116 lines
7.3 KiB
HTML
116 lines
7.3 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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<title>std::srand</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_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"><span style="font-size:0.7em; line-height:130%">std::</span>srand</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-numeric.html">numeric</a>‎ | <a href="cpp-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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<div>定义于头文件 <code><a href="cpp-header-cstdlib.html"><cstdlib></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><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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<p>以值 <code>seed</code> 播种 <span class="t-lc"><a href="cpp-numeric-random-rand.html">std::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</td>
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<td>-</td>
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<td>种子值</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 cpp source-cpp">time<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 cpp source-cpp">time_t</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <cstdlib></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <ctime></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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std<span class="sy4">::</span><span class="me2">srand</span><span class="br0">(</span><a href="cpp-chrono-c-time.html"><span class="kw1011">std::<span class="me2">time</span></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="cpp-numeric-random-rand.html"><span class="kw1497">std::<span class="me2">rand</span></span></a><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Random value on [0 "</span> <span class="sy1"><<</span> <a href="cpp-numeric-random-RAND_MAX.html"><span class="kw1498">RAND_MAX</span></a> <span class="sy1"><<</span> <span class="st0">"]: "</span>
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<span class="sy1"><<</span> random_variable <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</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=".E5.8F.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="cpp-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>生成伪随机数<br />
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<span class="t-mark">(函数)</span></td>
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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="cpp-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-lc"><a href="cpp-numeric-random-rand.html">std::rand</a></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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<div class="t-dsc-member-div">
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<div><a href="cpp-experimental-reseed.html"><span class="t-lines"><span>reseed</span></span></a></div>
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</div>
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</td>
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<td>重播种单个线程的随机引擎<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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</table>
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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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