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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 04:12:47 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

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)

-- 
__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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