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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" <pjb@informatimago.com> writes:
> Navy Cheng <navych@126.com> 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
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