From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!not-for-mail From: "Pascal J. Bourguignon" Newsgroups: gmane.emacs.help Subject: Re: How to use a symbol and its value to create alist? Date: Wed, 12 Aug 2015 04:18:04 +0200 Organization: Informatimago Message-ID: <87zj1xdzc3.fsf@kuiper.lan.informatimago.com> References: <20150811135254.GA20200@debian> <20150811151533.17219.21BE0C0D@ahiker.mooo.com> <874mk5fe5c.fsf@kuiper.lan.informatimago.com> NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Trace: ger.gmane.org 1439346021 10499 80.91.229.3 (12 Aug 2015 02:20:21 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 12 Aug 2015 02:20:21 +0000 (UTC) To: help-gnu-emacs@gnu.org Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Wed Aug 12 04:20:21 2015 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZPLeN-0000wx-MT for geh-help-gnu-emacs@m.gmane.org; Wed, 12 Aug 2015 04:20:19 +0200 Original-Received: from localhost ([::1]:36466 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZPLeM-0002Xu-Us for geh-help-gnu-emacs@m.gmane.org; Tue, 11 Aug 2015 22:20:18 -0400 Original-Path: usenet.stanford.edu!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail Original-Newsgroups: gnu.emacs.help Original-Lines: 73 Original-X-Trace: individual.net i40D+CXfKdYd2g9mjn/clws/OIJoqm/VBE5oBp/naANiKAUY3p Cancel-Lock: sha1:NDIxMDI1NGJjYzc0OTcxZmZkMDQzZTI3ZGJmMDRkYjIzZTg3OWM4Nw== sha1:omxZYLhdcJ3kJr98R1DxmoYmORk= Face: iVBORw0KGgoAAAANSUhEUgAAADAAAAAwAQMAAABtzGvEAAAABlBMVEUAAAD///+l2Z/dAAAA oElEQVR4nK3OsRHCMAwF0O8YQufUNIQRGIAja9CxSA55AxZgFO4coMgYrEDDQZWPIlNAjwq9 033pbOBPtbXuB6PKNBn5gZkhGa86Z4x2wE67O+06WxGD/HCOGR0deY3f9Ijwwt7rNGNf6Oac l/GuZTF1wFGKiYYHKSFAkjIo1b6sCYS1sVmFhhhahKQssRjRT90ITWUk6vvK3RsPGs+M1RuR mV+hO/VvFAAAAABJRU5ErkJggg== X-Accept-Language: fr, es, en User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) Original-Xref: usenet.stanford.edu gnu.emacs.help:214202 X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Users list for the GNU Emacs text editor List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Original-Sender: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane.org@gnu.org Xref: news.gmane.org gmane.emacs.help:106486 Archived-At: "Pascal J. Bourguignon" writes: > Navy Cheng 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