161 lines
12 KiB
HTML
161 lines
12 KiB
HTML
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<title>std::experimental::sample</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_experimental_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::experimental::</span>sample</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-experimental.html">experimental</a></span></div>
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<div>定义于头文件 <code><a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/header/experimental/algorithm&action=edit&redlink=1" class="new" title="cpp/header/experimental/algorithm（页面不存在）"><experimental/algorithm></a></code></div>
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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>(1)</td>
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<td><span class="t-mark-rev t-since-libfund-ts t-mark-ts">(库基础 TS)</span></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, <span class="kw1">class</span> Distance <span class="sy1">></span><br /></span>
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<p>SampleIterator sample<span class="br0">(</span> PopulationIterator first, PopulationIterator last,<br /></p>
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SampleIterator out, Distance n<span class="br0">)</span><span class="sy4">;</span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span></td>
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<td></td>
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<td></td>
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<td></td>
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<p>在序列 <span class="texhtml" style="white-space: nowrap;">[first; last)</span> 中选择 <code>n</code> 个元素，使得每个元素拥有相等的出现概率，并写入被选择元素到输出迭代器 <code>out</code> 。</p>
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<p>若<code>n</code> 大于序列中的元素数，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>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> 的要求，算法才稳定。</p>
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<div class="t-li1"><span class="t-li">1)</span> 用随机数生成器 <code>g</code> 生成随机数。</div>
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<div class="t-li1"><span class="t-li">2)</span> 用<a href="cpp-experimental-lib_extensions_2.html#.E7.AE.80.E5.8D.95.E7.9A.84.E9.9A.8F.E6.9C.BA.E6.95.B0.E7.94.9F.E6.88.90.E8.AE.BE.E6.96.BD">逐线程引擎</a>生成随机数。</div>
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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>指示从中抽样的范围（总体）的一对迭代器</td>
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</tr>
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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>要写入样本到其中的输出迭代器。必须不在 <span class="texhtml" style="white-space: nowrap;">[first;last)</span> 范围中</td>
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<td>n</td>
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<td>-</td>
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<td>样本数</td>
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<tr class="t-par">
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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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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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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> 必须亦满足<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> 必须为整数类型</td>
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<td colspan="3">-
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<code>URBG</code> 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> 的要求，而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>返回输出的最后样本后的 <code>out</code> 的副本，即样本范围的末尾。</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.E6.84.8F">注意</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=".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 <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 <experimental/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">"abcdefgh"</span>, out<span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">experimental</span><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="kw1758">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 abcdefgh : cdefg</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-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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</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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