130 lines
9.9 KiB
HTML
130 lines
9.9 KiB
HTML
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<title>std::rand</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_numeric_random_rand skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>rand</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>定义于头文件 <code><a href="cpp-header-cstdlib.html"><cstdlib></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> rand<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> 到 <span class="t-lc"><a href="cpp-numeric-random-RAND_MAX.html">RAND_MAX</a></span> （包含 0 与 <code>RAND_MAX</code> ）的随机数。</p>
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<p><span class="t-lc"><a href="cpp-numeric-random-srand.html">std::srand()</a></span> 播种 <code>rand()</code> 所用的伪随机数生成器。若在任何到 <code>srand()</code> 的调用前使用 <code>rand()</code> ，则 <code>rand()</code> 表现如同它以 <code>srand(1)</code> 播种。</p>
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<p>每次以 <code>srand()</code> 播种 <code>rand()</code> ，它必须在后续调用上产生相同的值数列。</p>
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<p>标准库中的其他函数可调用 <code>rand</code> 。哪个函数这么做是实现定义的。</p>
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<p><code>rand()</code> 是否线程安全是实现定义的。</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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<p>（无）</p>
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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><span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> 与 <span class="t-lc"><a href="cpp-numeric-random-RAND_MAX.html">RAND_MAX</a></span> 间的随机整数值。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>不保证生成的随机数列质量。过去，一些 <code>rand()</code> 在随机性、分布和产生的序列周期上有严重缺陷（在一个众所周知的例子中，调用之间最低位简单地在 <code>1</code> 与 <code>0</code> 间切换）。</p>
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<p>对于严肃的随机数生成需求不推荐 <code>rand()</code> 。<span class="t-rev-inl t-since-cxx11"><span>推荐用 C++11 的<a href="cpp-numeric-random.html">随机数生成</a>设施替换 rand() 。</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span></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="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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<a href="cpp-numeric-random-srand.html"><span class="kw1497">std::<span class="me2">srand</span></span></a><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>nullptr<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 以当前时间为随机生成器的种子</span>
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<span class="kw4">int</span> random_variable <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">rand</span><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="co1">// 扔 6 面色子 20 次</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">20</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">7</span><span class="sy4">;</span>
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<span class="kw1">while</span><span class="br0">(</span>x <span class="sy1">></span> <span class="nu0">6</span><span class="br0">)</span>
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x <span class="sy1">=</span> <span class="nu0">1</span> <span class="sy2">+</span> std<span class="sy4">::</span><span class="me2">rand</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">/</span> <span class="br0">(</span><span class="br0">(</span><a href="cpp-numeric-random-RAND_MAX.html"><span class="kw1498">RAND_MAX</span></a> <span class="sy2">+</span> 1u<span class="br0">)</span> <span class="sy2">/</span> <span class="nu0">6</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 注意： 1 + rand() % 6 有偏差！</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> x <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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<span class="br0">}</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]: 726295113
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6 3 6 2 6 5 6 3 1 1 1 6 6 6 4 1 3 6 4 2</pre></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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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-random-uniform_int_distribution.html"><span class="t-lines"><span>uniform_int_distribution</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>产生在一个范围上均匀分布的整数值<br />
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<span class="t-mark">(类模板)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-random-srand.html"><span class="t-lines"><span>srand</span></span></a></div>
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</div>
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</td>
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<td>初始化伪随机数生成器<br />
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<span class="t-mark">(函数)</span></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><span class="t-lc"><strong class="selflink">std::rand</strong></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-randint.html"><span class="t-lines"><span>randint</span></span></a></div>
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</div>
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</td>
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<td>生成指定范围中的随机整数<br />
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<span class="t-mark">(函数模板)</span></td>
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</div>
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<div class="visualClear"></div>
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