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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:46 2005 by faqproc version 2.7 --><!-- from source file fp.sgml dated Fri Jul 2 23:51:18 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/fp/strangefp.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:00 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 14.4a</title><link href="libm.html" rev=precedes><link href="degrees.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="libm.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="degrees.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 14.4a</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>My floating-point calculations are acting strangely and giving medifferent answers on different machines.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>First,see question <a href="fpdecl.html">14.2</a>.</p><p>If the problem isn't that simple,recall thatdigital computersusuallyuse floating-point formats which provide aclose but by no means exact simulation of real number arithmetic.Among other things, the associative and distributivelaws do nothold completely;that is,order of operation may be important,andrepeated addition is not necessarily equivalent to multiplication.Underflow,cumulative precision loss,and other anomalies are often troublesome.</p><p>Don't assume that floating-point results will be exact,and especially don't assume that floating-point values can becompared for equality.(Don't throw haphazard ``fuzz factors'' in, either;see question <a href="fpequal.html">14.5</a>.)Beware that some machines have more precision availableinfloating-point computation registersthan in <TT>double</TT> values stored in memory,which can leadto floating-point inequalitieswhen it would seem that two values just<em>have</em>to be equal.</p><p>These problems arenoworse for C than they are for any othercomputer language.Certain aspects offloating-pointare usually defined as ``howeverthe processor does them''(see also questions<a href="../ansi/undef.html">11.33</a> and<a href="../ansi/appalled.html">11.34</a>),otherwise a compiler for a machine without the ``right''model would have to do prohibitively expensive emulations.</p><p>This document cannot begin to list the pitfalls associated with,and workarounds appropriate for, floating-point work.A goodnumericalprogramming text should cover the basics;see also the references below.(Beware, though, that subtle problems can occupy numerical analysts foryears.)</p><p>References:Kernighan and Plauger,<I>The Elements of Programming Style</I> Sec. 6 pp. 115-8<br>Knuth, Volume 2 chapter 4<br>David Goldberg,``What Every Computer Scientist Should Knowabout Floating-Point Arithmetic''<br></p><!-- aend --><p><hr><a href="libm.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="degrees.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/fp/strangefp.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:00 GMT --></html>⌨️ 快捷键说明
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