156 lines
12 KiB
HTML
156 lines
12 KiB
HTML
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<title>std::sample</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_sample 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>sample</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-algorithm.html">algorithm</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-algorithm.html"><algorithm></a></code></div>
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<td></td>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> PopulationIterator, <span class="kw1">class</span> SampleIterator,<br /></span>
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<p> <span class="kw1">class</span> Distance, <span class="kw1">class</span> URBG <span class="sy1">></span><br />
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SampleIterator sample<span class="br0">(</span> PopulationIterator first, PopulationIterator last,<br />
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SampleIterator out, Distance n,<br /></p>
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URBG<span class="sy3">&&</span> g<span class="br0">)</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<p>从序列 <span class="texhtml" style="white-space: nowrap;">[first; last)</span> 选择 <code>n</code> 个元素（无替换），使得每个样本拥有相等的出现概率，并写入这些被选择元素到输出迭代器 <code>out</code> 。用随机数生成器 <code>g</code> 生成随机数。</p>
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<p>若 <code>n</code> 大于序列中的元素数量，则选择 <span class="t-c"><span class="mw-geshi cpp source-cpp">last<span class="sy2">-</span>first</span></span> 个元素。</p>
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<p>算法仅若 <code>PopulationIterator</code> 满足<a href="cpp-named_req-ForwardIterator.html"><span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span></a> 要求才稳定（保持被选择元素 的顺序）。</p>
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<p>若 <code>out</code> 在 <span class="texhtml" style="white-space: nowrap;">[first; last)</span> 中则行为未定义。</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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<td>first, last</td>
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<td>-</td>
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<td>一对组成自其采样的范围（总体）的迭代器</td>
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<tr class="t-par">
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<td>out</td>
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<td>-</td>
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<td>写入样本的输出迭代器</td>
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<tr class="t-par">
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<td>n</td>
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<td>-</td>
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<td>要抽取的样本数</td>
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<td>g</td>
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<td>-</td>
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<td>用作随机源的随机数生成器</td>
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<td colspan="3">类型要求</td>
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<td colspan="3">-
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<code>PopulationIterator</code> 必须满足<a href="cpp-named_req-InputIterator.html"><span style="font-style:italic">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</span></a> 的要求。</td>
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<td colspan="3">-
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<div class="spacer"></div>
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<code>SampleIterator</code> 必须满足<a href="cpp-named_req-OutputIterator.html"><span style="font-style:italic">老式输出迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyOutputIterator)</span></a> 的要求。</td>
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<td colspan="3">-
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若 <code>PopulationIterator</code> 不满足<a href="cpp-named_req-ForwardIterator.html"><span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span></a> ，则 <code>SampleIterator</code> 还必须满足<a href="cpp-named_req-RandomAccessIterator.html"><span style="font-style:italic">老式随机访问迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyRandomAccessIterator)</span></a> 的要求</td>
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<td colspan="3">-
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<code>PopulationIterator</code>的 value_type 必须可写入到 <code>out</code></td>
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<td colspan="3">-
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<code>Distance</code> 必须是整数类型</td>
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<td colspan="3">-
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<span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>URBG<span class="sy1">></span></span></span> 必须满足<a href="cpp-named_req-UniformRandomBitGenerator.html"><span style="font-style:italic">均匀随机位生成器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(UniformRandomBitGenerator)</span></a> ，且其返回类型必须可转换为 <code>Distance</code></td>
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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>在输出最后样本后返回 <code>out</code> 的副本，即采样范围的结尾。</p>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<p>与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-distance.html"><span class="kw673">std::<span class="me2">distance</span></span></a><span class="br0">(</span>first,last<span class="br0">)</span></span></span> 成线性</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>此函数可以实现选择抽样或蓄水池抽样。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<p>参阅 <a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/14d8a5ae472ca5743016f37da2dd4770d83dea21/libstdc%2B%2B-v3/include/bits/stl_algo.h#L5743-L5869">libstdc++</a>、 <a rel="nofollow" class="external text" href="https://github.com/llvm/llvm-project/blob/f221d905b131158cbe3cbc4320d1ecd1376c3f22/libcxx/include/__algorithm/sample.h">libc++</a> 与 <a rel="nofollow" class="external text" href="https://github.com/microsoft/STL/blob/472161105d596192194d4715ccad307c6c163b4a/stl/inc/algorithm#L4518-L4600">MSVC STL</a> 中的实现。</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 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 <iostream></span>
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<span class="co2">#include <random></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <iterator></span>
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<span class="co2">#include <algorithm></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-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> in <span class="sy1">=</span> <span class="st0">"hgfedcba"</span>, out<span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">sample</span><span class="br0">(</span>in.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, in.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, <a href="cpp-iterator-back_inserter.html"><span class="kw665">std::<span class="me2">back_inserter</span></span></a><span class="br0">(</span>out<span class="br0">)</span>,
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<span class="nu0">5</span>, <a href="cpp-numeric-random-mersenne_twister_engine.html"><span class="kw1489">std::<span class="me2">mt19937</span></span></a><span class="br0">{</span><a href="cpp-numeric-random-random_device.html"><span class="kw1486">std::<span class="me2">random_device</span></span></a><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span><span class="br0">)</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">"five random letters out of "</span> <span class="sy1"><<</span> in <span class="sy1"><<</span> <span class="st0">" : "</span> <span class="sy1"><<</span> out <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">five random letters out of hgfedcba: gfcba</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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<table class="t-dsc-begin">
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<div class="t-dsc-member-div">
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<div><a href="cpp-algorithm-random_shuffle.html"><span class="t-lines"><span>random_shuffle</span><span>shuffle</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span><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>随机重排范围中的元素<br />
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<span class="t-mark">(函数模板)</span></td>
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<div class="visualClear"></div>
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