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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!-- but may not be published or retransmitted without explicit permission.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 21:47:47 2005 by faqproc version 2.7 --><!-- from source file misclang.sgml dated Wed Dec 21 13:02:49 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/misc/funkyasgnop.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:05 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 20.21</title><link href="multibreak.html" rev=precedes><link href="circshift.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="multibreak.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="circshift.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 20.21</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>There seem to be a few missing operators, like <TT>^^</TT>, <TT>&&=</TT>, and <TT>->=</TT>.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>A logicalexclusive-or operator(hypothetically ``<TT>^^</TT>'')would be nice,but it couldn't possibly have short-circuitingbehavioranalogous to <TT>&&</TT> and <TT>||</TT>(see question <a href="../expr/shortcircuit.html">3.6</a>).Similarly,it's not clear how short-circuiting would apply tohypothetical assignment operators <TT>&&=</TT> and <TT>||=</TT>.(It's also not clearhow often <TT>&&=</TT> and <TT>||=</TT>would actually be needed.)</p><p>Though <TT>p = p->next</TT> is an extremely common idiom for traversing a linked list,<TT>-></TT> is not a binary arithmetic operator.Ahypothetical<TT>->=</TT> operatortherefore wouldn't really fit the patternof the other assignment operators.</p><p>You can write an exclusive-or macro in several ways:<pre> #define XOR(a, b) ((a) && !(b) || !(a) && (b)) /* 1 */ #define XOR(a, b) (!!(a) ^ !!(b)) /* 2 */ #define XOR(a, b) (!!(a) != !!(b)) /* 3 */ #define XOR(a, b) (!(a) ^ !(b)) /* 4 */ #define XOR(a, b) (!(a) != !(b)) /* 5 */ #define XOR(a, b) ((a) ? !(b) : !!(b)) /* 6 */</pre>The first is straight from the definition,but is poor because it may evaluate its arguments multiple times(see question <a href="../cpp/safemacros.html">10.1</a>).The second and third``normalize''their operands<a href="norml.html" rel=subdocument>[footnote]</a>to strict 0/1 by negating them twice--thesecond then applies bitwise exclusive or(to the single remaining bit);the third one implements exclusive-or as <TT>!=</TT>.The fourth and fifth are based on an elementaryidentity in Boolean algebra,namely that<blockquote> _ _<br>a (+) b = a (+) b</blockquote>(where (+) is exclusive-orand an overbar indicates negation).Finally,the sixth one,suggested by Lawrence Kirbyand Dan Pop,uses the <TT>?:</TT> operatorto guarantee a sequence point between the two operands,as for <TT>&&</TT> and <TT>||</TT>.(There is still no ``short circuiting'' behavior,though,nor can there be.)</p><p>Additional links:A<a href="xor.dmr.html">definitive answer</a>from Dennis Ritchie about <TT>^^</TT></p><!-- aend --><p><hr><a href="multibreak.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="circshift.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/misc/funkyasgnop.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:05 GMT --></html>⌨️ 快捷键说明
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