277 lines
22 KiB
HTML
277 lines
22 KiB
HTML
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<title>std::accumulate</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_accumulate 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>accumulate</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>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="kw1">class</span> T <span class="sy1">></span><br />
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T accumulate<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-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="kw1">class</span> T <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> T accumulate<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><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<td rowspan="142">(2)</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> BinaryOperation <span class="sy1">></span><br /></span>
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<p>T accumulate<span class="br0">(</span> InputIt first, InputIt last, T init,<br /></p>
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BinaryOperation op <span class="br0">)</span><span class="sy4">;</span></div>
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<td><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="kw1">class</span> T, <span class="kw1">class</span> BinaryOperation <span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> T accumulate<span class="br0">(</span> InputIt first, InputIt last, T init,<br /></p>
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BinaryOperation op <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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<p>计算给定值 <code>init</code> 与给定范围 <code>[first, last)</code> 中元素的和。第一版本用 <code>operator+</code> ，第二版本用二元函数 <code>op</code> 求和元素<span class="t-rev-inl t-since-cxx20"><span>，均将 <a href="cpp-utility-move.html"><tt>std::move</tt></a> 应用到其左侧运算数</span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>。</p>
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<p><code>op</code> 必须无副作用。</p>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
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<p><code>op</code> 必须不非法化涉及范围的任何迭代器，含尾迭代器，且不修改其所涉及范围的任何元素及 <code>*last</code> 。</p>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td>first, last</td>
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<td>-</td>
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<td>要求和的元素范围</td>
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<td>init</td>
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<td>-</td>
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<td>和的初值</td>
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<td>op</td>
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<td>-</td>
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<td>被使用的二元函数对象。接收当前积累值 <code>a</code> （初始化为 <code>init</code> ）和当前元素 <code>b</code> 的二元运算符。<br />
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<p>该函数的签名应当等价于：</p>
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<p><span class="t-cc"><span class="mw-geshi cpp source-cpp"> Ret fun<span class="br0">(</span><span class="kw4">const</span> Type1 <span class="sy3">&</span>a, <span class="kw4">const</span> Type2 <span class="sy3">&</span>b<span class="br0">)</span><span class="sy4">;</span></span></span></p>
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<p>签名中并不需要有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="sy3">&</span></span></span>。<br />
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类型 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type1</span></span> 必须使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">T</span></span> 类型的对象能隐式转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type1</span></span> 。类型 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type2</span></span> 必须使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">InputIt</span></span> 类型的对象能在解引用后隐式转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type2</span></span> 。 类型 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ret</span></span> 必须使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">T</span></span> 类型对象能被赋 <span class="t-c"><span class="mw-geshi cpp source-cpp">Ret</span></span> 类型值。 ​</p>
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<td colspan="3">类型要求</td>
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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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<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-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a> 和 <a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 的要求。</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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<div class="t-li1"><span class="t-li">1)</span> 给定值与给定范围中的元素的和。</div>
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<div class="t-li1"><span class="t-li">2)</span> 给定范围在 <code>op</code> 上<a href="https://en.wikipedia.org/wiki/Fold_(higher-order_function)" class="extiw" title="enwiki:Fold (higher-order function)">左折叠</a>的结果</div>
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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><code>std::accumulate</code> 进行左折叠。为进行右折叠，必须逆转二元运算符的参数顺序，并使用逆序迭代器。</p>
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<h4><span class="mw-headline" id=".E6.9C.80.E5.B8.B8.E8.A7.81.E7.9A.84.E9.94.99.E8.AF.AF">最常见的错误</span></h4>
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<p>在 <code>init</code> 的类型上进行 <code>op</code> ，这能引入不期望的元素转换，例如 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">accumulate</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="nu0">0</span><span class="br0">)</span></span></span> 在 <code>v</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><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></span></span> 时给出错误的结果。</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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<table class="eq-fun-cpp-table">
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<th>版本一</th>
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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="kw4">constexpr</span> <span class="co1">// C++20 起</span>
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T accumulate<span class="br0">(</span>InputIt first, InputIt last, T init<span class="br0">)</span>
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<span class="br0">{</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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init <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>init<span class="br0">)</span> <span class="sy2">+</span> <span class="sy2">*</span>first<span class="sy4">;</span> <span class="co1">// C++20 起有 std::move</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> init<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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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> T, <span class="kw1">class</span> BinaryOperation<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="co1">// C++20 起</span>
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T accumulate<span class="br0">(</span>InputIt first, InputIt last, T init,
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BinaryOperation op<span class="br0">)</span>
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<span class="br0">{</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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init <span class="sy1">=</span> op<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>init<span class="br0">)</span>, <span class="sy2">*</span>first<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C++20 起有 std::move</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> init<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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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 <vector></span>
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<span class="co2">#include <numeric></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <functional></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">5</span>, <span class="nu0">6</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="kw4">int</span> sum <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">accumulate</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="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> product <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">accumulate</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="nu0">1</span>, <a href="cpp-utility-functional-multiplies.html"><span class="kw1031">std::<span class="me2">multiplies</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></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> dash_fold <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> a, <span class="kw4">int</span> b<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>a<span class="br0">)</span> <span class="sy2">+</span> <span class="st0">'-'</span> <span class="sy2">+</span> <a href="cpp-string-basic_string-to_string.html"><span class="kw1265">std::<span class="me2">to_string</span></span></a><span class="br0">(</span>b<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">accumulate</span><span class="br0">(</span><a href="cpp-iterator-next.html"><span class="kw674">std::<span class="me2">next</span></span></a><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>,
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<a href="cpp-string-basic_string-to_string.html"><span class="kw1265">std::<span class="me2">to_string</span></span></a><span class="br0">(</span>v<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="br0">)</span>, <span class="co1">// 用首元素开始</span>
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dash_fold<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 使用逆向迭代器右折叠</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> rs <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">accumulate</span><span class="br0">(</span><a href="cpp-iterator-next.html"><span class="kw674">std::<span class="me2">next</span></span></a><span class="br0">(</span>v.<span class="me1">rbegin</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>, v.<span class="me1">rend</span><span class="br0">(</span><span class="br0">)</span>,
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<a href="cpp-string-basic_string-to_string.html"><span class="kw1265">std::<span class="me2">to_string</span></span></a><span class="br0">(</span>v.<span class="me1">back</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>, <span class="co1">// 用首元素开始</span>
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dash_fold<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">"sum: "</span> <span class="sy1"><<</span> sum <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"product: "</span> <span class="sy1"><<</span> product <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"dash-separated string: "</span> <span class="sy1"><<</span> s <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"dash-separated string (right-folded): "</span> <span class="sy1"><<</span> rs <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">sum: 55
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product: 3628800
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dash-separated string: 1-2-3-4-5-6-7-8-9-10
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dash-separated string (right-folded): 10-9-8-7-6-5-4-3-2-1</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-adjacent_difference.html"><span class="t-lines"><span>adjacent_difference</span></span></a></div>
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</div>
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</td>
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<td>计算范围内各相邻元素之间的差<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-inner_product.html"><span class="t-lines"><span>inner_product</span></span></a></div>
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</div>
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</td>
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<td>计算两个范围的元素的内积<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-partial_sum.html"><span class="t-lines"><span>partial_sum</span></span></a></div>
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</div>
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</td>
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<td>计算范围内元素的部分和<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-reduce.html"><span class="t-lines"><span>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>类似 <span class="t-lc"><strong class="selflink">std::accumulate</strong></span>，但不依序执行<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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