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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!-- but may not be published or retransmitted without explicit permission.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/srand.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 13.17</title><link href="randrange.html" rev=precedes><link href="notveryrand.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="randrange.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="notveryrand.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 13.17</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Each time I run my program,I get the same sequence of numbers back from <TT>rand()</TT>.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>It's a characteristic of mostpseudo-random number generators(and a defined property of the C library <TT>rand</TT>)thattheyalways start with the same numberand go through the same sequence.(Among other things,a bit of predictability can make debuggingmuch easier.)When you don't want this predictability,youcan call <TT>srand</TT> to seed the pseudo-random numbergenerator with a truly random(or at least variable)initial value.Popular seed values are the time of day,or a process ID number,orthe elapsed timebefore the user presses a key,or some combination of these.Here's an example call,using the timeof dayas a seed:<pre> #include <stdlib.h> #include <time.h> srand((unsigned int)time((time_t *)NULL));</pre>(There remain several difficulties:the <TT>time_t</TT> returned by <TT>time</TT>might be a floating-point type,hence not portably convertible to <TT>unsigned int</TT>without the possibility of overflow.Furthermore,if time of day is available with 1-second resolution,using it by itself means thatsuccessive runs of the program can easily get the same seed.Subsecond resolution,of time-of-dayorkeystroke presses,ishard to achieve portably;seequestion<a href="../osdep/subsecond.html">19.37</a>.)</p><p>Note also thatit'srarely useful to call <TT>srand</TT> more than once during a run of a program;in particular,don't try calling <TT>srand</TT>before each call to <TT>rand</TT>,in an attempt to get ``really random''numbers.</p><p>References:K&R2 Sec. 7.8.7 p. 168<br>ISO Sec. 7.10.2.2<br>H&S Sec. 17.7 p. 393<br></p><!-- aend --><p><hr><a href="randrange.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="notveryrand.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/srand.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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