From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Michael Heerdegen Newsgroups: gmane.emacs.help Subject: Re: Chopping the last element of a list Date: Thu, 28 Apr 2022 23:33:03 +0200 Message-ID: <87a6c4j3lc.fsf@web.de> References: Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="29268"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux) To: help-gnu-emacs@gnu.org Cancel-Lock: sha1:4E30NVTfZ12Aa60CaBhPytwNDW8= Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Thu Apr 28 23:33:49 2022 Return-path: Envelope-to: geh-help-gnu-emacs@m.gmane-mx.org Original-Received: from lists.gnu.org ([209.51.188.17]) by ciao.gmane.io with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1nkBll-0007QR-Hy for geh-help-gnu-emacs@m.gmane-mx.org; Thu, 28 Apr 2022 23:33:49 +0200 Original-Received: from localhost ([::1]:35028 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nkBlk-0003lZ-6W for geh-help-gnu-emacs@m.gmane-mx.org; Thu, 28 Apr 2022 17:33:48 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:47342) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nkBl9-0003lL-Si for help-gnu-emacs@gnu.org; Thu, 28 Apr 2022 17:33:11 -0400 Original-Received: from ciao.gmane.io ([116.202.254.214]:38748) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nkBl8-0000OC-9K for help-gnu-emacs@gnu.org; Thu, 28 Apr 2022 17:33:11 -0400 Original-Received: from list by ciao.gmane.io with local (Exim 4.92) (envelope-from ) id 1nkBl6-0006Wx-C3 for help-gnu-emacs@gnu.org; Thu, 28 Apr 2022 23:33:08 +0200 X-Injected-Via-Gmane: http://gmane.org/ Received-SPF: pass client-ip=116.202.254.214; envelope-from=geh-help-gnu-emacs@m.gmane-mx.org; helo=ciao.gmane.io X-Spam_score_int: -13 X-Spam_score: -1.4 X-Spam_bar: - X-Spam_report: (-1.4 / 5.0 requ) BAYES_00=-1.9, FREEMAIL_FORGED_FROMDOMAIN=0.249, FREEMAIL_FROM=0.001, HEADER_FROM_DIFFERENT_DOMAINS=0.249, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.29 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-mx.org@gnu.org Original-Sender: "help-gnu-emacs" Xref: news.gmane.io gmane.emacs.help:137074 Archived-At: emacsq via Users list for the GNU Emacs text editor writes: > If I want to do this then I can do, for example: > > (prog1 > (car (last mylist)) (setq mylist (nbutlast mylist))) > > But here last and nbutlast walks the list twice unnecessarily. I would do that like #+begin_src emacs-lisp (let ((last2 (last my-list 2))) (prog1 (cons (cadr last2) my-list) (setcdr last2 nil))) #+end_src > Is there an existing function which does this in one step? If not, > shouldn't there be one built-in emacs, for efficient manipulation of > the list's end? Dunno. Does one need that so often? I remember that I once implemented `rotate-left' and `rotate-right' functions for lists, but I never used them. Most of the time you need a pointer to the last cdr more than once, and then it is more efficient to remember that pointer in a variable. What's your use case? Michael.