143 lines
7.4 KiB
HTML
143 lines
7.4 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>srand</title>
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<link rel="stylesheet" href="site_modules.css" />
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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_numeric_random_srand skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading">srand</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-numeric.html">numeric</a>‎ | <a href="c-numeric-random.html">random</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<tbody>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><stdlib.h></code></div>
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</td>
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<td></td>
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<td></td>
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<tr class="t-dcl">
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<td class="t-dcl-nopad">
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<div><span class="mw-geshi c source-c"><span class="kw4">void</span> srand<span class="br0">(</span> <span class="kw4">unsigned</span> seed <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td class="t-dcl-nopad"></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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</table>
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<p>以值 <code>seed</code> 播种 <span class="t-lc"><a href="c-numeric-random-rand.html">rand()</a></span> 所用的随机数生成器。</p>
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<p>若在任何到 <code>srand()</code> 的调用前使用 <code>rand()</code> ，则 <code>rand()</code> 表现为如同它被以 <code>srand(1)</code> 播种。</p>
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<p>每次以同一 <code>seed</code> 播种 <code>rand()</code> 时，它必须产生相同的值数列。</p>
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<p><code>srand()</code> 不保证为线程安全。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>seed 种子值</td>
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<td>-</td>
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<td></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>（无）</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>通常来说，应该只播种一次随机数生成器，在程序开始出，任何到 <code>rand()</code> 的调用前。不应重复播种，或每次冀愿生成新一批随机数时重播种。</p>
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<p>标准实践是使用以 <span class="t-c"><span class="mw-geshi c source-c"><a href="c-chrono-time.html"><span class="kw513">time</span></a><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span></span></span> 为种子调用的结果。然而 <code>time()</code> 返回 <span class="t-c"><span class="mw-geshi c source-c"><a href="c-chrono-time_t.html"><span class="kw524">time_t</span></a></span></span> 值，而不保证 <code>time_t</code> 是整数类型。尽管实践中，主流实现都定义 <code>time_t</code> 为整数类型，且此亦为 POSIX 所要求。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="c source-c">
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<pre class="de1"><span class="co2">#include <stdio.h></span>
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<span class="co2">#include <stdlib.h></span>
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<span class="co2">#include <time.h></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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srand<span class="br0">(</span><a href="c-chrono-time.html"><span class="kw513">time</span></a><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 以当前时间为随机数生成器的种子</span>
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<span class="kw4">int</span> random_variable <span class="sy1">=</span> <a href="c-numeric-random-rand.html"><span class="kw740">rand</span></a><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"Random value on [0,%d]: %d<span class="es1">\n</span>"</span>, <a href="c-numeric-random-RAND_MAX.html"><span class="kw741">RAND_MAX</span></a>, random_variable<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">Random value on [0 2147483647]: 1373858591</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
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<div class="t-ref-std-11">
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<ul>
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<li>C11 标准（ISO/IEC 9899:2011）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.22.2.2 The srand function (p: 346-347)</li>
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</ul>
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</dd>
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</dl>
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<div class="t-ref-std-c99">
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<ul>
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<li>C99 标准（ISO/IEC 9899:1999）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.20.2.2 The srand function (p: 312-313)</li>
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</ul>
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</dd>
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</dl>
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<div class="t-ref-std-c89">
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<ul>
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<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>4.10.2.2 The srand function</li>
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</ul>
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</dd>
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</dl>
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</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="c-numeric-random-rand.html"><span class="t-lines"><span>rand</span></span></a></div>
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</div>
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</td>
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<td>产生一个伪随机数<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="c-numeric-random-RAND_MAX.html"><span class="t-lines"><span>RAND_MAX</span></span></a></div>
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</div>
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</td>
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<td><span class="t-c"><span class="mw-geshi c source-c"><a href="c-numeric-random-rand.html"><span class="kw740">rand</span></a><span class="br0">(</span><span class="br0">)</span></span></span> 生成的最大可能值<br />
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<span class="t-mark">(宏常量)</span></td>
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</tr>
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</table>
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</div>
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</div>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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