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


From: Pascal J. Bourguignon
Subject: Re: How to use a symbol and its value to create alist?
Date: Wed, 12 Aug 2015 04:18:04 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

"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)

and if you wonder why cc, it's because I have a constant named c ->
299792458.0 and this is the reason why you should name your global
variables, which are special, with stars: *a* *b* *c*.


-- 
__Pascal Bourguignon__                 http://www.informatimago.com/
“The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.” -- Carl Bass CEO Autodesk


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