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Re: How to use a symbol and its value to create alist?


From: Navy Cheng
Subject: Re: How to use a symbol and its value to create alist?
Date: Wed, 12 Aug 2015 13:06:12 +0800
User-agent: Mutt/1.5.23 (2014-03-12)

On Wed, Aug 12, 2015 at 04:18:04AM +0200, Pascal J. Bourguignon wrote:
> "Pascal J. Bourguignon" <address@hidden> writes:
> 
> > Navy Cheng <address@hidden> writes:
> >
> >> On Tue, Aug 11, 2015 at 08:21:53AM -0700, Ian Zimmerman wrote:
> >>> On 2015-08-11 21:52 +0800, Navy Cheng wrote:
> >>> 
> >>> > (setq a 1)
> >>> > (setq b 2)
> >>> > (setq c 3)
> >>> > 
> >>> > How can I a alist, like:
> >>> > ((a . 1) (b . 2) (c .3))
> >>> > 
> >>> > The value of a, b and c may change, so don't do this like
> >>> > (setq tree ((a . 1) (b . 2) (c .3)))
> >>> 
> >>> That's a strange question.  Why would you want such a list, how would
> >>> it be useful?  To look up the value a a symbol, you just use it, for
> >>> example: 
> >>
> >> I need to push some global variable to a "stack" and pop them later. If
> >> I don't do like this, the global variables will be changed by program
> >
> >     (defvar a 1)
> >     (defvar b 2)
> >     (defvar cc 3)
> >
> >     (defun do-something ()
> >       (print (list 'before a b cc))
> >       (setf a 0 b 0 cc 0)
> >       (print (list 'after a b cc)))
> >
> >     (progn
> >       (let ((a a)
> >             (b b)
> >             (cc cc))
> >          (do-something))
> >       (list 'finally a b cc))
> >     prints:
> >
> >     (before 1 2 3)
> >
> >     (after 0 0 0)
> >     --> (finally 1 2 3)
> 
> If you have a lot of global variables you want to preserve like this, or
> in a lot of places, you can write a macro:
> 
>     (defmacro with-saved-variables (variables &rest body)
>       `(let ,(mapcar (lambda (var) (list var var)) variables) 
>          ,@body))
> 
>      (progn
>        (with-saved-variables (a b cc)
>           (do-something))
>        (list 'finally a b cc))
>     prints:
>     (before 1 2 3)
> 
>     (after 0 0 0)
>     --> (finally 1 2 3)
> 
Thank you for your answers. As I'm not familar with macros in elisp, I think 
your
first answer is good for me.

By the way, you define global variable by (defval). But I always use (setq). 
What's
the difference bewteen (defval) and (setq)? And with one is recommand to define 
a
global variable?

     




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