270 lines
23 KiB
HTML
270 lines
23 KiB
HTML
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<title>std::count, std::count_if</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_count 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>count, <span style="font-size:0.7em; line-height:130%">std::</span>count_if</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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<table class="t-dcl-begin">
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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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<td rowspan="142">(1)</td>
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<td></td>
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<tr class="t-dcl t-until-cxx20">
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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> InputIt, <span class="kw1">class</span> T <span class="sy1">></span><br /></span>
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<p><span class="kw1">typename</span> iterator_traits<span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span><br /></p>
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count<span class="br0">(</span> InputIt first, InputIt last, <span class="kw4">const</span> T <span class="sy3">&</span>value <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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<tr class="t-dcl t-since-cxx20">
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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> InputIt, <span class="kw1">class</span> T <span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> <span class="kw1">typename</span> iterator_traits<span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span><br /></p>
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count<span class="br0">(</span> InputIt first, InputIt last, <span class="kw4">const</span> T <span class="sy3">&</span>value <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</tbody>
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<tbody>
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<tr class="t-dcl t-since-cxx17">
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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> ExecutionPolicy, <span class="kw1">class</span> ForwardIt, <span class="kw1">class</span> T <span class="sy1">></span><br /></span>
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<p><span class="kw1">typename</span> iterator_traits<span class="sy1"><</span>ForwardIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span><br /></p>
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count<span class="br0">(</span> ExecutionPolicy<span class="sy3">&&</span> policy, ForwardIt first, ForwardIt last, <span class="kw4">const</span> T <span class="sy3">&</span>value <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<td rowspan="142">(3)</td>
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<td></td>
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<tr class="t-dcl t-until-cxx20">
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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> InputIt, <span class="kw1">class</span> UnaryPredicate <span class="sy1">></span><br /></span>
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<p><span class="kw1">typename</span> iterator_traits<span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span><br /></p>
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count_if<span class="br0">(</span> InputIt first, InputIt last, UnaryPredicate p <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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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> InputIt, <span class="kw1">class</span> UnaryPredicate <span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> <span class="kw1">typename</span> iterator_traits<span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span><br /></p>
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count_if<span class="br0">(</span> InputIt first, InputIt last, UnaryPredicate p <span class="br0">)</span><span class="sy4">;</span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl t-since-cxx17">
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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> ExecutionPolicy, <span class="kw1">class</span> ForwardIt, <span class="kw1">class</span> UnaryPredicate <span class="sy1">></span><br /></span>
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<p><span class="kw1">typename</span> iterator_traits<span class="sy1"><</span>ForwardIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span><br /></p>
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count_if<span class="br0">(</span> ExecutionPolicy<span class="sy3">&&</span> policy, ForwardIt first, ForwardIt last, UnaryPredicate p <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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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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<p>返回范围 <code>[first, last)</code> 中满足特定判别标准的元素数。</p>
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<div class="t-li1"><span class="t-li">1)</span> 计数等于 <code>value</code> 的元素。</div>
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<div class="t-li1"><span class="t-li">3)</span> 计数谓词 <code>p</code> 对其返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 的元素。</div>
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<div class="t-li1"><span class="t-li">2,4)</span> 同 <span class="t-v">(1,3)</span> ，但按照 <code>policy</code> 执行。这些重载仅若 <span class="t-rev-inl t-until-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-is_execution_policy.html"><span class="kw1564">std::<span class="me2">is_execution_policy_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ExecutionPolicy<span class="sy1">>></span></span></span></span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span><span class="t-rev-inl t-since-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-is_execution_policy.html"><span class="kw1564">std::<span class="me2">is_execution_policy_v</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>ExecutionPolicy<span class="sy1">>></span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span> 为 true 才参与重载决议。</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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<tr class="t-par">
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<td>first, last</td>
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<td>-</td>
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<td>要检验的元素范围</td>
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</tr>
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<tr class="t-par">
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<td>value</td>
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<td>-</td>
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<td>要搜索的值</td>
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</tr>
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<tr class="t-par">
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<td>policy</td>
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<td>-</td>
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<td>所用的执行策略。细节见<a href="cpp-algorithm-execution_policy_tag_t.html">执行策略</a>。</td>
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</tr>
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<tr class="t-par">
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<td>p</td>
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<td>-</td>
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<td>对要求的元素则返回 ​<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 的一元谓词。<br />
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<p>对每个（可为 const 的） <code>VT</code> 类型参数 <code>v</code> ，其中 <code>VT</code> 是 <code>InputIt</code> 的值类型，表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">p<span class="br0">(</span>v<span class="br0">)</span></span></span> 必须可转换为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> ，无关乎<a href="cpp-language-value_category.html">值类别</a>，而且必须不修改 <code>v</code> 。从而不允许 <span class="t-c"><span class="mw-geshi cpp source-cpp">VT<span class="sy3">&</span></span></span> 类型参数<span class="t-rev-inl t-since-cxx11"><span>，亦不允许 <span class="t-c"><span class="mw-geshi cpp source-cpp">VT</span></span> ，除非对 <code>VT</code> 而言移动等价于复制</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>。 ​</p>
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</td>
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</tr>
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<tr class="t-par-hitem">
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<td colspan="3">类型要求</td>
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</tr>
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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>InputIt</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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</tr>
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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>ForwardIt</code> 必须满足<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。</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>满足条件的元素数。</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>准确 <code>last</code> - <code>first</code> 次比较/应用谓词。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>拥有名为 <code>ExecutionPolicy</code> 的模板形参的重载按下列方式报告错误：</p>
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<ul>
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<li>若作为算法一部分调用的函数的执行抛出异常，且 <code>ExecutionPolicy</code> 为<a href="cpp-algorithm-execution_policy_tag_t.html">标准策略</a>之一，则调用 <span class="t-lc"><a href="cpp-error-terminate.html">std::terminate</a></span> 。对于任何其他 <code>ExecutionPolicy</code> ，行为是实现定义的。</li>
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<li>若算法无法分配内存，则抛出 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。</li>
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</ul>
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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>对于无任何额外判别标准的 <code>[first, last)</code> 中的元素数，见 <span class="t-lc"><a href="cpp-iterator-distance.html">std::distance</a></span> 。</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 <span class="t-c"><span class="mw-geshi cpp source-cpp">count</span></span> 在 <a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/d9375e490072d1aae73a93949aa158fcd2a27018/libstdc%2B%2B-v3/include/bits/stl_algo.h#L4056">libstdc++</a> 与 <a rel="nofollow" class="external text" href="https://github.com/llvm-mirror/libcxx/blob/a12cb9d211019d99b5875b6d8034617cbc24c2cc/include/algorithm#L1171">libc++</a> 中的实现。</p>
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<p>参阅 <span class="t-c"><span class="mw-geshi cpp source-cpp">count_if</span></span> 在 <a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/d9375e490072d1aae73a93949aa158fcd2a27018/libstdc%2B%2B-v3/include/bits/stl_algo.h#L4079">libstdc++</a> 与 <a rel="nofollow" class="external text" href="https://github.com/llvm-mirror/libcxx/blob/a12cb9d211019d99b5875b6d8034617cbc24c2cc/include/algorithm#L1186">libc++</a> 中的实现。</p>
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<table class="eq-fun-cpp-table">
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<tr>
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<th>版本一</th>
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</tr>
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<tr>
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<td>
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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="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> InputIt, <span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw1">typename</span> iterator_traits<span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span>
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count<span class="br0">(</span>InputIt first, InputIt last, <span class="kw4">const</span> T<span class="sy3">&</span> value<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">typename</span> iterator_traits<span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span> ret <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="sy4">;</span> first <span class="sy3">!</span><span class="sy1">=</span> last<span class="sy4">;</span> <span class="sy2">++</span>first<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span><span class="sy2">*</span>first <span class="sy1">==</span> value<span class="br0">)</span> <span class="br0">{</span>
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ret<span class="sy2">++</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> ret<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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</td>
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</tr>
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<tr>
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<th>版本二</th>
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</tr>
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<tr>
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<td>
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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="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> InputIt, <span class="kw1">class</span> UnaryPredicate<span class="sy1">></span>
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<span class="kw1">typename</span> iterator_traits<span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span>
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count_if<span class="br0">(</span>InputIt first, InputIt last, UnaryPredicate p<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">typename</span> iterator_traits<span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span> ret <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="sy4">;</span> first <span class="sy3">!</span><span class="sy1">=</span> last<span class="sy4">;</span> <span class="sy2">++</span>first<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span>p<span class="br0">(</span><span class="sy2">*</span>first<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
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ret<span class="sy2">++</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> ret<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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</td>
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</table>
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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 <algorithm></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <vector></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-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="br0">{</span> <span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="nu0">4</span>, <span class="nu0">4</span>, <span class="nu0">3</span>, <span class="nu0">7</span>, <span class="nu0">8</span>, <span class="nu0">9</span>, <span class="nu0">10</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// 确定 std::vector 中有多少个整数匹配目标值。</span>
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<span class="kw4">int</span> target1 <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span>
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<span class="kw4">int</span> target2 <span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span>
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<span class="kw4">int</span> num_items1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">count</span><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, target1<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> num_items2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">count</span><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, target2<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"number: "</span> <span class="sy1"><<</span> target1 <span class="sy1"><<</span> <span class="st0">" count: "</span> <span class="sy1"><<</span> num_items1 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"number: "</span> <span class="sy1"><<</span> target2 <span class="sy1"><<</span> <span class="st0">" count: "</span> <span class="sy1"><<</span> num_items2 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="co1">// 用 lambda 表达式计量能被 3 整除的元素数。</span>
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<span class="kw4">int</span> num_items3 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">count_if</span><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span><span class="br0">{</span><span class="kw1">return</span> i <span class="sy2">%</span> <span class="nu0">3</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="sy4">;</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="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"number divisible by three: "</span> <span class="sy1"><<</span> num_items3 <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>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">number: 3 count: 2
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number: 5 count: 0
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number divisible by three: 3</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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<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-iterator-distance.html"><span class="t-lines"><span>distance</span></span></a></div>
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</div>
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</td>
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<td>返回两个迭代器间的距离<br />
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<span class="t-mark">(函数模板)</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="cpp-algorithm-ranges-count.html"><span class="t-lines"><span>ranges::count</span><span>ranges::count_if</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回满足指定判别标准的元素数<br />
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<span class="t-mark">(niebloid)</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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</html>
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