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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!-- but may not be published or retransmitted without explicit permission.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 21:47:45 2005 by faqproc version 2.7 --><!-- from source file expr.sgml dated Sat Jul 3 17:10:33 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/expr/precvsooe.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:47 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 3.4</title><link href="xorswapexpr.html" rev=precedes><link href="seqpointops.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="xorswapexpr.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="seqpointops.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 3.4</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Can I use explicit parentheses to forcethe order of evaluation I want,and control these side effects?Even if I don't, doesn't precedence dictate it?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Not in general.</p><p>Operator precedence and explicit parentheses impose only apartial ordering on the evaluation of anexpression.Inthe expression<pre> f() + g() * h()</pre>although we know thatthe multiplication will happenbefore the addition,there is no telling whichof the three functionswill be called first.In other words, precedence only partially specifies order ofevaluation, where ``partially'' emphatically does<I>not</I>cover evaluation of operands.</p><p>Parentheses tell the compiler which operands go with which operators;theydo <em>not</em> force the compiler to evaluate everything within the parentheses first.Adding explicit parentheses to the above expressionto make it<pre> f() + (g() * h())</pre>would make no difference in the order of the function calls.Similarly,adding explicit parentheses tothe expression from question <a href="evalorder2.html">3.2</a>to make it<pre> (i++) * (i++) /* WRONG */</pre>accomplishes nothing(since <TT>++</TT> alreadyhas higher precedencethan <TT>*</TT>);the expression remains undefined with or without them.</p><p>When you need to ensure the order of subexpression evaluation,you may need to use explicit temporary variablesand separate statements.</p><p>References:K&R1 Sec. 2.12 p. 49, Sec. A.7 p. 185<br>K&R2 Sec. 2.12 pp. 52-3, Sec. A.7 p. 200<br></p><!-- aend --><p><hr><a href="xorswapexpr.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="seqpointops.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/expr/precvsooe.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:47 GMT --></html>⌨️ 快捷键说明
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