From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Stefan Monnier via Users list for the GNU Emacs text editor Newsgroups: gmane.emacs.help Subject: Re: [External] : Use of an associated list with completing-read Date: Fri, 19 Apr 2024 21:52:23 -0400 Message-ID: References: <1ZznhcKDmGMQcjaA3pyQaBOnYG81YwX4pE1YSoGxi_Vj_UFqz_DzTcdhZ50XnL3lEDivoq5THtGr-ShAq1PMbkmNHxajYrXWeRmti6_BSD4=@protonmail.com> <6IDPPDE6NaK72davfT7rnK8S553m9H8sBdCDy8UqV81_uCoMjrGuIjErQYACxxx5LZjd2Np0R9S2RPCqIFanRLNpVZhEGvXNYfqBrYI0ahw=@protonmail.com> Reply-To: Stefan Monnier Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="10478"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) To: help-gnu-emacs@gnu.org Cancel-Lock: sha1:TRR1jD5/GfV/cE8BSDckjLmKyeM= Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Sat Apr 20 03:53:12 2024 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 1rxzuh-0002cx-NU for geh-help-gnu-emacs@m.gmane-mx.org; Sat, 20 Apr 2024 03:53:11 +0200 Original-Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rxzuC-0002mz-SC; Fri, 19 Apr 2024 21:52:40 -0400 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rxzu6-0002m9-NF for help-gnu-emacs@gnu.org; Fri, 19 Apr 2024 21:52:36 -0400 Original-Received: from ciao.gmane.io ([116.202.254.214]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rxzu4-0002r0-9h for help-gnu-emacs@gnu.org; Fri, 19 Apr 2024 21:52:33 -0400 Original-Received: from list by ciao.gmane.io with local (Exim 4.92) (envelope-from ) id 1rxzu1-0001d1-9a for help-gnu-emacs@gnu.org; Sat, 20 Apr 2024 03:52:29 +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: -16 X-Spam_score: -1.7 X-Spam_bar: - X-Spam_report: (-1.7 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=0.248, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Xref: news.gmane.io gmane.emacs.help:146395 Archived-At: >> > (setq-local tema-lugar >> > (append tema--lugar (list (cons lnum strg)))) >> >> This is the classic recipe for poor scaling performance since the above >> operation takes time proportional to the length of the list, so if you >> execute this N times (in a loop), the loop builds a list of length N >> but takes time Nē to do it. When N is small, noone notices, and as it gets >> large the performance starts to suck. > > Heime: > > One programming cliche for this is to > (1) start with a list that you create > (e.g., a let-binding to nil), so you > don't modify any existing list that > you might not want to mess with, (2) > use `nconc' instead of `append', to > append quickly (the _list structure_ > is modified - destructive), (3) being > sure to set your list variable to the > result of each modification. While this is slightly better because it avoid the O(Nē) memory allocation, it's still O(Nē) operations. > An even more common cliche for doing > the same thing is to do #1, then (2) > cons instead of append, and (3) when > finished adding list elements, do an > `nreverse' of the list you created. > That too is a destructive operation. That's the usual solution, with the desired linear (i.e. optimal) complexity, indeed. Stefan