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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!-- but may not be published or retransmitted without explicit permission.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/intoverflow1.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 3.14</title><link href="transitivity.html" rev=precedes><link href="intovf3.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="transitivity.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="intovf3.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 3.14</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Why doesn't the code<pre>int a = 1000, b = 1000;long int c = a * b;</pre>work?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Under C's integral promotion rules,the multiplication is carried out using <TT>int</TT> arithmetic,and the result may overflowor be truncated before beingpromoted andassigned to the <TT>long int</TT> left-hand side.Use an explicit caston at least one of the operandsto force long arithmetic:<pre> long int c = (long int)a * b;</pre>or perhaps<pre> long int c = (long int)a * (long int)b;</pre>(both forms are equivalent).</p><p>Notice thatthe expression<TT>(long int)(a * b)</TT> would<em>not</em>have the desired effect.Anexplicit cast of this form(i.e.applied to the result ofthe multiplication)is equivalent to the implicit conversion which would occur anywaywhen the value is assigned to the <TT>long int</TT>left-hand side,and like the implicit conversion,it happens too late,after the damage has been done.</p><p>See also question <a href="truncation1.html">3.15</a>.</p><p>References:K&R1 Sec. 2.7 p. 41<br>K&R2 Sec. 2.7 p. 44<br>ISO Sec. 6.2.1.5<br>H&S Sec. 6.3.4 p. 176<br>CT&P Sec. 3.9 pp. 49-50<br></p><!-- aend --><p><hr><a href="transitivity.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="intovf3.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/intoverflow1.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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