of the second
+word, a space. (For more information about regular expressions, see
address@hidden Search, , Regular Expression Searches}, as well as
address@hidden, , Syntax of Regular Expressions, emacs, The GNU Emacs
+Manual}, and @ref{Regular Expressions, , Regular Expressions, elisp,
+The GNU Emacs Lisp Reference Manual}.)
+
+You might try searching just for duplicated word-constituent
+characters but that does not work since the pattern detects doubles
+such as the two occurrences of `th' in `with the'.
+
+Another possible regexp searches for word-constituent characters
+followed by non-word-constituent characters, reduplicated. Here,
address@hidden@samp{\\w+}} matches one or more word-constituent characters and
address@hidden@samp{\\W*}} matches zero or more non-word-constituent characters.
+
address@hidden
+\\(\\(\\w+\\)\\W*\\)\\1
address@hidden smallexample
+
address@hidden
+Again, not useful.
+
+Here is the pattern that I use. It is not perfect, but good enough.
address@hidden@samp{\\b}} matches the empty string, provided it is at the
beginning
+or end of a word; @address@hidden@@ \n\t]+}} matches one or more occurrences of
+any characters that are @emph{not} an @@-sign, space, newline, or tab.
+
address@hidden
+\\b\\([^@@ \n\t]+\\)[ \n\t]+\\1\\b
address@hidden smallexample
+
+One can write more complicated expressions, but I found that this
+expression is good enough, so I use it.
+
+Here is the @code{the-the} function, as I include it in my
address@hidden file, along with a handy global key binding:
+
address@hidden
address@hidden
+(defun the-the ()
+ "Search forward for for a duplicated word."
+ (interactive)
+ (message "Searching for for duplicated words ...")
+ (push-mark)
address@hidden group
address@hidden
+ ;; This regexp is not perfect
+ ;; but is fairly good over all:
+ (if (re-search-forward
+ "\\b\\([^@@ \n\t]+\\)[ \n\t]+\\1\\b" nil 'move)
+ (message "Found duplicated word.")
+ (message "End of buffer")))
address@hidden group
+
address@hidden
+;; Bind `the-the' to C-c \
+(global-set-key "\C-c\\" 'the-the)
address@hidden group
address@hidden smallexample
+
address@hidden 1
+Here is test text:
+
address@hidden
address@hidden
+one two two three four five
+five six seven
address@hidden group
address@hidden smallexample
+
+You can substitute the other regular expressions shown above in the
+function definition and try each of them on this list.
+
address@hidden Kill Ring, Full Graph, the-the, Top
address@hidden Handling the Kill Ring
address@hidden Kill ring handling
address@hidden Handling the kill ring
address@hidden Ring, making a list like a
+
+The kill ring is a list that is transformed into a ring by the
+workings of the @code{current-kill} function. The @code{yank} and
address@hidden commands use the @code{current-kill} function.
+
+This appendix describes the @code{current-kill} function as well as
+both the @code{yank} and the @code{yank-pop} commands, but first,
+consider the workings of the kill ring.
+
address@hidden
+* What the Kill Ring Does::
+* current-kill::
+* yank:: Paste a copy of a clipped element.
+* yank-pop:: Insert element pointed to.
+* ring file::
address@hidden menu
+
address@hidden What the Kill Ring Does, current-kill, Kill Ring, Kill Ring
address@hidden
address@hidden What the Kill Ring Does
address@hidden ifnottex
+
address@hidden 1250
+The kill ring has a default maximum length of sixty items; this number
+is too large for an explanation. Instead, set it to four. Please
+evaluate the following:
+
address@hidden
address@hidden
+(setq old-kill-ring-max kill-ring-max)
+(setq kill-ring-max 4)
address@hidden group
address@hidden smallexample
+
address@hidden
+Then, please copy each line of the following indented example into the
+kill ring. You may kill each line with @kbd{C-k} or mark it and copy
+it with @kbd{M-w}.
+
address@hidden
+(In a read-only buffer, such as the @file{*info*} buffer, the kill
+command, @kbd{C-k} (@code{kill-line}), will not remove the text,
+merely copy it to the kill ring. However, your machine may beep at
+you. Alternatively, for silence, you may copy the region of each line
+with the @kbd{M-w} (@code{kill-ring-save}) command. You must mark
+each line for this command to succeed, but it does not matter at which
+end you put point or mark.)
+
address@hidden 1250
address@hidden
+Please invoke the calls in order, so that five elements attempt to
+fill the kill ring:
+
address@hidden
address@hidden
+first some text
+second piece of text
+third line
+fourth line of text
+fifth bit of text
address@hidden group
address@hidden smallexample
+
address@hidden 1250
address@hidden
+Then find the value of @code{kill-ring} by evaluating
+
address@hidden
+kill-ring
address@hidden smallexample
+
address@hidden 800
address@hidden
+It is:
+
address@hidden
address@hidden
+("fifth bit of text" "fourth line of text"
+"third line" "second piece of text")
address@hidden group
address@hidden smallexample
+
address@hidden
+The first element, @samp{first some text}, was dropped.
+
address@hidden 1250
+To return to the old value for the length of the kill ring, evaluate:
+
address@hidden
+(setq kill-ring-max old-kill-ring-max)
address@hidden smallexample
+
address@hidden current-kill, yank, What the Kill Ring Does, Kill Ring
address@hidden node-name, next, previous, up
address@hidden The @code{current-kill} Function
address@hidden current-kill
+
+The @code{current-kill} function changes the element in the kill ring
+to which @code{kill-ring-yank-pointer} points. (Also, the
address@hidden function sets @code{kill-ring-yank-pointer} to point
+to the latest element of the the kill ring. The @code{kill-new}
+function is used directly or indirectly by @code{kill-append},
address@hidden, @code{kill-ring-save}, @code{kill-line},
+and @code{kill-region}.)
+
address@hidden
+* Code for current-kill::
+* Understanding current-kill::
address@hidden menu
+
address@hidden Code for current-kill, Understanding current-kill, current-kill,
current-kill
address@hidden
address@hidden The code for @code{current-kill}
address@hidden ifnottex
+
+
address@hidden 1500
+The @code{current-kill} function is used by @code{yank} and by
address@hidden Here is the code for @code{current-kill}:
+
address@hidden
address@hidden
+(defun current-kill (n &optional do-not-move)
+ "Rotate the yanking point by N places, and then return that kill.
+If N is zero, `interprogram-paste-function' is set, and calling it
+returns a string, then that string is added to the front of the
+kill ring and returned as the latest kill.
address@hidden group
address@hidden
+If optional arg DO-NOT-MOVE is non-nil, then don't actually move the
+yanking point; just return the Nth kill forward."
+ (let ((interprogram-paste (and (= n 0)
+ interprogram-paste-function
+ (funcall interprogram-paste-function))))
address@hidden group
address@hidden
+ (if interprogram-paste
+ (progn
+ ;; Disable the interprogram cut function when we add the new
+ ;; text to the kill ring, so Emacs doesn't try to own the
+ ;; selection, with identical text.
+ (let ((interprogram-cut-function nil))
+ (kill-new interprogram-paste))
+ interprogram-paste)
address@hidden group
address@hidden
+ (or kill-ring (error "Kill ring is empty"))
+ (let ((ARGth-kill-element
+ (nthcdr (mod (- n (length kill-ring-yank-pointer))
+ (length kill-ring))
+ kill-ring)))
+ (or do-not-move
+ (setq kill-ring-yank-pointer ARGth-kill-element))
+ (car ARGth-kill-element)))))
address@hidden group
address@hidden smallexample
+
+Remember also that the @code{kill-new} function sets
address@hidden to the latest element of the the kill
+ring, which means that all the functions that call it set the value
+indirectly: @code{kill-append}, @code{copy-region-as-kill},
address@hidden, @code{kill-line}, and @code{kill-region}.
+
address@hidden 1500
+Here is the line in @code{kill-new}, which is explained in
address@hidden function, , The @code{kill-new} function}.
+
address@hidden
+(setq kill-ring-yank-pointer kill-ring)
address@hidden smallexample
+
address@hidden Understanding current-kill, , Code for current-kill,
current-kill
address@hidden
address@hidden @code{current-kill} in Outline
address@hidden ifnottex
+
+The @code{current-kill} function looks complex, but as usual, it can
+be understood by taking it apart piece by piece. First look at it in
+skeletal form:
+
address@hidden
address@hidden
+(defun current-kill (n &optional do-not-move)
+ "Rotate the yanking point by N places, and then return that kill."
+ (let @var{varlist}
+ @address@hidden)
address@hidden group
address@hidden smallexample
+
+This function takes two arguments, one of which is optional. It has a
+documentation string. It is @emph{not} interactive.
+
address@hidden
+* Body of current-kill::
+* Digression concerning error:: How to mislead humans, but not computers.
+* Determining the Element::
address@hidden menu
+
address@hidden Body of current-kill, Digression concerning error, Understanding
current-kill, Understanding current-kill
address@hidden
address@hidden The Body of @code{current-kill}
address@hidden ifnottex
+
+The body of the function definition is a @code{let} expression, which
+itself has a body as well as a @var{varlist}.
+
+The @code{let} expression declares a variable that will be only usable
+within the bounds of this function. This variable is called
address@hidden and is for copying to another program. It
+is not for copying within this instance of GNU Emacs. Most window
+systems provide a facility for interprogram pasting. Sadly, that
+facility usually provides only for the last element. Most windowing
+systems have not adopted a ring of many possibilities, even though
+Emacs has provided it for decades.
+
+The @code{if} expression has two parts, one if there exists
address@hidden and one if not.
+
address@hidden 2000
+Let us consider the `if not' or else-part of the @code{current-kill}
+function. (The then-part uses the the @code{kill-new} function, which
+we have already described. @xref{kill-new function, , The
address@hidden function}.)
+
address@hidden
address@hidden
+(or kill-ring (error "Kill ring is empty"))
+(let ((ARGth-kill-element
+ (nthcdr (mod (- n (length kill-ring-yank-pointer))
+ (length kill-ring))
+ kill-ring)))
+ (or do-not-move
+ (setq kill-ring-yank-pointer ARGth-kill-element))
+ (car ARGth-kill-element))
address@hidden group
address@hidden smallexample
+
address@hidden
+The code first checks whether the kill ring has content; otherwise it
+signals an error.
+
address@hidden 1000
+Note that the @code{or} expression is very similar to testing length
+with an @code{if}:
+
address@hidden zerop
address@hidden error
address@hidden
address@hidden
+(if (zerop (length kill-ring)) ; @r{if-part}
+ (error "Kill ring is empty")) ; @r{then-part}
+ ;; No else-part
address@hidden group
address@hidden smallexample
+
address@hidden
+If there is not anything in the kill ring, its length must be zero and
+an error message sent to the user: @samp{Kill ring is empty}. The
address@hidden function uses an @code{or} expression which is
+simpler. But an @code{if} expression reminds us what goes on.
+
+This @code{if} expression uses the function @code{zerop} which returns
+true if the value it is testing is zero. When @code{zerop} tests
+true, the then-part of the @code{if} is evaluated. The then-part is a
+list starting with the function @code{error}, which is a function that
+is similar to the @code{message} function
+(@pxref{message, , The @code{message} Function}) in that
+it prints a one-line message in the echo area. However, in addition
+to printing a message, @code{error} also stops evaluation of the
+function within which it is embedded. This means that the rest of the
+function will not be evaluated if the length of the kill ring is zero.
+
+Then the @code{current-kill} function selects the element to return.
+The selection depends on the number of places that @code{current-kill}
+rotates and on where @code{kill-ring-yank-pointer} points.
+
+Next, either the optional @code{do-not-move} argument is true or the
+current value of @code{kill-ring-yank-pointer} is set to point to the
+list. Finally, another expression returns the first element of the
+list even if the @code{do-not-move} argument is true.
+
address@hidden Digression concerning error, Determining the Element, Body of
current-kill, Understanding current-kill
address@hidden
address@hidden Digression about the word `error'
address@hidden ifnottex
+
+In my opinion, it is slightly misleading, at least to humans, to use
+the term `error' as the name of the @code{error} function. A better
+term would be `cancel'. Strictly speaking, of course, you cannot
+point to, much less rotate a pointer to a list that has no length, so
+from the point of view of the computer, the word `error' is correct.
+But a human expects to attempt this sort of thing, if only to find out
+whether the kill ring is full or empty. This is an act of
+exploration.
+
+From the human point of view, the act of exploration and discovery is
+not necessarily an error, and therefore should not be labelled as one,
+even in the bowels of a computer. As it is, the code in Emacs implies
+that a human who is acting virtuously, by exploring his or her
+environment, is making an error. This is bad. Even though the computer
+takes the same steps as it does when there is an `error', a term such as
+`cancel' would have a clearer connotation.
+
address@hidden Determining the Element, , Digression concerning error,
Understanding current-kill
address@hidden
address@hidden Determining the Element
address@hidden ifnottex
+
+Among other actions, the else-part of the @code{if} expression sets
+the value of @code{kill-ring-yank-pointer} to
address@hidden when the kill ring has something in it and
+the value of @code{do-not-move} is @code{nil}.
+
address@hidden 800
+The code looks like this:
+
address@hidden
address@hidden
+(nthcdr (mod (- n (length kill-ring-yank-pointer))
+ (length kill-ring))
+ kill-ring)))
address@hidden group
address@hidden smallexample
+
+This needs some examination. Unless it is not supposed to move the
+pointer, the @code{current-kill} function changes where
address@hidden points.
+That is what the
address@hidden@code{(setq kill-ring-yank-pointer ARGth-kill-element))}}
+expression does. Also, clearly, @code{ARGth-kill-element} is being
+set to be equal to some @sc{cdr} of the kill ring, using the
address@hidden function that is described in an earlier section.
+(@xref{copy-region-as-kill}.) How does it do this?
+
+As we have seen before (@pxref{nthcdr}), the @code{nthcdr} function
+works by repeatedly taking the @sc{cdr} of a list---it takes the
address@hidden of the @sc{cdr} of the @sc{cdr} @dots{}
+
address@hidden 800
+The two following expressions produce the same result:
+
address@hidden
address@hidden
+(setq kill-ring-yank-pointer (cdr kill-ring))
+
+(setq kill-ring-yank-pointer (nthcdr 1 kill-ring))
address@hidden group
address@hidden smallexample
+
+However, the @code{nthcdr} expression is more complicated. It uses
+the @code{mod} function to determine which @sc{cdr} to select.
+
+(You will remember to look at inner functions first; indeed, we will
+have to go inside the @code{mod}.)
+
+The @code{mod} function returns the value of its first argument modulo
+the second; that is to say, it returns the remainder after dividing
+the first argument by the second. The value returned has the same
+sign as the second argument.
+
address@hidden 800
+Thus,
+
address@hidden
address@hidden
+(mod 12 4)
+ @result{} 0 ;; @r{because there is no remainder}
+(mod 13 4)
+ @result{} 1
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+In this case, the first argument is often smaller than the second.
+That is fine.
+
address@hidden
address@hidden
+(mod 0 4)
+ @result{} 0
+(mod 1 4)
+ @result{} 1
address@hidden group
address@hidden smallexample
+
+We can guess what the @code{-} function does. It is like @code{+} but
+subtracts instead of adds; the @code{-} function subtracts its second
+argument from its first. Also, we already know what the @code{length}
+function does (@pxref{length}). It returns the length of a list.
+
+And @code{n} is the name of the required argument to the
address@hidden function.
+
address@hidden 1250
+So when the first argument to @code{nthcdr} is zero, the @code{nthcdr}
+expression returns the whole list, as you can see by evaluating the
+following:
+
address@hidden
address@hidden
+;; kill-ring-yank-pointer @r{and} kill-ring @r{have a length of four}
+;; @r{and} (mod (- 0 4) 4) @result{} 0
+(nthcdr (mod (- 0 4) 4)
+ '("fourth line of text"
+ "third line"
+ "second piece of text"
+ "first some text"))
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+When the first argument to the @code{current-kill} function is one,
+the @code{nthcdr} expression returns the list without its first
+element.
+
address@hidden
address@hidden
+(nthcdr (mod (- 1 4) 4)
+ '("fourth line of text"
+ "third line"
+ "second piece of text"
+ "first some text"))
address@hidden group
address@hidden smallexample
+
address@hidden @samp{global variable} defined
address@hidden @samp{variable, global}, defined
+Incidentally, both @code{kill-ring} and @code{kill-ring-yank-pointer}
+are @dfn{global variables}. That means that any expression in Emacs
+Lisp can access them. They are not like the local variables set by
address@hidden or like the symbols in an argument list.
+Local variables can only be accessed
+within the @code{let} that defines them or the function that specifies
+them in an argument list (and within expressions called by them).
+
address@hidden
address@hidden texi2dvi fails when the name of the section is within ifnottex
...
+(@xref{Prevent confusion, , @code{let} Prevents Confusion}, and
address@hidden, , The @code{defun} Special Form}.)
address@hidden ignore
+
address@hidden yank, yank-pop, current-kill, Kill Ring
address@hidden node-name, next, previous, up
address@hidden @code{yank}
address@hidden yank
+
+After learning about @code{current-kill}, the code for the
address@hidden function is almost easy.
+
+The @code{yank} function does not use the
address@hidden variable directly. It calls
address@hidden which calls @code{current-kill} which sets the
address@hidden variable.
+
address@hidden 1250
+The code looks like this:
+
address@hidden in GNU Emacs 22
address@hidden
address@hidden
+(defun yank (&optional arg)
+ "Reinsert (\"paste\") the last stretch of killed text.
+More precisely, reinsert the stretch of killed text most recently
+killed OR yanked. Put point at end, and set mark at beginning.
+With just \\[universal-argument] as argument, same but put point at
+beginning (and mark at end). With argument N, reinsert the Nth most
+recently killed stretch of killed text.
+
+When this command inserts killed text into the buffer, it honors
+`yank-excluded-properties' and `yank-handler' as described in the
+doc string for `insert-for-yank-1', which see.
+
+See also the command \\[yank-pop]."
address@hidden group
address@hidden
+ (interactive "*P")
+ (setq yank-window-start (window-start))
+ ;; If we don't get all the way thru, make last-command indicate that
+ ;; for the following command.
+ (setq this-command t)
+ (push-mark (point))
address@hidden group
address@hidden
+ (insert-for-yank (current-kill (cond
+ ((listp arg) 0)
+ ((eq arg '-) -2)
+ (t (1- arg)))))
+ (if (consp arg)
+ ;; This is like exchange-point-and-mark,
+ ;; but doesn't activate the mark.
+ ;; It is cleaner to avoid activation, even though the command
+ ;; loop would deactivate the mark because we inserted text.
+ (goto-char (prog1 (mark t)
+ (set-marker (mark-marker) (point) (current-buffer)))))
address@hidden group
address@hidden
+ ;; If we do get all the way thru, make this-command indicate that.
+ (if (eq this-command t)
+ (setq this-command 'yank))
+ nil)
address@hidden group
address@hidden smallexample
+
+The key expression is @code{insert-for-yank}, which inserts the string
+returned by @code{current-kill}, but removes some text properties from
+it.
+
+However, before getting to that expression, the function sets the value
+of @code{yank-window-start} to the position returned by the
address@hidden(window-start)} expression, the position at which the display
+currently starts. The @code{yank} function also sets
address@hidden and pushes the mark.
+
+After it yanks the appropriate element, if the optional argument is a
address@hidden rather than a number or nothing, it puts point at beginning
+of the yanked text and mark at its end.
+
+(The @code{prog1} function is like @code{progn} but returns the value
+of its first argument rather than the value of its last argument. Its
+first argument is forced to return the buffer's mark as an integer.
+You can see the documentation for these functions by placing point
+over them in this buffer and then typing @kbd{C-h f}
+(@code{describe-function}) followed by a @kbd{RET}; the default is the
+function.)
+
+The last part of the function tells what to do when it succeeds.
+
address@hidden yank-pop, ring file, yank, Kill Ring
address@hidden node-name, next, previous, up
address@hidden @code{yank-pop}
address@hidden yank-pop
+
+After understanding @code{yank} and @code{current-kill}, you know how
+to approach the @code{yank-pop} function. Leaving out the
+documentation to save space, it looks like this:
+
address@hidden GNU Emacs 22
address@hidden
address@hidden
+(defun yank-pop (&optional arg)
+ "@dots{}"
+ (interactive "*p")
+ (if (not (eq last-command 'yank))
+ (error "Previous command was not a yank"))
address@hidden group
address@hidden
+ (setq this-command 'yank)
+ (unless arg (setq arg 1))
+ (let ((inhibit-read-only t)
+ (before (< (point) (mark t))))
address@hidden group
address@hidden
+ (if before
+ (funcall (or yank-undo-function 'delete-region) (point) (mark t))
+ (funcall (or yank-undo-function 'delete-region) (mark t) (point)))
+ (setq yank-undo-function nil)
address@hidden group
address@hidden
+ (set-marker (mark-marker) (point) (current-buffer))
+ (insert-for-yank (current-kill arg))
+ ;; Set the window start back where it was in the yank command,
+ ;; if possible.
+ (set-window-start (selected-window) yank-window-start t)
address@hidden group
address@hidden
+ (if before
+ ;; This is like exchange-point-and-mark,
+ ;; but doesn't activate the mark.
+ ;; It is cleaner to avoid activation, even though the command
+ ;; loop would deactivate the mark because we inserted text.
+ (goto-char (prog1 (mark t)
+ (set-marker (mark-marker)
+ (point)
+ (current-buffer))))))
+ nil)
address@hidden group
address@hidden smallexample
+
+The function is interactive with a small @samp{p} so the prefix
+argument is processed and passed to the function. The command can
+only be used after a previous yank; otherwise an error message is
+sent. This check uses the variable @code{last-command} which is set
+by @code{yank} and is discussed elsewhere.
+(@xref{copy-region-as-kill}.)
+
+The @code{let} clause sets the variable @code{before} to true or false
+depending whether point is before or after mark and then the region
+between point and mark is deleted. This is the region that was just
+inserted by the previous yank and it is this text that will be
+replaced.
+
address@hidden calls its first argument as a function, passing
+remaining arguments to it. The first argument is whatever the
address@hidden expression returns. The two remaining arguments are the
+positions of point and mark set by the preceding @code{yank} command.
+
+There is more, but that is the hardest part.
+
address@hidden ring file, , yank-pop, Kill Ring
address@hidden node-name, next, previous, up
address@hidden The @file{ring.el} File
address@hidden @file{ring.el} file
+
+Interestingly, GNU Emacs posses a file called @file{ring.el} that
+provides many of the features we just discussed. But functions such
+as @code{kill-ring-yank-pointer} do not use this library, possibly
+because they were written earlier.
+
address@hidden Full Graph, Free Software and Free Manuals, Kill Ring, Top
address@hidden A Graph with Labelled Axes
+
+Printed axes help you understand a graph. They convey scale. In an
+earlier chapter (@pxref{Readying a Graph, , Readying a Graph}), we
+wrote the code to print the body of a graph. Here we write the code
+for printing and labelling vertical and horizontal axes, along with the
+body itself.
+
address@hidden
+* Labelled Example::
+* print-graph Varlist:: @code{let} expression in @code{print-graph}.
+* print-Y-axis:: Print a label for the vertical axis.
+* print-X-axis:: Print a horizontal label.
+* Print Whole Graph:: The function to print a complete graph.
address@hidden menu
+
address@hidden Labelled Example, print-graph Varlist, Full Graph, Full Graph
address@hidden
address@hidden Labelled Example Graph
address@hidden ifnottex
+
+Since insertions fill a buffer to the right and below point, the new
+graph printing function should first print the Y or vertical axis,
+then the body of the graph, and finally the X or horizontal axis.
+This sequence lays out for us the contents of the function:
+
address@hidden
address@hidden
+Set up code.
+
address@hidden
+Print Y axis.
+
address@hidden
+Print body of graph.
+
address@hidden
+Print X axis.
address@hidden enumerate
+
address@hidden 800
+Here is an example of how a finished graph should look:
+
address@hidden
address@hidden
+ 10 -
+ *
+ * *
+ * **
+ * ***
+ 5 - * *******
+ * *** *******
+ *************
+ ***************
+ 1 - ****************
+ | | | |
+ 1 5 10 15
address@hidden group
address@hidden smallexample
+
address@hidden
+In this graph, both the vertical and the horizontal axes are labelled
+with numbers. However, in some graphs, the horizontal axis is time
+and would be better labelled with months, like this:
+
address@hidden
address@hidden
+ 5 - *
+ * ** *
+ *******
+ ********** **
+ 1 - **************
+ | ^ |
+ Jan June Jan
address@hidden group
address@hidden smallexample
+
+Indeed, with a little thought, we can easily come up with a variety of
+vertical and horizontal labelling schemes. Our task could become
+complicated. But complications breed confusion. Rather than permit
+this, it is better choose a simple labelling scheme for our first
+effort, and to modify or replace it later.
+
address@hidden 1200
+These considerations suggest the following outline for the
address@hidden function:
+
address@hidden
address@hidden
+(defun print-graph (numbers-list)
+ "@address@hidden"
+ (let ((height @dots{}
+ @dots{}))
address@hidden group
address@hidden
+ (print-Y-axis height @dots{} )
+ (graph-body-print numbers-list)
+ (print-X-axis @dots{} )))
address@hidden group
address@hidden smallexample
+
+We can work on each part of the @code{print-graph} function definition
+in turn.
+
address@hidden print-graph Varlist, print-Y-axis, Labelled Example, Full Graph
address@hidden node-name, next, previous, up
address@hidden The @code{print-graph} Varlist
address@hidden @code{print-graph} varlist
+
+In writing the @code{print-graph} function, the first task is to write
+the varlist in the @code{let} expression. (We will leave aside for the
+moment any thoughts about making the function interactive or about the
+contents of its documentation string.)
+
+The varlist should set several values. Clearly, the top of the label
+for the vertical axis must be at least the height of the graph, which
+means that we must obtain this information here. Note that the
address@hidden function also requires this information. There
+is no reason to calculate the height of the graph in two different
+places, so we should change @code{print-graph-body} from the way we
+defined it earlier to take advantage of the calculation.
+
+Similarly, both the function for printing the X axis labels and the
address@hidden function need to learn the value of the width of
+each symbol. We can perform the calculation here and change the
+definition for @code{print-graph-body} from the way we defined it in the
+previous chapter.
+
+The length of the label for the horizontal axis must be at least as long
+as the graph. However, this information is used only in the function
+that prints the horizontal axis, so it does not need to be calculated here.
+
+These thoughts lead us directly to the following form for the varlist
+in the @code{let} for @code{print-graph}:
+
address@hidden
address@hidden
+(let ((height (apply 'max numbers-list)) ; @r{First version.}
+ (symbol-width (length graph-blank)))
address@hidden group
address@hidden smallexample
+
address@hidden
+As we shall see, this expression is not quite right.
+
address@hidden 2000
address@hidden print-Y-axis, print-X-axis, print-graph Varlist, Full Graph
address@hidden node-name, next, previous, up
address@hidden The @code{print-Y-axis} Function
address@hidden Axis, print vertical
address@hidden Y axis printing
address@hidden Vertical axis printing
address@hidden Print vertical axis
+
+The job of the @code{print-Y-axis} function is to print a label for
+the vertical axis that looks like this:
+
address@hidden
address@hidden
+ 10 -
+
+
+
+
+ 5 -
+
+
+
+ 1 -
address@hidden group
address@hidden smallexample
+
address@hidden
+The function should be passed the height of the graph, and then should
+construct and insert the appropriate numbers and marks.
+
address@hidden
+* print-Y-axis in Detail::
+* Height of label:: What height for the Y axis?
+* Compute a Remainder:: How to compute the remainder of a division.
+* Y Axis Element:: Construct a line for the Y axis.
+* Y-axis-column:: Generate a list of Y axis labels.
+* print-Y-axis Penultimate:: A not quite final version.
address@hidden menu
+
address@hidden print-Y-axis in Detail, Height of label, print-Y-axis,
print-Y-axis
address@hidden
address@hidden The @code{print-Y-axis} Function in Detail
address@hidden ifnottex
+
+It is easy enough to see in the figure what the Y axis label should
+look like; but to say in words, and then to write a function
+definition to do the job is another matter. It is not quite true to
+say that we want a number and a tic every five lines: there are only
+three lines between the @samp{1} and the @samp{5} (lines 2, 3, and 4),
+but four lines between the @samp{5} and the @samp{10} (lines 6, 7, 8,
+and 9). It is better to say that we want a number and a tic mark on
+the base line (number 1) and then that we want a number and a tic on
+the fifth line from the bottom and on every line that is a multiple of
+five.
+
address@hidden Height of label, Compute a Remainder, print-Y-axis in Detail,
print-Y-axis
address@hidden
address@hidden What height should the label be?
address@hidden ifnottex
+
+The next issue is what height the label should be? Suppose the maximum
+height of tallest column of the graph is seven. Should the highest
+label on the Y axis be @samp{5 -}, and should the graph stick up above
+the label? Or should the highest label be @samp{7 -}, and mark the peak
+of the graph? Or should the highest label be @code{10 -}, which is a
+multiple of five, and be higher than the topmost value of the graph?
+
+The latter form is preferred. Most graphs are drawn within rectangles
+whose sides are an integral number of steps long---5, 10, 15, and so
+on for a step distance of five. But as soon as we decide to use a
+step height for the vertical axis, we discover that the simple
+expression in the varlist for computing the height is wrong. The
+expression is @code{(apply 'max numbers-list)}. This returns the
+precise height, not the maximum height plus whatever is necessary to
+round up to the nearest multiple of five. A more complex expression
+is required.
+
+As usual in cases like this, a complex problem becomes simpler if it is
+divided into several smaller problems.
+
+First, consider the case when the highest value of the graph is an
+integral multiple of five---when it is 5, 10, 15, or some higher
+multiple of five. We can use this value as the Y axis height.
+
+A fairly simply way to determine whether a number is a multiple of
+five is to divide it by five and see if the division results in a
+remainder. If there is no remainder, the number is a multiple of
+five. Thus, seven divided by five has a remainder of two, and seven
+is not an integral multiple of five. Put in slightly different
+language, more reminiscent of the classroom, five goes into seven
+once, with a remainder of two. However, five goes into ten twice,
+with no remainder: ten is an integral multiple of five.
+
address@hidden Compute a Remainder, Y Axis Element, Height of label,
print-Y-axis
address@hidden Side Trip: Compute a Remainder
+
address@hidden % @r{(remainder function)}
address@hidden Remainder function, @code{%}
+In Lisp, the function for computing a remainder is @code{%}. The
+function returns the remainder of its first argument divided by its
+second argument. As it happens, @code{%} is a function in Emacs Lisp
+that you cannot discover using @code{apropos}: you find nothing if you
+type @kbd{M-x apropos @key{RET} remainder @key{RET}}. The only way to
+learn of the existence of @code{%} is to read about it in a book such
+as this or in the Emacs Lisp sources.
+
+You can try the @code{%} function by evaluating the following two
+expressions:
+
address@hidden
address@hidden
+(% 7 5)
+
+(% 10 5)
address@hidden group
address@hidden smallexample
+
address@hidden
+The first expression returns 2 and the second expression returns 0.
+
+To test whether the returned value is zero or some other number, we
+can use the @code{zerop} function. This function returns @code{t} if
+its argument, which must be a number, is zero.
+
address@hidden
address@hidden
+(zerop (% 7 5))
+ @result{} nil
+
+(zerop (% 10 5))
+ @result{} t
address@hidden group
address@hidden smallexample
+
+Thus, the following expression will return @code{t} if the height
+of the graph is evenly divisible by five:
+
address@hidden
+(zerop (% height 5))
address@hidden smallexample
+
address@hidden
+(The value of @code{height}, of course, can be found from @code{(apply
+'max numbers-list)}.)
+
+On the other hand, if the value of @code{height} is not a multiple of
+five, we want to reset the value to the next higher multiple of five.
+This is straightforward arithmetic using functions with which we are
+already familiar. First, we divide the value of @code{height} by five
+to determine how many times five goes into the number. Thus, five
+goes into twelve twice. If we add one to this quotient and multiply by
+five, we will obtain the value of the next multiple of five that is
+larger than the height. Five goes into twelve twice. Add one to two,
+and multiply by five; the result is fifteen, which is the next multiple
+of five that is higher than twelve. The Lisp expression for this is:
+
address@hidden
+(* (1+ (/ height 5)) 5)
address@hidden smallexample
+
address@hidden
+For example, if you evaluate the following, the result is 15:
+
address@hidden
+(* (1+ (/ 12 5)) 5)
address@hidden smallexample
+
+All through this discussion, we have been using `five' as the value
+for spacing labels on the Y axis; but we may want to use some other
+value. For generality, we should replace `five' with a variable to
+which we can assign a value. The best name I can think of for this
+variable is @code{Y-axis-label-spacing}.
+
address@hidden 1250
+Using this term, and an @code{if} expression, we produce the
+following:
+
address@hidden
address@hidden
+(if (zerop (% height Y-axis-label-spacing))
+ height
+ ;; @r{else}
+ (* (1+ (/ height Y-axis-label-spacing))
+ Y-axis-label-spacing))
address@hidden group
address@hidden smallexample
+
address@hidden
+This expression returns the value of @code{height} itself if the height
+is an even multiple of the value of the @code{Y-axis-label-spacing} or
+else it computes and returns a value of @code{height} that is equal to
+the next higher multiple of the value of the @code{Y-axis-label-spacing}.
+
+We can now include this expression in the @code{let} expression of the
address@hidden function (after first setting the value of
address@hidden):
address@hidden Y-axis-label-spacing
+
address@hidden
address@hidden
+(defvar Y-axis-label-spacing 5
+ "Number of lines from one Y axis label to next.")
address@hidden group
+
address@hidden
address@hidden
+(let* ((height (apply 'max numbers-list))
+ (height-of-top-line
+ (if (zerop (% height Y-axis-label-spacing))
+ height
address@hidden group
address@hidden
+ ;; @r{else}
+ (* (1+ (/ height Y-axis-label-spacing))
+ Y-axis-label-spacing)))
+ (symbol-width (length graph-blank))))
address@hidden
address@hidden group
address@hidden smallexample
+
address@hidden
+(Note use of the @code{let*} function: the initial value of height is
+computed once by the @code{(apply 'max numbers-list)} expression and
+then the resulting value of @code{height} is used to compute its
+final value. @xref{fwd-para let, , The @code{let*} expression}, for
+more about @code{let*}.)
+
address@hidden Y Axis Element, Y-axis-column, Compute a Remainder, print-Y-axis
address@hidden Construct a Y Axis Element
+
+When we print the vertical axis, we want to insert strings such as
address@hidden@samp{5 -}} and @address@hidden - }} every five lines.
+Moreover, we want the numbers and dashes to line up, so shorter
+numbers must be padded with leading spaces. If some of the strings
+use two digit numbers, the strings with single digit numbers must
+include a leading blank space before the number.
+
address@hidden number-to-string
+To figure out the length of the number, the @code{length} function is
+used. But the @code{length} function works only with a string, not with
+a number. So the number has to be converted from being a number to
+being a string. This is done with the @code{number-to-string} function.
+For example,
+
address@hidden
address@hidden
+(length (number-to-string 35))
+ @result{} 2
+
+(length (number-to-string 100))
+ @result{} 3
address@hidden group
address@hidden smallexample
+
address@hidden
+(@code{number-to-string} is also called @code{int-to-string}; you will
+see this alternative name in various sources.)
+
+In addition, in each label, each number is followed by a string such
+as @address@hidden - }}, which we will call the @code{Y-axis-tic} marker.
+This variable is defined with @code{defvar}:
+
address@hidden Y-axis-tic
address@hidden
address@hidden
+(defvar Y-axis-tic " - "
+ "String that follows number in a Y axis label.")
address@hidden group
address@hidden smallexample
+
+The length of the Y label is the sum of the length of the Y axis tic
+mark and the length of the number of the top of the graph.
+
address@hidden
+(length (concat (number-to-string height) Y-axis-tic)))
address@hidden smallexample
+
+This value will be calculated by the @code{print-graph} function in
+its varlist as @code{full-Y-label-width} and passed on. (Note that we
+did not think to include this in the varlist when we first proposed it.)
+
+To make a complete vertical axis label, a tic mark is concatenated
+with a number; and the two together may be preceded by one or more
+spaces depending on how long the number is. The label consists of
+three parts: the (optional) leading spaces, the number, and the tic
+mark. The function is passed the value of the number for the specific
+row, and the value of the width of the top line, which is calculated
+(just once) by @code{print-graph}.
+
address@hidden
address@hidden
+(defun Y-axis-element (number full-Y-label-width)
+ "Construct a NUMBERed label element.
+A numbered element looks like this ` 5 - ',
+and is padded as needed so all line up with
+the element for the largest number."
address@hidden group
address@hidden
+ (let* ((leading-spaces
+ (- full-Y-label-width
+ (length
+ (concat (number-to-string number)
+ Y-axis-tic)))))
address@hidden group
address@hidden
+ (concat
+ (make-string leading-spaces ? )
+ (number-to-string number)
+ Y-axis-tic)))
address@hidden group
address@hidden smallexample
+
+The @code{Y-axis-element} function concatenates together the leading
+spaces, if any; the number, as a string; and the tic mark.
+
+To figure out how many leading spaces the label will need, the
+function subtracts the actual length of the label---the length of the
+number plus the length of the tic mark---from the desired label width.
+
address@hidden make-string
+Blank spaces are inserted using the @code{make-string} function. This
+function takes two arguments: the first tells it how long the string
+will be and the second is a symbol for the character to insert, in a
+special format. The format is a question mark followed by a blank
+space, like this, @samp{? }. @xref{Character Type, , Character Type,
+elisp, The GNU Emacs Lisp Reference Manual}, for a description of the
+syntax for characters. (Of course, you might want to replace the
+blank space by some other character @dots{} You know what to do.)
+
+The @code{number-to-string} function is used in the concatenation
+expression, to convert the number to a string that is concatenated
+with the leading spaces and the tic mark.
+
address@hidden Y-axis-column, print-Y-axis Penultimate, Y Axis Element,
print-Y-axis
address@hidden Create a Y Axis Column
+
+The preceding functions provide all the tools needed to construct a
+function that generates a list of numbered and blank strings to insert
+as the label for the vertical axis:
+
address@hidden Y-axis-column
address@hidden
address@hidden
+(defun Y-axis-column (height width-of-label)
+ "Construct list of Y axis labels and blank strings.
+For HEIGHT of line above base and WIDTH-OF-LABEL."
+ (let (Y-axis)
address@hidden
address@hidden group
+ (while (> height 1)
+ (if (zerop (% height Y-axis-label-spacing))
+ ;; @r{Insert label.}
+ (setq Y-axis
+ (cons
+ (Y-axis-element height width-of-label)
+ Y-axis))
address@hidden
address@hidden group
+ ;; @r{Else, insert blanks.}
+ (setq Y-axis
+ (cons
+ (make-string width-of-label ? )
+ Y-axis)))
+ (setq height (1- height)))
+ ;; @r{Insert base line.}
+ (setq Y-axis
+ (cons (Y-axis-element 1 width-of-label) Y-axis))
+ (nreverse Y-axis)))
address@hidden group
address@hidden smallexample
+
+In this function, we start with the value of @code{height} and
+repetitively subtract one from its value. After each subtraction, we
+test to see whether the value is an integral multiple of the
address@hidden If it is, we construct a numbered label
+using the @code{Y-axis-element} function; if not, we construct a
+blank label using the @code{make-string} function. The base line
+consists of the number one followed by a tic mark.
+
address@hidden 2000
address@hidden print-Y-axis Penultimate, , Y-axis-column, print-Y-axis
address@hidden The Not Quite Final Version of @code{print-Y-axis}
+
+The list constructed by the @code{Y-axis-column} function is passed to
+the @code{print-Y-axis} function, which inserts the list as a column.
+
address@hidden print-Y-axis
address@hidden
address@hidden
+(defun print-Y-axis (height full-Y-label-width)
+ "Insert Y axis using HEIGHT and FULL-Y-LABEL-WIDTH.
+Height must be the maximum height of the graph.
+Full width is the width of the highest label element."
+;; Value of height and full-Y-label-width
+;; are passed by `print-graph'.
address@hidden group
address@hidden
+ (let ((start (point)))
+ (insert-rectangle
+ (Y-axis-column height full-Y-label-width))
+ ;; @r{Place point ready for inserting graph.}
+ (goto-char start)
+ ;; @r{Move point forward by value of} full-Y-label-width
+ (forward-char full-Y-label-width)))
address@hidden group
address@hidden smallexample
+
+The @code{print-Y-axis} uses the @code{insert-rectangle} function to
+insert the Y axis labels created by the @code{Y-axis-column} function.
+In addition, it places point at the correct position for printing the body of
+the graph.
+
+You can test @code{print-Y-axis}:
+
address@hidden
address@hidden
+Install
+
address@hidden
address@hidden
+Y-axis-label-spacing
+Y-axis-tic
+Y-axis-element
+Y-axis-column
+print-Y-axis
address@hidden group
address@hidden smallexample
+
address@hidden
+Copy the following expression:
+
address@hidden
+(print-Y-axis 12 5)
address@hidden smallexample
+
address@hidden
+Switch to the @file{*scratch*} buffer and place the cursor where you
+want the axis labels to start.
+
address@hidden
+Type @kbd{M-:} (@code{eval-expression}).
+
address@hidden
+Yank the @code{graph-body-print} expression into the minibuffer
+with @kbd{C-y} (@code{yank)}.
+
address@hidden
+Press @key{RET} to evaluate the expression.
address@hidden enumerate
+
+Emacs will print labels vertically, the top one being @address@hidden
address@hidden
+}}}. (The @code{print-graph} function will pass the value of
address@hidden, which in this case will end up as 15,
+thereby getting rid of what might appear as a bug.)
+
address@hidden 2000
address@hidden print-X-axis, Print Whole Graph, print-Y-axis, Full Graph
address@hidden The @code{print-X-axis} Function
address@hidden Axis, print horizontal
address@hidden X axis printing
address@hidden Print horizontal axis
address@hidden Horizontal axis printing
+
+X axis labels are much like Y axis labels, except that the ticks are on a
+line above the numbers. Labels should look like this:
+
address@hidden
address@hidden
+ | | | |
+ 1 5 10 15
address@hidden group
address@hidden smallexample
+
+The first tic is under the first column of the graph and is preceded by
+several blank spaces. These spaces provide room in rows above for the Y
+axis labels. The second, third, fourth, and subsequent ticks are all
+spaced equally, according to the value of @code{X-axis-label-spacing}.
+
+The second row of the X axis consists of numbers, preceded by several
+blank spaces and also separated according to the value of the variable
address@hidden
+
+The value of the variable @code{X-axis-label-spacing} should itself be
+measured in units of @code{symbol-width}, since you may want to change
+the width of the symbols that you are using to print the body of the
+graph without changing the ways the graph is labelled.
+
address@hidden
+* Similarities differences:: Much like @code{print-Y-axis}, but not exactly.
+* X Axis Tic Marks:: Create tic marks for the horizontal axis.
address@hidden menu
+
address@hidden Similarities differences, X Axis Tic Marks, print-X-axis,
print-X-axis
address@hidden
address@hidden Similarities and differences
address@hidden ifnottex
+
+The @code{print-X-axis} function is constructed in more or less the
+same fashion as the @code{print-Y-axis} function except that it has
+two lines: the line of tic marks and the numbers. We will write a
+separate function to print each line and then combine them within the
address@hidden function.
+
+This is a three step process:
+
address@hidden
address@hidden
+Write a function to print the X axis tic marks, @code{print-X-axis-tic-line}.
+
address@hidden
+Write a function to print the X numbers, @code{print-X-axis-numbered-line}.
+
address@hidden
+Write a function to print both lines, the @code{print-X-axis} function,
+using @code{print-X-axis-tic-line} and
address@hidden
address@hidden enumerate
+
address@hidden X Axis Tic Marks, , Similarities differences, print-X-axis
address@hidden X Axis Tic Marks
+
+The first function should print the X axis tic marks. We must specify
+the tic marks themselves and their spacing:
+
address@hidden
address@hidden
+(defvar X-axis-label-spacing
+ (if (boundp 'graph-blank)
+ (* 5 (length graph-blank)) 5)
+ "Number of units from one X axis label to next.")
address@hidden group
address@hidden smallexample
+
address@hidden
+(Note that the value of @code{graph-blank} is set by another
address@hidden The @code{boundp} predicate checks whether it has
+already been set; @code{boundp} returns @code{nil} if it has not. If
address@hidden were unbound and we did not use this conditional
+construction, in a recent GNU Emacs, we would enter the debugger and
+see an error message saying @address@hidden entered--Lisp error:}
address@hidden(void-variable graph-blank)}}.)
+
address@hidden 1200
+Here is the @code{defvar} for @code{X-axis-tic-symbol}:
+
address@hidden
address@hidden
+(defvar X-axis-tic-symbol "|"
+ "String to insert to point to a column in X axis.")
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+The goal is to make a line that looks like this:
+
address@hidden
+ | | | |
address@hidden smallexample
+
+The first tic is indented so that it is under the first column, which is
+indented to provide space for the Y axis labels.
+
+A tic element consists of the blank spaces that stretch from one tic to
+the next plus a tic symbol. The number of blanks is determined by the
+width of the tic symbol and the @code{X-axis-label-spacing}.
+
address@hidden 1250
+The code looks like this:
+
address@hidden
address@hidden
+;;; X-axis-tic-element
address@hidden
+(concat
+ (make-string
+ ;; @r{Make a string of blanks.}
+ (- (* symbol-width X-axis-label-spacing)
+ (length X-axis-tic-symbol))
+ ? )
+ ;; @r{Concatenate blanks with tic symbol.}
+ X-axis-tic-symbol)
address@hidden
address@hidden group
address@hidden smallexample
+
+Next, we determine how many blanks are needed to indent the first tic
+mark to the first column of the graph. This uses the value of
address@hidden passed it by the @code{print-graph} function.
+
address@hidden 1250
+The code to make @code{X-axis-leading-spaces}
+looks like this:
+
address@hidden
address@hidden
+;; X-axis-leading-spaces
address@hidden
+(make-string full-Y-label-width ? )
address@hidden
address@hidden group
address@hidden smallexample
+
+We also need to determine the length of the horizontal axis, which is
+the length of the numbers list, and the number of ticks in the horizontal
+axis:
+
address@hidden
address@hidden
+;; X-length
address@hidden
+(length numbers-list)
address@hidden group
+
address@hidden
+;; tic-width
address@hidden
+(* symbol-width X-axis-label-spacing)
address@hidden group
+
address@hidden
+;; number-of-X-ticks
+(if (zerop (% (X-length tic-width)))
+ (/ (X-length tic-width))
+ (1+ (/ (X-length tic-width))))
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+All this leads us directly to the function for printing the X axis tic line:
+
address@hidden print-X-axis-tic-line
address@hidden
address@hidden
+(defun print-X-axis-tic-line
+ (number-of-X-tics X-axis-leading-spaces X-axis-tic-element)
+ "Print ticks for X axis."
+ (insert X-axis-leading-spaces)
+ (insert X-axis-tic-symbol) ; @r{Under first column.}
address@hidden group
address@hidden
+ ;; @r{Insert second tic in the right spot.}
+ (insert (concat
+ (make-string
+ (- (* symbol-width X-axis-label-spacing)
+ ;; @r{Insert white space up to second tic symbol.}
+ (* 2 (length X-axis-tic-symbol)))
+ ? )
+ X-axis-tic-symbol))
address@hidden group
address@hidden
+ ;; @r{Insert remaining ticks.}
+ (while (> number-of-X-tics 1)
+ (insert X-axis-tic-element)
+ (setq number-of-X-tics (1- number-of-X-tics))))
address@hidden group
address@hidden smallexample
+
+The line of numbers is equally straightforward:
+
address@hidden 1250
+First, we create a numbered element with blank spaces before each number:
+
address@hidden X-axis-element
address@hidden
address@hidden
+(defun X-axis-element (number)
+ "Construct a numbered X axis element."
+ (let ((leading-spaces
+ (- (* symbol-width X-axis-label-spacing)
+ (length (number-to-string number)))))
+ (concat (make-string leading-spaces ? )
+ (number-to-string number))))
address@hidden group
address@hidden smallexample
+
+Next, we create the function to print the numbered line, starting with
+the number ``1'' under the first column:
+
address@hidden print-X-axis-numbered-line
address@hidden
address@hidden
+(defun print-X-axis-numbered-line
+ (number-of-X-tics X-axis-leading-spaces)
+ "Print line of X-axis numbers"
+ (let ((number X-axis-label-spacing))
+ (insert X-axis-leading-spaces)
+ (insert "1")
address@hidden group
address@hidden
+ (insert (concat
+ (make-string
+ ;; @r{Insert white space up to next number.}
+ (- (* symbol-width X-axis-label-spacing) 2)
+ ? )
+ (number-to-string number)))
address@hidden group
address@hidden
+ ;; @r{Insert remaining numbers.}
+ (setq number (+ number X-axis-label-spacing))
+ (while (> number-of-X-tics 1)
+ (insert (X-axis-element number))
+ (setq number (+ number X-axis-label-spacing))
+ (setq number-of-X-tics (1- number-of-X-tics)))))
address@hidden group
address@hidden smallexample
+
+Finally, we need to write the @code{print-X-axis} that uses
address@hidden and
address@hidden
+
+The function must determine the local values of the variables used by both
address@hidden and @code{print-X-axis-numbered-line}, and
+then it must call them. Also, it must print the carriage return that
+separates the two lines.
+
+The function consists of a varlist that specifies five local variables,
+and calls to each of the two line printing functions:
+
address@hidden print-X-axis
address@hidden
address@hidden
+(defun print-X-axis (numbers-list)
+ "Print X axis labels to length of NUMBERS-LIST."
+ (let* ((leading-spaces
+ (make-string full-Y-label-width ? ))
address@hidden group
address@hidden
+ ;; symbol-width @r{is provided by} graph-body-print
+ (tic-width (* symbol-width X-axis-label-spacing))
+ (X-length (length numbers-list))
address@hidden group
address@hidden
+ (X-tic
+ (concat
+ (make-string
address@hidden group
address@hidden
+ ;; @r{Make a string of blanks.}
+ (- (* symbol-width X-axis-label-spacing)
+ (length X-axis-tic-symbol))
+ ? )
address@hidden group
address@hidden
+ ;; @r{Concatenate blanks with tic symbol.}
+ X-axis-tic-symbol))
address@hidden group
address@hidden
+ (tic-number
+ (if (zerop (% X-length tic-width))
+ (/ X-length tic-width)
+ (1+ (/ X-length tic-width)))))
address@hidden group
address@hidden
+ (print-X-axis-tic-line tic-number leading-spaces X-tic)
+ (insert "\n")
+ (print-X-axis-numbered-line tic-number leading-spaces)))
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+You can test @code{print-X-axis}:
+
address@hidden
address@hidden
+Install @code{X-axis-tic-symbol}, @code{X-axis-label-spacing},
address@hidden, as well as @code{X-axis-element},
address@hidden, and @code{print-X-axis}.
+
address@hidden
+Copy the following expression:
+
address@hidden
address@hidden
+(progn
+ (let ((full-Y-label-width 5)
+ (symbol-width 1))
+ (print-X-axis
+ '(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16))))
address@hidden group
address@hidden smallexample
+
address@hidden
+Switch to the @file{*scratch*} buffer and place the cursor where you
+want the axis labels to start.
+
address@hidden
+Type @kbd{M-:} (@code{eval-expression}).
+
address@hidden
+Yank the test expression into the minibuffer
+with @kbd{C-y} (@code{yank)}.
+
address@hidden
+Press @key{RET} to evaluate the expression.
address@hidden enumerate
+
address@hidden 1250
+Emacs will print the horizontal axis like this:
address@hidden 1
+
address@hidden
address@hidden
+ | | | | |
+ 1 5 10 15 20
address@hidden group
address@hidden smallexample
+
address@hidden Print Whole Graph, , print-X-axis, Full Graph
address@hidden Printing the Whole Graph
address@hidden Printing the whole graph
address@hidden Whole graph printing
address@hidden Graph, printing all
+
+Now we are nearly ready to print the whole graph.
+
+The function to print the graph with the proper labels follows the
+outline we created earlier (@pxref{Full Graph, , A Graph with Labelled
+Axes}), but with additions.
+
address@hidden 1250
+Here is the outline:
+
address@hidden
address@hidden
+(defun print-graph (numbers-list)
+ "@address@hidden"
+ (let ((height @dots{}
+ @dots{}))
address@hidden group
address@hidden
+ (print-Y-axis height @dots{} )
+ (graph-body-print numbers-list)
+ (print-X-axis @dots{} )))
address@hidden group
address@hidden smallexample
+
address@hidden
+* The final version:: A few changes.
+* Test print-graph:: Run a short test.
+* Graphing words in defuns:: Executing the final code.
+* lambda:: How to write an anonymous function.
+* mapcar:: Apply a function to elements of a list.
+* Another Bug:: Yet another bug @dots{} most insidious.
+* Final printed graph:: The graph itself!
address@hidden menu
+
address@hidden The final version, Test print-graph, Print Whole Graph, Print
Whole Graph
address@hidden
address@hidden Changes for the Final Version
address@hidden ifnottex
+
+The final version is different from what we planned in two ways:
+first, it contains additional values calculated once in the varlist;
+second, it carries an option to specify the labels' increment per row.
+This latter feature turns out to be essential; otherwise, a graph may
+have more rows than fit on a display or on a sheet of paper.
+
address@hidden 1500
+This new feature requires a change to the @code{Y-axis-column}
+function, to add @code{vertical-step} to it. The function looks like
+this:
+
address@hidden Y-axis-column @r{Final version.}
address@hidden
address@hidden
+;;; @r{Final version.}
+(defun Y-axis-column
+ (height width-of-label &optional vertical-step)
+ "Construct list of labels for Y axis.
+HEIGHT is maximum height of graph.
+WIDTH-OF-LABEL is maximum width of label.
+VERTICAL-STEP, an option, is a positive integer
+that specifies how much a Y axis label increments
+for each line. For example, a step of 5 means
+that each line is five units of the graph."
address@hidden group
address@hidden
+ (let (Y-axis
+ (number-per-line (or vertical-step 1)))
+ (while (> height 1)
+ (if (zerop (% height Y-axis-label-spacing))
address@hidden group
address@hidden
+ ;; @r{Insert label.}
+ (setq Y-axis
+ (cons
+ (Y-axis-element
+ (* height number-per-line)
+ width-of-label)
+ Y-axis))
address@hidden group
address@hidden
+ ;; @r{Else, insert blanks.}
+ (setq Y-axis
+ (cons
+ (make-string width-of-label ? )
+ Y-axis)))
+ (setq height (1- height)))
address@hidden group
address@hidden
+ ;; @r{Insert base line.}
+ (setq Y-axis (cons (Y-axis-element
+ (or vertical-step 1)
+ width-of-label)
+ Y-axis))
+ (nreverse Y-axis)))
address@hidden group
address@hidden smallexample
+
+The values for the maximum height of graph and the width of a symbol
+are computed by @code{print-graph} in its @code{let} expression; so
address@hidden must be changed to accept them.
+
address@hidden graph-body-print @r{Final version.}
address@hidden
address@hidden
+;;; @r{Final version.}
+(defun graph-body-print (numbers-list height symbol-width)
+ "Print a bar graph of the NUMBERS-LIST.
+The numbers-list consists of the Y-axis values.
+HEIGHT is maximum height of graph.
+SYMBOL-WIDTH is number of each column."
address@hidden group
address@hidden
+ (let (from-position)
+ (while numbers-list
+ (setq from-position (point))
+ (insert-rectangle
+ (column-of-graph height (car numbers-list)))
+ (goto-char from-position)
+ (forward-char symbol-width)
address@hidden group
address@hidden
+ ;; @r{Draw graph column by column.}
+ (sit-for 0)
+ (setq numbers-list (cdr numbers-list)))
+ ;; @r{Place point for X axis labels.}
+ (forward-line height)
+ (insert "\n")))
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+Finally, the code for the @code{print-graph} function:
+
address@hidden print-graph @r{Final version.}
address@hidden
address@hidden
+;;; @r{Final version.}
+(defun print-graph
+ (numbers-list &optional vertical-step)
+ "Print labelled bar graph of the NUMBERS-LIST.
+The numbers-list consists of the Y-axis values.
address@hidden group
+
address@hidden
+Optionally, VERTICAL-STEP, a positive integer,
+specifies how much a Y axis label increments for
+each line. For example, a step of 5 means that
+each row is five units."
address@hidden group
address@hidden
+ (let* ((symbol-width (length graph-blank))
+ ;; @code{height} @r{is both the largest number}
+ ;; @r{and the number with the most digits.}
+ (height (apply 'max numbers-list))
address@hidden group
address@hidden
+ (height-of-top-line
+ (if (zerop (% height Y-axis-label-spacing))
+ height
+ ;; @r{else}
+ (* (1+ (/ height Y-axis-label-spacing))
+ Y-axis-label-spacing)))
address@hidden group
address@hidden
+ (vertical-step (or vertical-step 1))
+ (full-Y-label-width
+ (length
address@hidden group
address@hidden
+ (concat
+ (number-to-string
+ (* height-of-top-line vertical-step))
+ Y-axis-tic))))
address@hidden group
+
address@hidden
+ (print-Y-axis
+ height-of-top-line full-Y-label-width vertical-step)
address@hidden group
address@hidden
+ (graph-body-print
+ numbers-list height-of-top-line symbol-width)
+ (print-X-axis numbers-list)))
address@hidden group
address@hidden smallexample
+
address@hidden Test print-graph, Graphing words in defuns, The final version,
Print Whole Graph
address@hidden Testing @code{print-graph}
+
address@hidden 1250
+We can test the @code{print-graph} function with a short list of numbers:
+
address@hidden
address@hidden
+Install the final versions of @code{Y-axis-column},
address@hidden, and @code{print-graph} (in addition to the
+rest of the code.)
+
address@hidden
+Copy the following expression:
+
address@hidden
+(print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1))
address@hidden smallexample
+
address@hidden
+Switch to the @file{*scratch*} buffer and place the cursor where you
+want the axis labels to start.
+
address@hidden
+Type @kbd{M-:} (@code{eval-expression}).
+
address@hidden
+Yank the test expression into the minibuffer
+with @kbd{C-y} (@code{yank)}.
+
address@hidden
+Press @key{RET} to evaluate the expression.
address@hidden enumerate
+
address@hidden 1250
+Emacs will print a graph that looks like this:
+
address@hidden
address@hidden
+10 -
+
+
+ *
+ ** *
+ 5 - **** *
+ **** ***
+ * *********
+ ************
+ 1 - *************
+
+ | | | |
+ 1 5 10 15
address@hidden group
address@hidden smallexample
+
address@hidden 1200
+On the other hand, if you pass @code{print-graph} a
address@hidden value of 2, by evaluating this expression:
+
address@hidden
+(print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1) 2)
address@hidden smallexample
+
address@hidden 1250
address@hidden
+The graph looks like this:
+
address@hidden
address@hidden
+20 -
+
+
+ *
+ ** *
+10 - **** *
+ **** ***
+ * *********
+ ************
+ 2 - *************
+
+ | | | |
+ 1 5 10 15
address@hidden group
address@hidden smallexample
+
address@hidden
+(A question: is the `2' on the bottom of the vertical axis a bug or a
+feature? If you think it is a bug, and should be a `1' instead, (or
+even a `0'), you can modify the sources.)
+
address@hidden Graphing words in defuns, lambda, Test print-graph, Print Whole
Graph
address@hidden Graphing Numbers of Words and Symbols
+
+Now for the graph for which all this code was written: a graph that
+shows how many function definitions contain fewer than 10 words and
+symbols, how many contain between 10 and 19 words and symbols, how
+many contain between 20 and 29 words and symbols, and so on.
+
+This is a multi-step process. First make sure you have loaded all the
+requisite code.
+
address@hidden 1500
+It is a good idea to reset the value of @code{top-of-ranges} in case
+you have set it to some different value. You can evaluate the
+following:
+
address@hidden
address@hidden
+(setq top-of-ranges
+ '(10 20 30 40 50
+ 60 70 80 90 100
+ 110 120 130 140 150
+ 160 170 180 190 200
+ 210 220 230 240 250
+ 260 270 280 290 300)
address@hidden group
address@hidden smallexample
+
address@hidden
+Next create a list of the number of words and symbols in each range.
+
address@hidden 1500
address@hidden
+Evaluate the following:
+
address@hidden
address@hidden
+(setq list-for-graph
+ (defuns-per-range
+ (sort
+ (recursive-lengths-list-many-files
+ (directory-files "/usr/local/emacs/lisp"
+ t ".+el$"))
+ '<)
+ top-of-ranges))
address@hidden group
address@hidden smallexample
+
address@hidden
+On my old machine, this took about an hour. It looked though 303 Lisp
+files in my copy of Emacs version 19.23. After all that computing,
+the @code{list-for-graph} had this value:
+
address@hidden
address@hidden
+(537 1027 955 785 594 483 349 292 224 199 166 120 116 99
+90 80 67 48 52 45 41 33 28 26 25 20 12 28 11 13 220)
address@hidden group
address@hidden smallexample
+
address@hidden
+This means that my copy of Emacs had 537 function definitions with
+fewer than 10 words or symbols in them, 1,027 function definitions
+with 10 to 19 words or symbols in them, 955 function definitions with
+20 to 29 words or symbols in them, and so on.
+
+Clearly, just by looking at this list we can see that most function
+definitions contain ten to thirty words and symbols.
+
+Now for printing. We do @emph{not} want to print a graph that is
+1,030 lines high @dots{} Instead, we should print a graph that is
+fewer than twenty-five lines high. A graph that height can be
+displayed on almost any monitor, and easily printed on a sheet of paper.
+
+This means that each value in @code{list-for-graph} must be reduced to
+one-fiftieth its present value.
+
+Here is a short function to do just that, using two functions we have
+not yet seen, @code{mapcar} and @code{lambda}.
+
address@hidden
address@hidden
+(defun one-fiftieth (full-range)
+ "Return list, each number one-fiftieth of previous."
+ (mapcar '(lambda (arg) (/ arg 50)) full-range))
address@hidden group
address@hidden smallexample
+
address@hidden lambda, mapcar, Graphing words in defuns, Print Whole Graph
address@hidden A @code{lambda} Expression: Useful Anonymity
address@hidden Anonymous function
address@hidden lambda
+
address@hidden is the symbol for an anonymous function, a function
+without a name. Every time you use an anonymous function, you need to
+include its whole body.
+
address@hidden 1250
address@hidden
+Thus,
+
address@hidden
+(lambda (arg) (/ arg 50))
address@hidden smallexample
+
address@hidden
+is a function definition that says `return the value resulting from
+dividing whatever is passed to me as @code{arg} by 50'.
+
address@hidden 1200
+Earlier, for example, we had a function @code{multiply-by-seven}; it
+multiplied its argument by 7. This function is similar, except it
+divides its argument by 50; and, it has no name. The anonymous
+equivalent of @code{multiply-by-seven} is:
+
address@hidden
+(lambda (number) (* 7 number))
address@hidden smallexample
+
address@hidden
+(@xref{defun, , The @code{defun} Special Form}.)
+
address@hidden 1250
address@hidden
+If we want to multiply 3 by 7, we can write:
+
address@hidden !!! Clear print-postscript-figures if the computer formatting
this
address@hidden document is too small and cannot handle all the diagrams and
figures.
address@hidden clear print-postscript-figures
address@hidden set print-postscript-figures
address@hidden lambda example diagram #1
address@hidden
address@hidden
address@hidden
+(multiply-by-seven 3)
+ \_______________/ ^
+ | |
+ function argument
address@hidden group
address@hidden smallexample
address@hidden ifnottex
address@hidden print-postscript-figures
address@hidden 1
address@hidden
address@hidden @image{lambda-1}
+%%%% old method of including an image
+% \input /usr/local/lib/tex/inputs/psfig.tex
+% \centerline{\psfig{figure=/usr/local/lib/emacs/man/lambda-1.eps}}
+% address@hidden %
address@hidden tex
address@hidden 1
address@hidden ifset
address@hidden print-postscript-figures
address@hidden
address@hidden
address@hidden
+(multiply-by-seven 3)
+ \_______________/ ^
+ | |
+ function argument
address@hidden group
address@hidden smallexample
address@hidden iftex
address@hidden ifclear
+
address@hidden
+This expression returns 21.
+
address@hidden 1250
address@hidden
+Similarly, we can write:
+
address@hidden lambda example diagram #2
address@hidden
address@hidden
address@hidden
+((lambda (number) (* 7 number)) 3)
+ \____________________________/ ^
+ | |
+ anonymous function argument
address@hidden group
address@hidden smallexample
address@hidden ifnottex
address@hidden print-postscript-figures
address@hidden 1
address@hidden
address@hidden @image{lambda-2}
+%%%% old method of including an image
+% \input /usr/local/lib/tex/inputs/psfig.tex
+% \centerline{\psfig{figure=/usr/local/lib/emacs/man/lambda-2.eps}}
+% address@hidden %
address@hidden tex
address@hidden 1
address@hidden ifset
address@hidden print-postscript-figures
address@hidden
address@hidden
address@hidden
+((lambda (number) (* 7 number)) 3)
+ \____________________________/ ^
+ | |
+ anonymous function argument
address@hidden group
address@hidden smallexample
address@hidden iftex
address@hidden ifclear
+
address@hidden 1250
address@hidden
+If we want to divide 100 by 50, we can write:
+
address@hidden lambda example diagram #3
address@hidden
address@hidden
address@hidden
+((lambda (arg) (/ arg 50)) 100)
+ \______________________/ \_/
+ | |
+ anonymous function argument
address@hidden group
address@hidden smallexample
address@hidden ifnottex
address@hidden print-postscript-figures
address@hidden 1
address@hidden
address@hidden @image{lambda-3}
+%%%% old method of including an image
+% \input /usr/local/lib/tex/inputs/psfig.tex
+% \centerline{\psfig{figure=/usr/local/lib/emacs/man/lambda-3.eps}}
+% address@hidden %
address@hidden tex
address@hidden 1
address@hidden ifset
address@hidden print-postscript-figures
address@hidden
address@hidden
address@hidden
+((lambda (arg) (/ arg 50)) 100)
+ \______________________/ \_/
+ | |
+ anonymous function argument
address@hidden group
address@hidden smallexample
address@hidden iftex
address@hidden ifclear
+
address@hidden
+This expression returns 2. The 100 is passed to the function, which
+divides that number by 50.
+
address@hidden Expressions, , Lambda Expressions, elisp, The GNU Emacs
+Lisp Reference Manual}, for more about @code{lambda}. Lisp and lambda
+expressions derive from the Lambda Calculus.
+
address@hidden mapcar, Another Bug, lambda, Print Whole Graph
address@hidden The @code{mapcar} Function
address@hidden mapcar
+
address@hidden is a function that calls its first argument with each
+element of its second argument, in turn. The second argument must be
+a sequence.
+
+The @samp{map} part of the name comes from the mathematical phrase,
+`mapping over a domain', meaning to apply a function to each of the
+elements in a domain. The mathematical phrase is based on the
+metaphor of a surveyor walking, one step at a time, over an area he is
+mapping. And @samp{car}, of course, comes from the Lisp notion of the
+first of a list.
+
address@hidden 1250
address@hidden
+For example,
+
address@hidden
address@hidden
+(mapcar '1+ '(2 4 6))
+ @result{} (3 5 7)
address@hidden group
address@hidden smallexample
+
address@hidden
+The function @code{1+} which adds one to its argument, is executed on
address@hidden element of the list, and a new list is returned.
+
+Contrast this with @code{apply}, which applies its first argument to
+all the remaining.
+(@xref{Readying a Graph, , Readying a Graph}, for a explanation of
address@hidden)
+
address@hidden 1250
+In the definition of @code{one-fiftieth}, the first argument is the
+anonymous function:
+
address@hidden
+(lambda (arg) (/ arg 50))
address@hidden smallexample
+
address@hidden
+and the second argument is @code{full-range}, which will be bound to
address@hidden
+
address@hidden 1250
+The whole expression looks like this:
+
address@hidden
+(mapcar '(lambda (arg) (/ arg 50)) full-range))
address@hidden smallexample
+
address@hidden Functions, , Mapping Functions, elisp, The GNU Emacs
+Lisp Reference Manual}, for more about @code{mapcar}.
+
+Using the @code{one-fiftieth} function, we can generate a list in
+which each element is one-fiftieth the size of the corresponding
+element in @code{list-for-graph}.
+
address@hidden
address@hidden
+(setq fiftieth-list-for-graph
+ (one-fiftieth list-for-graph))
address@hidden group
address@hidden smallexample
+
address@hidden 1250
+The resulting list looks like this:
+
address@hidden
address@hidden
+(10 20 19 15 11 9 6 5 4 3 3 2 2
+1 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 4)
address@hidden group
address@hidden smallexample
+
address@hidden
+This, we are almost ready to print! (We also notice the loss of
+information: many of the higher ranges are 0, meaning that fewer than
+50 defuns had that many words or symbols---but not necessarily meaning
+that none had that many words or symbols.)
+
address@hidden Another Bug, Final printed graph, mapcar, Print Whole Graph
address@hidden Another Bug @dots{} Most Insidious
address@hidden Bug, most insidious type
address@hidden Insidious type of bug
+
+I said `almost ready to print'! Of course, there is a bug in the
address@hidden function @dots{} It has a @code{vertical-step}
+option, but not a @code{horizontal-step} option. The
address@hidden scale goes from 10 to 300 by tens. But the
address@hidden function will print only by ones.
+
+This is a classic example of what some consider the most insidious
+type of bug, the bug of omission. This is not the kind of bug you can
+find by studying the code, for it is not in the code; it is an omitted
+feature. Your best actions are to try your program early and often;
+and try to arrange, as much as you can, to write code that is easy to
+understand and easy to change. Try to be aware, whenever you can,
+that whatever you have written, @emph{will} be rewritten, if not soon,
+eventually. A hard maxim to follow.
+
+It is the @code{print-X-axis-numbered-line} function that needs the
+work; and then the @code{print-X-axis} and the @code{print-graph}
+functions need to be adapted. Not much needs to be done; there is one
+nicety: the numbers ought to line up under the tic marks. This takes
+a little thought.
+
address@hidden 1250
+Here is the corrected @code{print-X-axis-numbered-line}:
+
address@hidden
address@hidden
+(defun print-X-axis-numbered-line
+ (number-of-X-tics X-axis-leading-spaces
+ &optional horizontal-step)
+ "Print line of X-axis numbers"
+ (let ((number X-axis-label-spacing)
+ (horizontal-step (or horizontal-step 1)))
address@hidden group
address@hidden
+ (insert X-axis-leading-spaces)
+ ;; @r{Delete extra leading spaces.}
+ (delete-char
+ (- (1-
+ (length (number-to-string horizontal-step)))))
+ (insert (concat
+ (make-string
address@hidden group
address@hidden
+ ;; @r{Insert white space.}
+ (- (* symbol-width
+ X-axis-label-spacing)
+ (1-
+ (length
+ (number-to-string horizontal-step)))
+ 2)
+ ? )
+ (number-to-string
+ (* number horizontal-step))))
address@hidden group
address@hidden
+ ;; @r{Insert remaining numbers.}
+ (setq number (+ number X-axis-label-spacing))
+ (while (> number-of-X-tics 1)
+ (insert (X-axis-element
+ (* number horizontal-step)))
+ (setq number (+ number X-axis-label-spacing))
+ (setq number-of-X-tics (1- number-of-X-tics)))))
address@hidden group
address@hidden smallexample
+
address@hidden 1500
+If you are reading this in Info, you can see the new versions of
address@hidden @code{print-graph} and evaluate them. If you are
+reading this in a printed book, you can see the changed lines here
+(the full text is too much to print).
+
address@hidden
address@hidden
address@hidden
+(defun print-X-axis (numbers-list horizontal-step)
+ @dots{}
+ (print-X-axis-numbered-line
+ tic-number leading-spaces horizontal-step))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun print-graph
+ (numbers-list
+ &optional vertical-step horizontal-step)
+ @dots{}
+ (print-X-axis numbers-list horizontal-step))
address@hidden group
address@hidden smallexample
address@hidden iftex
+
address@hidden
address@hidden
address@hidden
+(defun print-X-axis (numbers-list horizontal-step)
+ "Print X axis labels to length of NUMBERS-LIST.
+Optionally, HORIZONTAL-STEP, a positive integer,
+specifies how much an X axis label increments for
+each column."
address@hidden group
address@hidden
+;; Value of symbol-width and full-Y-label-width
+;; are passed by `print-graph'.
+ (let* ((leading-spaces
+ (make-string full-Y-label-width ? ))
+ ;; symbol-width @r{is provided by} graph-body-print
+ (tic-width (* symbol-width X-axis-label-spacing))
+ (X-length (length numbers-list))
address@hidden group
address@hidden
+ (X-tic
+ (concat
+ (make-string
+ ;; @r{Make a string of blanks.}
+ (- (* symbol-width X-axis-label-spacing)
+ (length X-axis-tic-symbol))
+ ? )
address@hidden group
address@hidden
+ ;; @r{Concatenate blanks with tic symbol.}
+ X-axis-tic-symbol))
+ (tic-number
+ (if (zerop (% X-length tic-width))
+ (/ X-length tic-width)
+ (1+ (/ X-length tic-width)))))
address@hidden group
+
address@hidden
+ (print-X-axis-tic-line
+ tic-number leading-spaces X-tic)
+ (insert "\n")
+ (print-X-axis-numbered-line
+ tic-number leading-spaces horizontal-step)))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun print-graph
+ (numbers-list &optional vertical-step horizontal-step)
+ "Print labelled bar graph of the NUMBERS-LIST.
+The numbers-list consists of the Y-axis values.
address@hidden group
+
address@hidden
+Optionally, VERTICAL-STEP, a positive integer,
+specifies how much a Y axis label increments for
+each line. For example, a step of 5 means that
+each row is five units.
address@hidden group
+
address@hidden
+Optionally, HORIZONTAL-STEP, a positive integer,
+specifies how much an X axis label increments for
+each column."
+ (let* ((symbol-width (length graph-blank))
+ ;; @code{height} @r{is both the largest number}
+ ;; @r{and the number with the most digits.}
+ (height (apply 'max numbers-list))
address@hidden group
address@hidden
+ (height-of-top-line
+ (if (zerop (% height Y-axis-label-spacing))
+ height
+ ;; @r{else}
+ (* (1+ (/ height Y-axis-label-spacing))
+ Y-axis-label-spacing)))
address@hidden group
address@hidden
+ (vertical-step (or vertical-step 1))
+ (full-Y-label-width
+ (length
+ (concat
+ (number-to-string
+ (* height-of-top-line vertical-step))
+ Y-axis-tic))))
address@hidden group
address@hidden
+ (print-Y-axis
+ height-of-top-line full-Y-label-width vertical-step)
+ (graph-body-print
+ numbers-list height-of-top-line symbol-width)
+ (print-X-axis numbers-list horizontal-step)))
address@hidden group
address@hidden smallexample
address@hidden ifnottex
+
address@hidden qqq
address@hidden
+Graphing Definitions Re-listed
+
address@hidden 1250
+Here are all the graphing definitions in their final form:
+
address@hidden
address@hidden
+(defvar top-of-ranges
+ '(10 20 30 40 50
+ 60 70 80 90 100
+ 110 120 130 140 150
+ 160 170 180 190 200
+ 210 220 230 240 250)
+ "List specifying ranges for `defuns-per-range'.")
address@hidden group
+
address@hidden
+(defvar graph-symbol "*"
+ "String used as symbol in graph, usually an asterisk.")
address@hidden group
+
address@hidden
+(defvar graph-blank " "
+ "String used as blank in graph, usually a blank space.
+graph-blank must be the same number of columns wide
+as graph-symbol.")
address@hidden group
+
address@hidden
+(defvar Y-axis-tic " - "
+ "String that follows number in a Y axis label.")
address@hidden group
+
address@hidden
+(defvar Y-axis-label-spacing 5
+ "Number of lines from one Y axis label to next.")
address@hidden group
+
address@hidden
+(defvar X-axis-tic-symbol "|"
+ "String to insert to point to a column in X axis.")
address@hidden group
+
address@hidden
+(defvar X-axis-label-spacing
+ (if (boundp 'graph-blank)
+ (* 5 (length graph-blank)) 5)
+ "Number of units from one X axis label to next.")
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun count-words-in-defun ()
+ "Return the number of words and symbols in a defun."
+ (beginning-of-defun)
+ (let ((count 0)
+ (end (save-excursion (end-of-defun) (point))))
address@hidden group
+
address@hidden
+ (while
+ (and (< (point) end)
+ (re-search-forward
+ "\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"
+ end t))
+ (setq count (1+ count)))
+ count))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun lengths-list-file (filename)
+ "Return list of definitions' lengths within FILE.
+The returned list is a list of numbers.
+Each number is the number of words or
+symbols in one function definition."
address@hidden group
+
address@hidden
+ (message "Working on `%s' ... " filename)
+ (save-excursion
+ (let ((buffer (find-file-noselect filename))
+ (lengths-list))
+ (set-buffer buffer)
+ (setq buffer-read-only t)
+ (widen)
+ (goto-char (point-min))
address@hidden group
+
address@hidden
+ (while (re-search-forward "^(defun" nil t)
+ (setq lengths-list
+ (cons (count-words-in-defun) lengths-list)))
+ (kill-buffer buffer)
+ lengths-list)))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun lengths-list-many-files (list-of-files)
+ "Return list of lengths of defuns in LIST-OF-FILES."
+ (let (lengths-list)
+;;; @r{true-or-false-test}
+ (while list-of-files
+ (setq lengths-list
+ (append
+ lengths-list
address@hidden group
address@hidden
+;;; @r{Generate a lengths' list.}
+ (lengths-list-file
+ (expand-file-name (car list-of-files)))))
+;;; @r{Make files' list shorter.}
+ (setq list-of-files (cdr list-of-files)))
+;;; @r{Return final value of lengths' list.}
+ lengths-list))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun defuns-per-range (sorted-lengths top-of-ranges)
+ "SORTED-LENGTHS defuns in each TOP-OF-RANGES range."
+ (let ((top-of-range (car top-of-ranges))
+ (number-within-range 0)
+ defuns-per-range-list)
address@hidden group
+
address@hidden
+ ;; @r{Outer loop.}
+ (while top-of-ranges
+
+ ;; @r{Inner loop.}
+ (while (and
+ ;; @r{Need number for numeric test.}
+ (car sorted-lengths)
+ (< (car sorted-lengths) top-of-range))
+
+ ;; @r{Count number of definitions within current range.}
+ (setq number-within-range (1+ number-within-range))
+ (setq sorted-lengths (cdr sorted-lengths)))
address@hidden group
+
address@hidden
+ ;; @r{Exit inner loop but remain within outer loop.}
+
+ (setq defuns-per-range-list
+ (cons number-within-range defuns-per-range-list))
+ (setq number-within-range 0) ; @r{Reset count to zero.}
+
+ ;; @r{Move to next range.}
+ (setq top-of-ranges (cdr top-of-ranges))
+ ;; @r{Specify next top of range value.}
+ (setq top-of-range (car top-of-ranges)))
address@hidden group
+
address@hidden
+ ;; @r{Exit outer loop and count the number of defuns larger than}
+ ;; @r{ the largest top-of-range value.}
+ (setq defuns-per-range-list
+ (cons
+ (length sorted-lengths)
+ defuns-per-range-list))
+
+ ;; @r{Return a list of the number of definitions within each range,}
+ ;; @r{ smallest to largest.}
+ (nreverse defuns-per-range-list)))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun column-of-graph (max-graph-height actual-height)
+ "Return list of MAX-GRAPH-HEIGHT strings;
+ACTUAL-HEIGHT are graph-symbols.
+The graph-symbols are contiguous entries at the end
+of the list.
+The list will be inserted as one column of a graph.
+The strings are either graph-blank or graph-symbol."
address@hidden group
+
address@hidden
+ (let ((insert-list nil)
+ (number-of-top-blanks
+ (- max-graph-height actual-height)))
+
+ ;; @r{Fill in @code{graph-symbols}.}
+ (while (> actual-height 0)
+ (setq insert-list (cons graph-symbol insert-list))
+ (setq actual-height (1- actual-height)))
address@hidden group
+
address@hidden
+ ;; @r{Fill in @code{graph-blanks}.}
+ (while (> number-of-top-blanks 0)
+ (setq insert-list (cons graph-blank insert-list))
+ (setq number-of-top-blanks
+ (1- number-of-top-blanks)))
+
+ ;; @r{Return whole list.}
+ insert-list))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun Y-axis-element (number full-Y-label-width)
+ "Construct a NUMBERed label element.
+A numbered element looks like this ` 5 - ',
+and is padded as needed so all line up with
+the element for the largest number."
address@hidden group
address@hidden
+ (let* ((leading-spaces
+ (- full-Y-label-width
+ (length
+ (concat (number-to-string number)
+ Y-axis-tic)))))
address@hidden group
address@hidden
+ (concat
+ (make-string leading-spaces ? )
+ (number-to-string number)
+ Y-axis-tic)))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun print-Y-axis
+ (height full-Y-label-width &optional vertical-step)
+ "Insert Y axis by HEIGHT and FULL-Y-LABEL-WIDTH.
+Height must be the maximum height of the graph.
+Full width is the width of the highest label element.
+Optionally, print according to VERTICAL-STEP."
address@hidden group
address@hidden
+;; Value of height and full-Y-label-width
+;; are passed by `print-graph'.
+ (let ((start (point)))
+ (insert-rectangle
+ (Y-axis-column height full-Y-label-width vertical-step))
address@hidden group
address@hidden
+ ;; @r{Place point ready for inserting graph.}
+ (goto-char start)
+ ;; @r{Move point forward by value of} full-Y-label-width
+ (forward-char full-Y-label-width)))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun print-X-axis-tic-line
+ (number-of-X-tics X-axis-leading-spaces X-axis-tic-element)
+ "Print ticks for X axis."
+ (insert X-axis-leading-spaces)
+ (insert X-axis-tic-symbol) ; @r{Under first column.}
address@hidden group
address@hidden
+ ;; @r{Insert second tic in the right spot.}
+ (insert (concat
+ (make-string
+ (- (* symbol-width X-axis-label-spacing)
+ ;; @r{Insert white space up to second tic symbol.}
+ (* 2 (length X-axis-tic-symbol)))
+ ? )
+ X-axis-tic-symbol))
address@hidden group
address@hidden
+ ;; @r{Insert remaining ticks.}
+ (while (> number-of-X-tics 1)
+ (insert X-axis-tic-element)
+ (setq number-of-X-tics (1- number-of-X-tics))))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun X-axis-element (number)
+ "Construct a numbered X axis element."
+ (let ((leading-spaces
+ (- (* symbol-width X-axis-label-spacing)
+ (length (number-to-string number)))))
+ (concat (make-string leading-spaces ? )
+ (number-to-string number))))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun graph-body-print (numbers-list height symbol-width)
+ "Print a bar graph of the NUMBERS-LIST.
+The numbers-list consists of the Y-axis values.
+HEIGHT is maximum height of graph.
+SYMBOL-WIDTH is number of each column."
address@hidden group
address@hidden
+ (let (from-position)
+ (while numbers-list
+ (setq from-position (point))
+ (insert-rectangle
+ (column-of-graph height (car numbers-list)))
+ (goto-char from-position)
+ (forward-char symbol-width)
address@hidden group
address@hidden
+ ;; @r{Draw graph column by column.}
+ (sit-for 0)
+ (setq numbers-list (cdr numbers-list)))
+ ;; @r{Place point for X axis labels.}
+ (forward-line height)
+ (insert "\n")))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun Y-axis-column
+ (height width-of-label &optional vertical-step)
+ "Construct list of labels for Y axis.
+HEIGHT is maximum height of graph.
+WIDTH-OF-LABEL is maximum width of label.
address@hidden group
address@hidden
+VERTICAL-STEP, an option, is a positive integer
+that specifies how much a Y axis label increments
+for each line. For example, a step of 5 means
+that each line is five units of the graph."
+ (let (Y-axis
+ (number-per-line (or vertical-step 1)))
address@hidden group
address@hidden
+ (while (> height 1)
+ (if (zerop (% height Y-axis-label-spacing))
+ ;; @r{Insert label.}
+ (setq Y-axis
+ (cons
+ (Y-axis-element
+ (* height number-per-line)
+ width-of-label)
+ Y-axis))
address@hidden group
address@hidden
+ ;; @r{Else, insert blanks.}
+ (setq Y-axis
+ (cons
+ (make-string width-of-label ? )
+ Y-axis)))
+ (setq height (1- height)))
address@hidden group
address@hidden
+ ;; @r{Insert base line.}
+ (setq Y-axis (cons (Y-axis-element
+ (or vertical-step 1)
+ width-of-label)
+ Y-axis))
+ (nreverse Y-axis)))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun print-X-axis-numbered-line
+ (number-of-X-tics X-axis-leading-spaces
+ &optional horizontal-step)
+ "Print line of X-axis numbers"
+ (let ((number X-axis-label-spacing)
+ (horizontal-step (or horizontal-step 1)))
address@hidden group
address@hidden
+ (insert X-axis-leading-spaces)
+ ;; line up number
+ (delete-char (- (1- (length (number-to-string horizontal-step)))))
+ (insert (concat
+ (make-string
+ ;; @r{Insert white space up to next number.}
+ (- (* symbol-width X-axis-label-spacing)
+ (1- (length (number-to-string horizontal-step)))
+ 2)
+ ? )
+ (number-to-string (* number horizontal-step))))
address@hidden group
address@hidden
+ ;; @r{Insert remaining numbers.}
+ (setq number (+ number X-axis-label-spacing))
+ (while (> number-of-X-tics 1)
+ (insert (X-axis-element (* number horizontal-step)))
+ (setq number (+ number X-axis-label-spacing))
+ (setq number-of-X-tics (1- number-of-X-tics)))))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun print-X-axis (numbers-list horizontal-step)
+ "Print X axis labels to length of NUMBERS-LIST.
+Optionally, HORIZONTAL-STEP, a positive integer,
+specifies how much an X axis label increments for
+each column."
address@hidden group
address@hidden
+;; Value of symbol-width and full-Y-label-width
+;; are passed by `print-graph'.
+ (let* ((leading-spaces
+ (make-string full-Y-label-width ? ))
+ ;; symbol-width @r{is provided by} graph-body-print
+ (tic-width (* symbol-width X-axis-label-spacing))
+ (X-length (length numbers-list))
address@hidden group
address@hidden
+ (X-tic
+ (concat
+ (make-string
+ ;; @r{Make a string of blanks.}
+ (- (* symbol-width X-axis-label-spacing)
+ (length X-axis-tic-symbol))
+ ? )
address@hidden group
address@hidden
+ ;; @r{Concatenate blanks with tic symbol.}
+ X-axis-tic-symbol))
+ (tic-number
+ (if (zerop (% X-length tic-width))
+ (/ X-length tic-width)
+ (1+ (/ X-length tic-width)))))
address@hidden group
+
address@hidden
+ (print-X-axis-tic-line
+ tic-number leading-spaces X-tic)
+ (insert "\n")
+ (print-X-axis-numbered-line
+ tic-number leading-spaces horizontal-step)))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun one-fiftieth (full-range)
+ "Return list, each number of which is 1/50th previous."
+ (mapcar '(lambda (arg) (/ arg 50)) full-range))
address@hidden group
address@hidden smallexample
+
address@hidden
address@hidden
+(defun print-graph
+ (numbers-list &optional vertical-step horizontal-step)
+ "Print labelled bar graph of the NUMBERS-LIST.
+The numbers-list consists of the Y-axis values.
address@hidden group
+
address@hidden
+Optionally, VERTICAL-STEP, a positive integer,
+specifies how much a Y axis label increments for
+each line. For example, a step of 5 means that
+each row is five units.
address@hidden group
+
address@hidden
+Optionally, HORIZONTAL-STEP, a positive integer,
+specifies how much an X axis label increments for
+each column."
+ (let* ((symbol-width (length graph-blank))
+ ;; @code{height} @r{is both the largest number}
+ ;; @r{and the number with the most digits.}
+ (height (apply 'max numbers-list))
address@hidden group
address@hidden
+ (height-of-top-line
+ (if (zerop (% height Y-axis-label-spacing))
+ height
+ ;; @r{else}
+ (* (1+ (/ height Y-axis-label-spacing))
+ Y-axis-label-spacing)))
address@hidden group
address@hidden
+ (vertical-step (or vertical-step 1))
+ (full-Y-label-width
+ (length
+ (concat
+ (number-to-string
+ (* height-of-top-line vertical-step))
+ Y-axis-tic))))
address@hidden group
address@hidden
+
+ (print-Y-axis
+ height-of-top-line full-Y-label-width vertical-step)
+ (graph-body-print
+ numbers-list height-of-top-line symbol-width)
+ (print-X-axis numbers-list horizontal-step)))
address@hidden group
address@hidden smallexample
address@hidden qqq
address@hidden ignore
+
address@hidden
address@hidden Final printed graph, , Another Bug, Print Whole Graph
address@hidden The Printed Graph
+
+When made and installed, you can call the @code{print-graph} command
+like this:
address@hidden 1
+
address@hidden
address@hidden
+(print-graph fiftieth-list-for-graph 50 10)
address@hidden group
address@hidden smallexample
address@hidden 1
+
address@hidden
+Here is the graph:
address@hidden 2
+
address@hidden
address@hidden
+1000 - *
+ **
+ **
+ **
+ **
+ 750 - ***
+ ***
+ ***
+ ***
+ ****
+ 500 - *****
+ ******
+ ******
+ ******
+ *******
+ 250 - ********
+ ********* *
+ *********** *
+ ************* *
+ 50 - ***************** * *
+ | | | | | | | |
+ 10 50 100 150 200 250 300 350
address@hidden group
address@hidden smallexample
+
address@hidden 2
+
address@hidden
+The largest group of functions contain 10 -- 19 words and symbols each.
+
address@hidden Free Software and Free Manuals, GNU Free Documentation License,
Full Graph, Top
address@hidden Free Software and Free Manuals
+
address@hidden Richard M. Stallman}
address@hidden 1
+
+The biggest deficiency in free operating systems is not in the
+software---it is the lack of good free manuals that we can include in
+these systems. Many of our most important programs do not come with
+full manuals. Documentation is an essential part of any software
+package; when an important free software package does not come with a
+free manual, that is a major gap. We have many such gaps today.
+
+Once upon a time, many years ago, I thought I would learn Perl. I got
+a copy of a free manual, but I found it hard to read. When I asked
+Perl users about alternatives, they told me that there were better
+introductory manuals---but those were not free.
+
+Why was this? The authors of the good manuals had written them for
+O'Reilly Associates, which published them with restrictive terms---no
+copying, no modification, source files not available---which exclude
+them from the free software community.
+
+That wasn't the first time this sort of thing has happened, and (to
+our community's great loss) it was far from the last. Proprietary
+manual publishers have enticed a great many authors to restrict their
+manuals since then. Many times I have heard a GNU user eagerly tell me
+about a manual that he is writing, with which he expects to help the
+GNU project---and then had my hopes dashed, as he proceeded to explain
+that he had signed a contract with a publisher that would restrict it
+so that we cannot use it.
+
+Given that writing good English is a rare skill among programmers, we
+can ill afford to lose manuals this way.
+
address@hidden (texinfo)uref
+(The Free Software Foundation
address@hidden://www.gnu.org/doc/doc.html#DescriptionsOfGNUDocumentation, ,
+sells printed copies} of free @uref{http://www.gnu.org/doc/doc.html,
+GNU manuals}, too.)
+
+Free documentation, like free software, is a matter of freedom, not
+price. The problem with these manuals was not that O'Reilly Associates
+charged a price for printed copies---that in itself is fine. (The Free
+Software Foundation sells printed copies of free GNU manuals, too.)
+But GNU manuals are available in source code form, while these manuals
+are available only on paper. GNU manuals come with permission to copy
+and modify; the Perl manuals do not. These restrictions are the
+problems.
+
+The criterion for a free manual is pretty much the same as for free
+software: it is a matter of giving all users certain
+freedoms. Redistribution (including commercial redistribution) must be
+permitted, so that the manual can accompany every copy of the program,
+on-line or on paper. Permission for modification is crucial too.
+
+As a general rule, I don't believe that it is essential for people to
+have permission to modify all sorts of articles and books. The issues
+for writings are not necessarily the same as those for software. For
+example, I don't think you or I are obliged to give permission to
+modify articles like this one, which describe our actions and our
+views.
+
+But there is a particular reason why the freedom to modify is crucial
+for documentation for free software. When people exercise their right
+to modify the software, and add or change its features, if they are
+conscientious they will change the manual too---so they can provide
+accurate and usable documentation with the modified program. A manual
+which forbids programmers to be conscientious and finish the job, or
+more precisely requires them to write a new manual from scratch if
+they change the program, does not fill our community's needs.
+
+While a blanket prohibition on modification is unacceptable, some
+kinds of limits on the method of modification pose no problem. For
+example, requirements to preserve the original author's copyright
+notice, the distribution terms, or the list of authors, are ok. It is
+also no problem to require modified versions to include notice that
+they were modified, even to have entire sections that may not be
+deleted or changed, as long as these sections deal with nontechnical
+topics. (Some GNU manuals have them.)
+
+These kinds of restrictions are not a problem because, as a practical
+matter, they don't stop the conscientious programmer from adapting the
+manual to fit the modified program. In other words, they don't block
+the free software community from making full use of the manual.
+
+However, it must be possible to modify all the technical content of
+the manual, and then distribute the result in all the usual media,
+through all the usual channels; otherwise, the restrictions do block
+the community, the manual is not free, and so we need another manual.
+
+Unfortunately, it is often hard to find someone to write another
+manual when a proprietary manual exists. The obstacle is that many
+users think that a proprietary manual is good enough---so they don't
+see the need to write a free manual. They do not see that the free
+operating system has a gap that needs filling.
+
+Why do users think that proprietary manuals are good enough? Some have
+not considered the issue. I hope this article will do something to
+change that.
+
+Other users consider proprietary manuals acceptable for the same
+reason so many people consider proprietary software acceptable: they
+judge in purely practical terms, not using freedom as a
+criterion. These people are entitled to their opinions, but since
+those opinions spring from values which do not include freedom, they
+are no guide for those of us who do value freedom.
+
+Please spread the word about this issue. We continue to lose manuals
+to proprietary publishing. If we spread the word that proprietary
+manuals are not sufficient, perhaps the next person who wants to help
+GNU by writing documentation will realize, before it is too late, that
+he must above all make it free.
+
+We can also encourage commercial publishers to sell free, copylefted
+manuals instead of proprietary ones. One way you can help this is to
+check the distribution terms of a manual before you buy it, and prefer
+copylefted manuals to non-copylefted ones.
+
address@hidden 2
address@hidden
+Note: The Free Software Foundation maintains a page on its Web site
+that lists free books available from other publishers:@*
address@hidden://www.gnu.org/doc/other-free-books.html}
+
address@hidden GNU Free Documentation License, Index, Free Software and Free
Manuals, Top
address@hidden GNU Free Documentation License
+
address@hidden FDL, GNU Free Documentation License
address@hidden Version 1.2, November 2002
+
address@hidden
+Copyright @copyright{} 2000,2001,2002 Free Software Foundation, Inc.
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
+
+Everyone is permitted to copy and distribute verbatim copies
+of this license document, but changing it is not allowed.
address@hidden display
+
address@hidden 0
address@hidden
+PREAMBLE
+
+The purpose of this License is to make a manual, textbook, or other
+functional and useful document @dfn{free} in the sense of freedom: to
+assure everyone the effective freedom to copy and redistribute it,
+with or without modifying it, either commercially or noncommercially.
+Secondarily, this License preserves for the author and publisher a way
+to get credit for their work, while not being considered responsible
+for modifications made by others.
+
+This License is a kind of ``copyleft'', which means that derivative
+works of the document must themselves be free in the same sense. It
+complements the GNU General Public License, which is a copyleft
+license designed for free software.
+
+We have designed this License in order to use it for manuals for free
+software, because free software needs free documentation: a free
+program should come with manuals providing the same freedoms that the
+software does. But this License is not limited to software manuals;
+it can be used for any textual work, regardless of subject matter or
+whether it is published as a printed book. We recommend this License
+principally for works whose purpose is instruction or reference.
+
address@hidden
+APPLICABILITY AND DEFINITIONS
+
+This License applies to any manual or other work, in any medium, that
+contains a notice placed by the copyright holder saying it can be
+distributed under the terms of this License. Such a notice grants a
+world-wide, royalty-free license, unlimited in duration, to use that
+work under the conditions stated herein. The ``Document'', below,
+refers to any such manual or work. Any member of the public is a
+licensee, and is addressed as ``you''. You accept the license if you
+copy, modify or distribute the work in a way requiring permission
+under copyright law.
+
+A ``Modified Version'' of the Document means any work containing the
+Document or a portion of it, either copied verbatim, or with
+modifications and/or translated into another language.
+
+A ``Secondary Section'' is a named appendix or a front-matter section
+of the Document that deals exclusively with the relationship of the
+publishers or authors of the Document to the Document's overall
+subject (or to related matters) and contains nothing that could fall
+directly within that overall subject. (Thus, if the Document is in
+part a textbook of mathematics, a Secondary Section may not explain
+any mathematics.) The relationship could be a matter of historical
+connection with the subject or with related matters, or of legal,
+commercial, philosophical, ethical or political position regarding
+them.
+
+The ``Invariant Sections'' are certain Secondary Sections whose titles
+are designated, as being those of Invariant Sections, in the notice
+that says that the Document is released under this License. If a
+section does not fit the above definition of Secondary then it is not
+allowed to be designated as Invariant. The Document may contain zero
+Invariant Sections. If the Document does not identify any Invariant
+Sections then there are none.
+
+The ``Cover Texts'' are certain short passages of text that are listed,
+as Front-Cover Texts or Back-Cover Texts, in the notice that says that
+the Document is released under this License. A Front-Cover Text may
+be at most 5 words, and a Back-Cover Text may be at most 25 words.
+
+A ``Transparent'' copy of the Document means a machine-readable copy,
+represented in a format whose specification is available to the
+general public, that is suitable for revising the document
+straightforwardly with generic text editors or (for images composed of
+pixels) generic paint programs or (for drawings) some widely available
+drawing editor, and that is suitable for input to text formatters or
+for automatic translation to a variety of formats suitable for input
+to text formatters. A copy made in an otherwise Transparent file
+format whose markup, or absence of markup, has been arranged to thwart
+or discourage subsequent modification by readers is not Transparent.
+An image format is not Transparent if used for any substantial amount
+of text. A copy that is not ``Transparent'' is called ``Opaque''.
+
+Examples of suitable formats for Transparent copies include plain
address@hidden without markup, Texinfo input format, address@hidden input
+format, @acronym{SGML} or @acronym{XML} using a publicly available
address@hidden, and standard-conforming simple @acronym{HTML},
+PostScript or @acronym{PDF} designed for human modification. Examples
+of transparent image formats include @acronym{PNG}, @acronym{XCF} and
address@hidden Opaque formats include proprietary formats that can be
+read and edited only by proprietary word processors, @acronym{SGML} or
address@hidden for which the @acronym{DTD} and/or processing tools are
+not generally available, and the machine-generated @acronym{HTML},
+PostScript or @acronym{PDF} produced by some word processors for
+output purposes only.
+
+The ``Title Page'' means, for a printed book, the title page itself,
+plus such following pages as are needed to hold, legibly, the material
+this License requires to appear in the title page. For works in
+formats which do not have any title page as such, ``Title Page'' means
+the text near the most prominent appearance of the work's title,
+preceding the beginning of the body of the text.
+
+A section ``Entitled XYZ'' means a named subunit of the Document whose
+title either is precisely XYZ or contains XYZ in parentheses following
+text that translates XYZ in another language. (Here XYZ stands for a
+specific section name mentioned below, such as ``Acknowledgements'',
+``Dedications'', ``Endorsements'', or ``History''.) To ``Preserve the Title''
+of such a section when you modify the Document means that it remains a
+section ``Entitled XYZ'' according to this definition.
+
+The Document may include Warranty Disclaimers next to the notice which
+states that this License applies to the Document. These Warranty
+Disclaimers are considered to be included by reference in this
+License, but only as regards disclaiming warranties: any other
+implication that these Warranty Disclaimers may have is void and has
+no effect on the meaning of this License.
+
address@hidden
+VERBATIM COPYING
+
+You may copy and distribute the Document in any medium, either
+commercially or noncommercially, provided that this License, the
+copyright notices, and the license notice saying this License applies
+to the Document are reproduced in all copies, and that you add no other
+conditions whatsoever to those of this License. You may not use
+technical measures to obstruct or control the reading or further
+copying of the copies you make or distribute. However, you may accept
+compensation in exchange for copies. If you distribute a large enough
+number of copies you must also follow the conditions in section 3.
+
+You may also lend copies, under the same conditions stated above, and
+you may publicly display copies.
+
address@hidden
+COPYING IN QUANTITY
+
+If you publish printed copies (or copies in media that commonly have
+printed covers) of the Document, numbering more than 100, and the
+Document's license notice requires Cover Texts, you must enclose the
+copies in covers that carry, clearly and legibly, all these Cover
+Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
+the back cover. Both covers must also clearly and legibly identify
+you as the publisher of these copies. The front cover must present
+the full title with all words of the title equally prominent and
+visible. You may add other material on the covers in addition.
+Copying with changes limited to the covers, as long as they preserve
+the title of the Document and satisfy these conditions, can be treated
+as verbatim copying in other respects.
+
+If the required texts for either cover are too voluminous to fit
+legibly, you should put the first ones listed (as many as fit
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+
+If you publish or distribute Opaque copies of the Document numbering
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+a computer-network location from which the general network-using
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+If you use the latter option, you must take reasonably prudent steps,
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+
+It is requested, but not required, that you contact the authors of the
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+
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+MODIFICATIONS
+
+You may copy and distribute a Modified Version of the Document under
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+
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+Use in the Title Page (and on the covers, if any) a title distinct
+from that of the Document, and from those of previous versions
+(which should, if there were any, be listed in the History section
+of the Document). You may use the same title as a previous version
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+
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+List on the Title Page, as authors, one or more persons or entities
+responsible for authorship of the modifications in the Modified
+Version, together with at least five of the principal authors of the
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+unless they release you from this requirement.
+
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+State on the Title page the name of the publisher of the
+Modified Version, as the publisher.
+
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+Preserve all the copyright notices of the Document.
+
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+Add an appropriate copyright notice for your modifications
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+Preserve in that license notice the full lists of Invariant Sections
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+
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+Include an unaltered copy of this License.
+
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+Preserve the section Entitled ``History'', Preserve its Title, and add
+to it an item stating at least the title, year, new authors, and
+publisher of the Modified Version as given on the Title Page. If
+there is no section Entitled ``History'' in the Document, create one
+stating the title, year, authors, and publisher of the Document as
+given on its Title Page, then add an item describing the Modified
+Version as stated in the previous sentence.
+
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+Preserve the network location, if any, given in the Document for
+public access to a Transparent copy of the Document, and likewise
+the network locations given in the Document for previous versions
+it was based on. These may be placed in the ``History'' section.
+You may omit a network location for a work that was published at
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+
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+For any section Entitled ``Acknowledgements'' or ``Dedications'', Preserve
+the Title of the section, and preserve in the section all the
+substance and tone of each of the contributor acknowledgements and/or
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+Preserve all the Invariant Sections of the Document,
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+Delete any section Entitled ``Endorsements''. Such a section
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+Do not retitle any existing section to be Entitled ``Endorsements'' or
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+Preserve any Warranty Disclaimers.
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+
+If the Modified Version includes new front-matter sections or
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+of these sections as invariant. To do this, add their titles to the
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+These titles must be distinct from any other section titles.
+
+You may add a section Entitled ``Endorsements'', provided it contains
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+
+You may add a passage of up to five words as a Front-Cover Text, and a
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+you may not add another; but you may replace the old one, on explicit
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+The author(s) and publisher(s) of the Document do not by this License
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+COMBINING DOCUMENTS
+
+You may combine the Document with other documents released under this
+License, under the terms defined in section 4 above for modified
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+list them all as Invariant Sections of your combined work in its
+license notice, and that you preserve all their Warranty Disclaimers.
+
+The combined work need only contain one copy of this License, and
+multiple identical Invariant Sections may be replaced with a single
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+Make the same adjustment to the section titles in the list of
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+
+In the combination, you must combine any sections Entitled ``History''
+in the various original documents, forming one section Entitled
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+and any sections Entitled ``Dedications''. You must delete all
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+
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+COLLECTIONS OF DOCUMENTS
+
+You may make a collection consisting of the Document and other documents
+released under this License, and replace the individual copies of this
+License in the various documents with a single copy that is included in
+the collection, provided that you follow the rules of this License for
+verbatim copying of each of the documents in all other respects.
+
+You may extract a single document from such a collection, and distribute
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+
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+AGGREGATION WITH INDEPENDENT WORKS
+
+A compilation of the Document or its derivatives with other separate
+and independent documents or works, in or on a volume of a storage or
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+resulting from the compilation is not used to limit the legal rights
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+When the Document is included in an aggregate, this License does not
+apply to the other works in the aggregate which are not themselves
+derivative works of the Document.
+
+If the Cover Text requirement of section 3 is applicable to these
+copies of the Document, then if the Document is less than one half of
+the entire aggregate, the Document's Cover Texts may be placed on
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+Otherwise they must appear on printed covers that bracket the whole
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+
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+TRANSLATION
+
+Translation is considered a kind of modification, so you may
+distribute translations of the Document under the terms of section 4.
+Replacing Invariant Sections with translations requires special
+permission from their copyright holders, but you may include
+translations of some or all Invariant Sections in addition to the
+original versions of these Invariant Sections. You may include a
+translation of this License, and all the license notices in the
+Document, and any Warranty Disclaimers, provided that you also include
+the original English version of this License and the original versions
+of those notices and disclaimers. In case of a disagreement between
+the translation and the original version of this License or a notice
+or disclaimer, the original version will prevail.
+
+If a section in the Document is Entitled ``Acknowledgements'',
+``Dedications'', or ``History'', the requirement (section 4) to Preserve
+its Title (section 1) will typically require changing the actual
+title.
+
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+TERMINATION
+
+You may not copy, modify, sublicense, or distribute the Document except
+as expressly provided for under this License. Any other attempt to
+copy, modify, sublicense or distribute the Document is void, and will
+automatically terminate your rights under this License. However,
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+
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+FUTURE REVISIONS OF THIS LICENSE
+
+The Free Software Foundation may publish new, revised versions
+of the GNU Free Documentation License from time to time. Such new
+versions will be similar in spirit to the present version, but may
+differ in detail to address new problems or concerns. See
address@hidden://www.gnu.org/copyleft/}.
+
+Each version of the License is given a distinguishing version number.
+If the Document specifies that a particular numbered version of this
+License ``or any later version'' applies to it, you have the option of
+following the terms and conditions either of that specified version or
+of any later version that has been published (not as a draft) by the
+Free Software Foundation. If the Document does not specify a version
+number of this License, you may choose any version ever published (not
+as a draft) by the Free Software Foundation.
address@hidden enumerate
+
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address@hidden ADDENDUM: How to use this License for your documents
+
+To use this License in a document you have written, include a copy of
+the License in the document and put the following copyright and
+license notices just after the title page:
+
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+Copyright (C) @var{year} @var{your name}.
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.2
+or any later version published by the Free Software Foundation;
+with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
+A copy of the license is included in the section entitled ``GNU
+Free Documentation License''.
address@hidden group
address@hidden smallexample
+
+If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
+replace the ``with...Texts.'' line with this:
+
address@hidden
address@hidden
+with the Invariant Sections being @var{list their titles}, with
+the Front-Cover Texts being @var{list}, and with the Back-Cover Texts
+being @var{list}.
address@hidden group
address@hidden smallexample
+
+If you have Invariant Sections without Cover Texts, or some other
+combination of the three, merge those two alternatives to suit the
+situation.
+
+If your document contains nontrivial examples of program code, we
+recommend releasing these examples in parallel under your choice of
+free software license, such as the GNU General Public License,
+to permit their use in free software.
+
address@hidden Index, About the Author, GNU Free Documentation License, Top
address@hidden node-name, next, previous, up
address@hidden Index
+
address@hidden
+MENU ENTRY: NODE NAME.
address@hidden ignore
+
address@hidden cp
+
address@hidden
address@hidden Place biographical information on right-hand (verso) page
+
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+\ifodd\pageno
+ \par\vfill\supereject
+ \global\evenheadline={\hfil} \global\evenfootline={\hfil}
+ \global\oddheadline={\hfil} \global\oddfootline={\hfil}
+ \page\hbox{}\page
+\else
+ \par\vfill\supereject
+ \par\vfill\supereject
+ \global\evenheadline={\hfil} \global\evenfootline={\hfil}
+ \global\oddheadline={\hfil} \global\oddfootline={\hfil}
+ \page\hbox{}\page
+ \page\hbox{}\page
+\fi
address@hidden tex
+
address@hidden
address@hidden }
+
address@hidden ================ Biographical information ================
+
address@hidden }
address@hidden 8
address@hidden About the Author
address@hidden 1
address@hidden iftex
+
address@hidden
address@hidden About the Author, , Index, Top
address@hidden About the Author
address@hidden ifnottex
+
address@hidden
+Robert J. Chassell has worked with GNU Emacs since 1985. He writes
+and edits, teaches Emacs and Emacs Lisp, and speaks throughout the
+world on software freedom. Chassell was a founding Director and
+Treasurer of the Free Software Foundation, Inc. He is co-author of
+the @cite{Texinfo} manual, and has edited more than a dozen other
+books. He graduated from Cambridge University, in England. He has an
+abiding interest in social and economic history and flies his own
+airplane.
address@hidden quotation
+
address@hidden
address@hidden }
+
address@hidden Prevent page number on blank verso, so eject it first.
address@hidden
+\par\vfill\supereject
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+
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address@hidden off
address@hidden @thispage @| @| @thistitle
address@hidden @| @| @thispage
address@hidden iftex
+
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+
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+ arch-tag: da1a2154-531f-43a8-8e33-fc7faad10acf
address@hidden ignore