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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!-- but may not be published or retransmitted without explicit permission.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/rand48.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 13.21</title><link href="gaussian.html" rev=precedes><link href="exitvsreturn.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="gaussian.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="exitvsreturn.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 13.21</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I'm porting this program, and it calls a routine<TT>drand48</TT>,which my library doesn't have.What isit?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font><TT>drand48</TT>is aUnix System Vroutinewhichreturns floating point random numbers(presumablywith 48 bits of precision)in the half-open interval[0, 1)<a href="fn76.html" rel=subdocument>[footnote]</a>.(Its companion seed routine is <TT>srand48</TT>;neither is in the C Standard.)It's easy to write a low-precision replacement:<pre>#include <stdlib.h>double drand48(){ return rand() / (RAND_MAX + 1.);}</pre></p><p>To more accurately simulate <TT>drand48</TT>'s semantics,you can try to give it closer to48 bitsworth of precision:<pre>#define PRECISION 2.82e14 /* 2**48, rounded up */double drand48(){ double x = 0; double denom = RAND_MAX + 1.; double need; for(need = PRECISION; need > 1; need /= (RAND_MAX + 1.)) { x += rand() / denom; denom *= RAND_MAX + 1.; } return x;}</pre></p><p>Before using code like this, though,beware that it is numerically suspect,particularly if(as is usually the case)the period of <TT>rand</TT> ison the order ofRAND_MAX.(If you have a longer-period random number generator available,such as BSD <TT>random</TT>,definitelyuse itwhen simulating <TT>drand48</TT>.)</p><p>References:PCS Sec. 11 p. 149<br></p><!-- aend --><p><hr><a href="gaussian.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="exitvsreturn.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/rand48.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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