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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!-- but may not be published or retransmitted without explicit permission.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 librand.sgml dated Sat Jul 3 13:09:31 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/lib/randrange.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:59 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 13.16</title><link href="rand.html" rev=precedes><link href="srand.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="rand.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="srand.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 13.16</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I get random integers in a certain range?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>The obvious way,<pre> rand() % N /* POOR */</pre>(which tries to return numbers from <TT>0</TT> to <TT>N-1</TT>)is poor,becausethe low-order bits of many randomnumbergenerators are distressingly <em>non</em>-random.(See question<a href="notveryrand.html">13.18</a>.)A bettermethod is something like<pre> (int)((double)rand() / ((double)RAND_MAX + 1) * N)</pre></p><p>If you'd rather not use floating point,another method is<pre> rand() / (RAND_MAX / N + 1)</pre>If you just need to do something with probability 1/<TT>N</TT>,you could use<pre> if(rand() < (RAND_MAX+1u) / N)</pre>All thesemethodsobviously require knowing <TT>RAND_MAX</TT>(which ANSI <TT>#define</TT>s in <TT><stdlib.h></TT>),andassumethat <TT>N</TT> is much less than <TT>RAND_MAX</TT>.</p><p>When <TT>N</TT> is close to <TT>RAND_MAX</TT>,and if the range of the random number generatoris not a multiple of <TT>N</TT>(i.e. if <TT>(RAND_MAX+1) % N != 0</TT>),all of these methods break down:some outputsoccur more often than others.(Using floating point does <em>not</em> help;the problem is that <TT>rand</TT> returns <TT>RAND_MAX+1</TT>distinct values,which cannotalways be evenlydivvied upinto <TT>N</TT> buckets.)If this is a problem, about the only thing you can dois to call <TT>rand</TT> multiple times,discarding certain values:<pre> unsigned int x = (RAND_MAX + 1u) / N; unsigned int y = x * N; unsigned int r; do { r = rand(); } while(r >= y); return r / x;</pre></p><p>For any of these techniques,it'sstraightforward to shiftthe range,if necessary;numbers in the range [M, N] could be generatedwith something like <pre> M + rand() / (RAND_MAX / (N - M + 1) + 1)</pre></p><p>(Note, by the way, that<TT>RAND_MAX</TT> is a <em>constant</em>telling you what the fixed rangeof the C library <TT>rand</TT> function is.You cannot set <TT>RAND_MAX</TT> to some other value,and there is no way of requesting that <TT>rand</TT>return numbers in some other range.)</p><p>If you're starting with a random number generatorwhich returns floating-point valuesbetween 0 and 1(such as the last version of <TT>PMrand</TT>alluded to in question <a href="rand.html">13.15</a>,or <TT>drand48</TT> in question <a href="rand48.html">13.21</a>),all you have to doto get integers from 0 to <TT>N-1</TT>is multiply the output of that generator by <TT>N</TT>:<pre> (int)(drand48() * N)</pre></p><p><a href="sd15.html" rel=subdocument>Additional links</a></p><p>References:K&R2 Sec. 7.8.7 p. 168<br>PCS Sec. 11 p. 172<br></p><!-- aend --><p><hr><a href="rand.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="srand.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/lib/randrange.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:59 GMT --></html>⌨️ 快捷键说明
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