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序号：39797 发表者：王莹 发表日期：2003-05-13 19:42:55
<br>主题：rand()函数一问
<br>内容：我需要一个随机数产生函数，范围是从0到0xffffff,可是编译器提供的最大值是0x7fff，我在函数中这样写：<BR> #ifdef RAND_MAX<BR> #undef RAND_MAX<BR> #endif<BR> #define RAND_MAX 0xffffff<BR>然后再调用rand(),可是没有起作用，产生的最大值还是0x7fff。<BR><BR>我该怎么办？：（
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<font color=red>答案被接受</font><br>回复者：Justin Le 回复日期：2003-05-13 19:50:23
<br>内容：rand() * (0xffffff/0x7fff)<BR><BR>//你看可以吗
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回复者：dr0 回复日期：2003-05-13 20:04:29
<br>内容：/***<BR>*rand.c - random number generator<BR>*<BR>*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Copyright (c) 1985-1997, Microsoft Corporation. All rights reserved.<BR>*<BR>*Purpose:<BR>*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; defines rand(), srand() - random number generator<BR>*<BR>*******************************************************************************/<BR><BR>#include &lt;cruntime.h&gt;<BR>#include &lt;mtdll.h&gt;<BR>#include &lt;stddef.h&gt;<BR>#include &lt;stdlib.h&gt;<BR><BR>#ifndef _MT<BR>static long holdrand = 1L;<BR>#endif&nbsp;&nbsp;/* _MT */<BR><BR>/***<BR>*void srand(seed) - seed the random number generator<BR>*<BR>*Purpose:<BR>*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Seeds the random number generator with the int given.&nbsp;&nbsp;Adapted from the<BR>*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; BASIC random number generator.<BR>*<BR>*Entry:<BR>*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; unsigned seed - seed to seed rand # generator with<BR>*<BR>*Exit:<BR>*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; None.<BR>*<BR>*Exceptions:<BR>*<BR>*******************************************************************************/<BR><BR>void __cdecl srand (<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;unsigned int seed<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;)<BR>{<BR>#ifdef _MT<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_getptd()-&gt;_holdrand = (unsigned long)seed;<BR><BR>#else&nbsp;&nbsp;/* _MT */<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;holdrand = (long)seed;<BR>#endif&nbsp;&nbsp;/* _MT */<BR>}<BR><BR><BR>/***<BR>*int rand() - returns a random number<BR>*<BR>*Purpose:<BR>*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; returns a pseudo-random number 0 through 32767.<BR>*<BR>*Entry:<BR>*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; None.<BR>*<BR>*Exit:<BR>*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Returns a pseudo-random number 0 through 32767.<BR>*<BR>*Exceptions:<BR>*<BR>*******************************************************************************/<BR><BR>int __cdecl rand (<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;void<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;)<BR>{<BR>#ifdef _MT<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;_ptiddata ptd = _getptd();<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return( ((ptd-&gt;_holdrand = ptd-&gt;_holdrand * 214013L<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ 2531011L) &gt;&gt; 16) &amp; 0x7fff );<BR><BR>#else&nbsp;&nbsp;/* _MT */<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return(((holdrand = holdrand * 214013L + 2531011L) &gt;&gt; 16) &amp; 0x7fff);<BR>#endif&nbsp;&nbsp;/* _MT */<BR>}<BR><BR>// 上面是 MS CRT 的源代码，自己可以看看他的实现，写一个新的函数，比如 rand64()
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回复者：dr0 回复日期：2003-05-13 20:13:13
<br>内容：还有一个方法，把 0xFFFFFF分成几段，可以用３个0xFF 表示，<BR>这样子，调用３次rand把结果用位操作结合起来即可。<BR>2003-5-13 21:13:52