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302 lines
25 KiB
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<title>std::reduce</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_reduce 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>reduce</h1>
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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-numeric.html"><numeric></a></code></div>
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<td rowspan="142">(1)</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> InputIt<span class="sy1">></span><br /></span>
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<p><span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span> reduce<span class="br0">(</span><br /></p>
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InputIt first, InputIt last<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-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</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> InputIt<span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> <span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span> reduce<span class="br0">(</span><br /></p>
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InputIt first, InputIt last<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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<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="sy1">></span><br /></span>
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<p><span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>ForwardIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span> reduce<span class="br0">(</span><br />
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ExecutionPolicy<span class="sy3">&&</span> policy,<br /></p>
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ForwardIt first, ForwardIt last<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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<td rowspan="142">(3)</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> InputIt, <span class="kw1">class</span> T<span class="sy1">></span><br />
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T reduce<span class="br0">(</span>InputIt first, InputIt last, T init<span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<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 />
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<span class="kw4">constexpr</span> T reduce<span class="br0">(</span>InputIt first, InputIt last, T init<span class="br0">)</span><span class="sy4">;</span></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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<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>T reduce<span class="br0">(</span>ExecutionPolicy<span class="sy3">&&</span> policy,<br /></p>
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ForwardIt first, ForwardIt last, T init<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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<td rowspan="142">(5)</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> InputIt, <span class="kw1">class</span> T, <span class="kw1">class</span> BinaryOp<span class="sy1">></span><br />
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T reduce<span class="br0">(</span>InputIt first, InputIt last, T init, BinaryOp binary_op<span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx17">(C++17 前)</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> InputIt, <span class="kw1">class</span> T, <span class="kw1">class</span> BinaryOp<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> T reduce<span class="br0">(</span>InputIt first, InputIt last, T init, BinaryOp binary_op<span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</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> ExecutionPolicy, <span class="kw1">class</span> ForwardIt, <span class="kw1">class</span> T, <span class="kw1">class</span> BinaryOp<span class="sy1">></span><br /></span>
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<p>T reduce<span class="br0">(</span>ExecutionPolicy<span class="sy3">&&</span> policy,<br /></p>
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ForwardIt first, ForwardIt last, T init, BinaryOp binary_op<span class="br0">)</span><span class="sy4">;</span></div>
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<td>(6)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 同 <span class="t-c"><span class="mw-geshi cpp source-cpp">reduce<span class="br0">(</span>first, last, <span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span></span></span></div>
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<div class="t-li1"><span class="t-li">3)</span> 同 <span class="t-c"><span class="mw-geshi cpp source-cpp">reduce<span class="br0">(</span>first, last, init, <a href="cpp-utility-functional-plus.html"><span class="kw1029">std::<span class="me2">plus</span></span></a><span class="sy1"><></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span></div>
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<div class="t-li1"><span class="t-li">5)</span> 在 <code>binary_op</code> 上以初值 <code>init</code> 规约范围 <span class="texhtml" style="white-space: nowrap;">[first; last)</span> ，可能以未指定方式排序聚合。</div>
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<div class="t-li1"><span class="t-li">2,4,6)</span> 同 <span class="t-v">(1,3,5)</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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<p>若 <code>binary_op</code> 非结合或非交换，则行为非确定。</p>
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<p>若 <code>binary_op</code> 修改 <span class="texhtml" style="white-space: nowrap;">[first; last]</span> 中任何元素或非法化范围中任何迭代器，含尾迭代器，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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<tr class="t-par">
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<td>first, last</td>
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<td>-</td>
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<td>要应用算法的元素范围</td>
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</tr>
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<tr class="t-par">
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<td>init</td>
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<td>-</td>
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<td>广义和的初值</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>使用的执行策略。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>binary_op</td>
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<td>-</td>
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<td>将以未指定顺序应用于解引用输入迭代器结果、其他 <code>binary_op</code> 结果及 <code>init</code> 上的二元<a href="cpp-named_req-FunctionObject.html"><span style="font-style:italic">函数对象</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(FunctionObject)</span></a> 。</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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<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>T</code> 必须满足<a href="cpp-named_req-MoveConstructible.html"><span style="font-style:italic">可移动构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveConstructible)</span></a> 的要求。而且 <code>binary_op(init, *first)</code> 、 <code>binary_op(*first, init)</code> 、 <code>binary_op(init, init)</code> 及 <code>binary_op(*first, *first)</code> 必须可转换到 <code>T</code> 。</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><code>init</code> 及 <code>*first</code> 、 <code>*(first+1)</code> 、…… <code>*(last-1)</code> 在 <code>binary_op</code> 上的广义和，</p>
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<p>其中广义和 <span class="texhtml" style="white-space: nowrap;">GSUM(op, a<span class="t-su t-su-b"><br />
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1</span>, ..., a<span class="t-su t-su-b"><br />
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N</span>)</span> 定义如下：</p>
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<ul>
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<li>若 <span class="texhtml" style="white-space: nowrap;">N=1</span> ，则为 <span class="texhtml" style="white-space: nowrap;">a<span class="t-su t-su-b"><br />
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1</span></span></li>
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<li>若 <span class="texhtml" style="white-space: nowrap;">N > 1</span> ，则为 <span class="texhtml" style="white-space: nowrap;">op(GSUM(op, b<span class="t-su t-su-b"><br />
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1</span>, ..., b<span class="t-su t-su-b"><br />
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K</span>), GSUM(op, b<span class="t-su t-su-b"><br />
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M</span>, ..., b<span class="t-su t-su-b"><br />
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N</span>))</span> ，其中</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li><span class="texhtml" style="white-space: nowrap;">b<span class="t-su t-su-b"><br />
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1</span>, ..., b<span class="t-su t-su-b"><br />
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N</span></span> 可以是任何 <span class="texhtml" style="white-space: nowrap;">a1, ..., aN</span> 的排列，且</li>
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<li><span class="texhtml" style="white-space: nowrap;">1 < K+1 = M ≤ N</span></li>
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</ul>
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</dd>
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</dl>
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<p>换言之， <code>reduce</code> 表现类似 <span class="t-lc"><a href="cpp-algorithm-accumulate.html">std::accumulate</a></span> ，除了范围中的元素可能以任意顺序分组并重排。</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><span class="texhtml" style="white-space: nowrap;">O(last - first)</span> 次应用 <code>binary_op</code>.</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>若范围为空，则返回不修改的 <code>init</code> 。</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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<p>reduce 与 <span class="t-lc"><a href="cpp-algorithm-accumulate.html">std::accumulate</a></span> 间并行的比较：</p>
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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 <chrono></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <numeric></span>
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<span class="co2">#include <execution></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">double</span><span class="sy1">></span> v<span class="br0">(</span><span class="nu0">10</span><span class="st0">'000'</span><span class="nu8">007</span>, <span class="nu16">0.5</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">{</span>
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<span class="kw4">auto</span> t1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">double</span> result <span class="sy1">=</span> <a href="cpp-algorithm-accumulate.html"><span class="kw1651">std::<span class="me2">accumulate</span></span></a><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="nu16">0.0</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> t2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-chrono-duration.html"><span class="kw993">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration</span></span></a><span class="sy1"><</span><span class="kw4">double</span>, <a href="cpp-numeric-ratio-ratio.html"><span class="kw1528">std::<span class="me2">milli</span></span></a><span class="sy1">></span> ms <span class="sy1">=</span> t2 <span class="sy2">-</span> t1<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> <a href="cpp-io-manip-fixed.html"><span class="kw1783">std::<span class="me2">fixed</span></span></a> <span class="sy1"><<</span> <span class="st0">"std::accumulate result "</span> <span class="sy1"><<</span> result
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<span class="sy1"><<</span> <span class="st0">" took "</span> <span class="sy1"><<</span> ms.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" ms<span class="es1">\n</span>"</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="kw4">auto</span> t1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">double</span> result <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">reduce</span><span class="br0">(</span><a href="cpp-algorithm-execution_policy_tag.html"><span class="kw1561">std::<span class="me2">execution</span><span class="sy4">::</span><span class="me2">par</span></span></a>, 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="sy4">;</span>
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<span class="kw4">auto</span> t2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-chrono-duration.html"><span class="kw993">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration</span></span></a><span class="sy1"><</span><span class="kw4">double</span>, <a href="cpp-numeric-ratio-ratio.html"><span class="kw1528">std::<span class="me2">milli</span></span></a><span class="sy1">></span> ms <span class="sy1">=</span> t2 <span class="sy2">-</span> t1<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">"std::reduce result "</span>
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<span class="sy1"><<</span> result <span class="sy1"><<</span> <span class="st0">" took "</span> <span class="sy1"><<</span> ms.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" ms<span class="es1">\n</span>"</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">std::accumulate result 5000003.50000 took 12.7365 ms
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std::reduce result 5000003.50000 took 5.06423 ms</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-algorithm-accumulate.html"><span class="t-lines"><span>accumulate</span></span></a></div>
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</div>
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</td>
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<td>对一个范围内的元素求和<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-transform.html"><span class="t-lines"><span>transform</span></span></a></div>
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</div>
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</td>
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<td>将一个函数应用于某一范围的各个元素，并在目标范围存储结果<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-transform_reduce.html"><span class="t-lines"><span>transform_reduce</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>应用一个函数对象，然后以乱序规约<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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</div>
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</div>
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</body>
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</html>
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