From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.ciao.gmane.io!not-for-mail From: Stefan Monnier Newsgroups: gmane.emacs.help Subject: Re: (- 2.8 1.6) ; 1.1999999999999997 Date: Mon, 10 Feb 2020 20:31:56 -0500 Message-ID: References: <8736biywcn.fsf@ebih.ebihd> Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="ciao.gmane.io:159.69.161.202"; logging-data="91497"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) To: help-gnu-emacs@gnu.org Cancel-Lock: sha1:4OW34toZRbdxAbQnsWAc+Hvl3Co= Original-X-From: help-gnu-emacs-bounces+geh-help-gnu-emacs=m.gmane-mx.org@gnu.org Tue Feb 11 02:32:57 2020 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 1j1KQ4-000NjG-WB for geh-help-gnu-emacs@m.gmane-mx.org; Tue, 11 Feb 2020 02:32:56 +0100 Original-Received: from localhost ([::1]:42194 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1j1KQ4-0000QD-1P for geh-help-gnu-emacs@m.gmane-mx.org; Mon, 10 Feb 2020 20:32:56 -0500 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]:53597) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1j1KPG-0000OS-3Q for help-gnu-emacs@gnu.org; Mon, 10 Feb 2020 20:32:07 -0500 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1j1KPF-00013e-0Z for help-gnu-emacs@gnu.org; Mon, 10 Feb 2020 20:32:06 -0500 Original-Received: from ciao.gmane.io ([159.69.161.202]:39898) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1j1KPE-0000xE-PJ for help-gnu-emacs@gnu.org; Mon, 10 Feb 2020 20:32:04 -0500 Original-Received: from list by ciao.gmane.io with local (Exim 4.92) (envelope-from ) id 1j1KPC-000Mi7-Mh for help-gnu-emacs@gnu.org; Tue, 11 Feb 2020 02:32:02 +0100 X-Injected-Via-Gmane: http://gmane.org/ X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 159.69.161.202 X-BeenThere: help-gnu-emacs@gnu.org X-Mailman-Version: 2.1.23 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:122396 Archived-At: > (- 2.8 1.6) ; 1.1999999999999997 Emacs Lisp, like most common programming language doesn't support rational numbers (and even less real numbers, which can't be faithfully represented in a computer) but only floating point numbers. And like most programming languages it uses the standard IEEE representation supported natively by most CPUs. This representation can't represent 2.8 nor 1.6 (and nor 1.2) exactly. It so happens that "the representable number closest to 2.8" minus "the representable number closest to 1.6" is not equal to "the representable number closest to 1.2". > With the TI-84 Plus, > > 2.8-1.6=1.2 IIUC those calculators used a decimal floating point representation, which suffers from the same kinds of problems of course, but works better in this specific case. Stefan