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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!-- but may not be published or retransmitted without explicit permission.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/rand.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 13.15</title><link href="y2k.html" rev=precedes><link href="randrange.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="y2k.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="randrange.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 13.15</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I need a random number generator.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>TheStandardC library has one: <TT>rand</TT>.The implementation on your system may not be perfect,but writing a better one isn't necessarily easy, either.</p><p>Ifyou do find yourself needing to implement your ownrandom number generator,there is plenty of literature out there;see the References below orthe sci.math.num-analysis FAQ list.There are also any number of packages on the net:old standbys arer250, RANLIB, and FSULTRA(see question <a href="../resources/sources.html">18.16</a>),and there is much recent workby<a href="http://stat.fsu.edu/~geo/">Marsaglia</a>,and Matumoto and Nishimura(the ``<a href="http://www.math.keio.ac.jp/~matumoto/emt.html">Mersenne Twister</a>''),andsome code collected by<a href="http://www-cs-faculty.stanford.edu/~knuth/">Don Knuth</a>on his<a href="http://www-cs-faculty.stanford.edu/~knuth/programs.html">web pages</a>.</p><p>Here isa portable C implementation ofthe ``minimal standard'' generator proposed byPark andMiller:<pre>#define a 16807#define m 2147483647#define q (m / a)#define r (m % a)static long int seed = 1;long int PMrand(){ long int hi = seed / q; long int lo = seed % q; long int test = a * lo - r * hi; if(test > 0) seed = test; else seed = test + m; return seed;}</pre>(The ``minimal standard'' is adequately good;it is something ``against which all others should be judged'';it is recommended for use ``unless one has access to a random number generator<I>known</I> to be better.'')</p><p>This code implements the generator<blockquote><I>X</I> <- (<I>aX</I> + <I>c</I>) mod <I>m</I></blockquote>for <I>a</I> = 16807,<I>m</I> = 2147483647(which is2**31-1),and <I>c</I> = 0.<a href="pmprimem.html" rel=subdocument>[footnote]</a>The multiplication is carried outusing a technique described bySchrage,ensuring that the intermediate result <I>aX</I> does not overflow.The implementation above returns <TT>long int</TT> valuesin the range [1, 2147483646];that is, it corresponds to C's <TT>rand</TT>with a <TT>RAND_MAX</TT> of 2147483646,<em>except</em> that it never returns 0.To alter it to return floating-point numbers in the range (0, 1)(as in the Park and Miller paper),change the declaration to<pre> double PMrand()</pre>and the last line to<pre> return (double)seed / m;</pre>For slightly better statistical properties,Park and Miller now recommend using <I>a</I> = 48271.</p><p>References:K&R2 Sec. 2.7 p. 46, Sec. 7.8.7 p. 168<br>ISO Sec. 7.10.2.1<br>H&S Sec. 17.7 p. 393<br>PCS Sec. 11 p. 172<br>Knuth Vol. 2 Chap. 3 pp. 1-177<br>Park and Miller, ``Random Number Generators: Good Ones are Hard to Find''<br></p><!-- aend --><p><hr><a href="y2k.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="randrange.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/rand.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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