From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Philip Kaludercic Newsgroups: gmane.emacs.devel Subject: Re: Negative nth index Date: Tue, 24 Dec 2024 13:09:22 +0000 Message-ID: <877c7pb531.fsf@posteo.net> References: <865xnc6u9n.fsf@gnu.org> <87jzbs9jlk.fsf@gnu.org> <87v7v9d1ae.fsf@posteo.net> Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="40748"; mail-complaints-to="usenet@ciao.gmane.io" Cc: Stefan Monnier , Stefan Kangas , Tassilo Horn , Eli Zaretskii , Anand Tamariya , emacs-devel@gnu.org To: Thierry Volpiatto Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Tue Dec 24 14:10:05 2024 Return-path: Envelope-to: ged-emacs-devel@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 1tQ4fl-000ATB-1e for ged-emacs-devel@m.gmane-mx.org; Tue, 24 Dec 2024 14:10:05 +0100 Original-Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1tQ4fD-0001nf-3D; Tue, 24 Dec 2024 08:09:31 -0500 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 1tQ4fA-0001nO-B3 for emacs-devel@gnu.org; Tue, 24 Dec 2024 08:09:28 -0500 Original-Received: from mout01.posteo.de ([185.67.36.65]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1tQ4f8-0003hW-G6 for emacs-devel@gnu.org; Tue, 24 Dec 2024 08:09:28 -0500 Original-Received: from submission (posteo.de [185.67.36.169]) by mout01.posteo.de (Postfix) with ESMTPS id C1A72240029 for ; Tue, 24 Dec 2024 14:09:23 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=posteo.net; s=2017; t=1735045763; bh=bUu7KTV80kGBsPikQXyjOxhy2PePiVn+zTEtA4YuOr8=; h=From:To:Cc:Subject:Autocrypt:OpenPGP:Date:Message-ID:MIME-Version: Content-Type:From; b=I7nAd2/R3KDM5BNgjeE7sJEOjvDbE8SeR6YI+R1nl0YtzMzY3UMN+F19TIDgtcQAj cGeK7GndsHGHUcVJlftMsYJRpliqwOJiUnWCNJraxUXN5keiuDfNqZl/5pHd4NH9ll oKtFMgVhKXf7lTHT/XUGCImYQt5mD3MCuMiO/Kvd1DlVIf4rM2EH48oTRL0K34Zmvr jz6gAS18p1u1rWVis6Rif5luDFOdpSAJs5E9xbP6jJ9heJgjz/GvB2cWN19Wq4C3FB d7UxhvAftjnZI4fg5SiKq3+8IGme50wHPlrzt9iCq9G30c9N6IjAiZ8CWo4zchu6Md YVPtabTRBTtrg== Original-Received: from customer (localhost [127.0.0.1]) by submission (posteo.de) with ESMTPSA id 4YHZwf69ksz9rxF; Tue, 24 Dec 2024 14:09:22 +0100 (CET) In-Reply-To: <87v7v9d1ae.fsf@posteo.net> (Thierry Volpiatto's message of "Tue, 24 Dec 2024 06:48:25 +0000") Autocrypt: addr=philipk@posteo.net; keydata= mDMEZBBQQhYJKwYBBAHaRw8BAQdAHJuofBrfqFh12uQu0Yi7mrl525F28eTmwUDflFNmdui0QlBo aWxpcCBLYWx1ZGVyY2ljIChnZW5lcmF0ZWQgYnkgYXV0b2NyeXB0LmVsKSA8cGhpbGlwa0Bwb3N0 ZW8ubmV0PoiWBBMWCAA+FiEEDg7HY17ghYlni8XN8xYDWXahwukFAmQQUEICGwMFCQHhM4AFCwkI BwIGFQoJCAsCBBYCAwECHgECF4AACgkQ8xYDWXahwulikAEA77hloUiSrXgFkUVJhlKBpLCHUjA0 mWZ9j9w5d08+jVwBAK6c4iGP7j+/PhbkxaEKa4V3MzIl7zJkcNNjHCXmvFcEuDgEZBBQQhIKKwYB BAGXVQEFAQEHQI5NLiLRjZy3OfSt1dhCmFyn+fN/QKELUYQetiaoe+MMAwEIB4h+BBgWCAAmFiEE Dg7HY17ghYlni8XN8xYDWXahwukFAmQQUEICGwwFCQHhM4AACgkQ8xYDWXahwukm+wEA8cml4JpK NeAu65rg+auKrPOP6TP/4YWRCTIvuYDm0joBALw98AMz7/qMHvSCeU/hw9PL6u6R2EScxtpKnWof z4oM OpenPGP: id=philipk@posteo.net; url="https://keys.openpgp.org/vks/v1/by-email/philipk@posteo.net"; preference=signencrypt Received-SPF: pass client-ip=185.67.36.65; envelope-from=philipk@posteo.net; helo=mout01.posteo.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Original-Sender: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Xref: news.gmane.io gmane.emacs.devel:327005 Archived-At: Thierry Volpiatto writes: > Stefan Monnier writes: > >>>>>> Should negative index for nth be valid? e.g. >>>>>> (nth -1 '(1 2)) returns 1 >> >> [ I'd make it return 2 otherwise you can't use it to return the >> last element. ] >> >>>>> What does this do in other Lisps? >>>> In CL, it errors (ditto for nthcdr with negative index). Same for (nth >>>> '(1 2 3) -1) in Clojure or (list-ref '(1 2 3) -1) in Scheme. >>> Stefan Monnier, any comments here? >> >> I don't see a strong argument in favor of making it index from the end >> rather than signal an error. What would the implementation look like? >> Would it be significantly more efficient than doing it "by hand" e.g.: >> >> (let ((l (length X))) >> (nth (if (< i 0) (- l i) i) X)) > > I guess you meant (+ l i) no? And do we need a check to see if (< (- i) (length l)), otherwise (nth -10 '(1 2 3)) could still return a non-nil value, while (nth 10 '(1 2 3)) just returns nil. Using (mod i l) would also be too DWIM-y, right? >> >> - Stefan >> >> >>