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最終更新日時(UTC):
<span itemprop="datePublished" content="2024-07-26T02:55:35">
2024年07月26日 02時55分35秒
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<span itemprop="author" itemscope itemtype="http://schema.org/Person">
<span itemprop="name">Akira Takahashi</span>
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<div class="identifier-type">function template</div><div class="header"><numeric></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">accumulate</span></h1>
<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="k">namespace</span><span class="w"> </span><span class="nn">std</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">InputIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span>
<span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="n">accumulate</span><span class="p">(</span><span class="n">InputIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">InputIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="n">init</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++03</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">InputIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="n">accumulate</span><span class="p">(</span><span class="n">InputIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">InputIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="n">init</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">InputIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">BinaryOperation</span><span class="o">></span>
<span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="n">accumulate</span><span class="p">(</span><span class="n">InputIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">InputIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="n">init</span><span class="p">,</span>
<span class="w"> </span><span class="n">BinaryOperation</span><span class="w"> </span><span class="n">binary_op</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++03</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">InputIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">BinaryOperation</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="n">accumulate</span><span class="p">(</span><span class="n">InputIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">InputIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="n">init</span><span class="p">,</span>
<span class="w"> </span><span class="n">BinaryOperation</span><span class="w"> </span><span class="n">binary_op</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++20</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p><code>accumulate()</code>は、<a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a><code>[first, last)</code>を集計する関数である。</p>
<p>初期値(<code>init</code>)に対して、<a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a><code>[first, last)</code>の各要素<code>i</code>を前から順番に、任意の二項演算関数<code>binary_op</code>を<code>init = f(init, *i)</code>のように適用していき、範囲の全ての要素を集計した結果を<a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a>として返す。</p>
<p>他の言語でこのような処理は、<code>foldL</code> (Haskell)、<code>reduce</code> (Common Lisp, Ruby)、<code>aggregate</code> (C#)などと呼ばれている。</p>
<ul>
<li>(1) : 二項演算をデフォルトで<code>operator+</code>とする。それによって、この<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a>は、範囲の合計値を求める処理となる</li>
<li>(2) : 任意の二項演算関数を受け取って集計処理を行う</li>
</ul>
<h2>要件</h2>
<ul>
<li>C++03まで : <code>binary_op</code>は副作用を起こしてはならない</li>
<li>C++11から : <code>binary_op</code>は、<a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a><code>[first, last]</code>の要素変更およびイテレータの無効化をしてはならない</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数等の意味論を構成する要素の1つ。Constraints。関数であればオーバーロード解決に参加するための条件、それ以外であれば受け付ける型の条件">テンプレートパラメータ制約</a></h2>
<ul>
<li><code>T</code>が<a href="../concepts/copy_constructible.html">コピー構築可能</a>であること</li>
<li><code>T</code>が<a href="../type_traits/is_copy_assignable.html">コピー代入可能</a>であること</li>
</ul>
<h2>効果</h2>
<ul>
<li>(1) : <code>binary_op</code>を<code>operator+</code>として (2) の効果を適用する</li>
<li>(2) :<ul>
<li>C++03 : <code>acc = init</code>、<code>[first, last)</code>の各イテレータを<code>i</code>とし、<code>acc = binary_op(acc, *i)</code>を範囲の全要素に対して適用し、その結果となる<code>acc</code>を返す</li>
<li>C++20 : <code>acc = init</code>、<code>[first, last)</code>の各イテレータを<code>i</code>とし、<code>acc = binary_op(<a href="../utility/move.html">std::move</a>(acc), *i)</code>を範囲の全要素に対して適用し、その結果となる<code>acc</code>を返す</li>
</ul>
</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a></h2>
<p>集計結果の値</p>
<h2><a class="cpprefjp-defined-word" data-desc="関数等の意味論を構成する要素の1つ。Complexity。関数が使うアルゴリズムの空間計算量・時間計算量">計算量</a></h2>
<p><a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a><code>[first, last)</code>の要素数に対して線形時間</p>
<h2>備考</h2>
<ul>
<li>この関数は並列アルゴリズムに対応していない。計算順序を規程せず、並列アルゴリズムに対応したバージョンとして<code><a href="reduce.html">std::reduce()</a></code>関数が定義されている</li>
</ul>
<h2>例</h2>
<h3>基本的な使い方</h3>
<p><div class="yata" id="db8d0b10b162cd6ace5d1763ee320328f6a36f77"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../vector.html"><vector></a></span>
<span class="cp">#include <a href="../string.html"><string></a></span>
<span class="cp">#include <a href="../numeric.html"><numeric></a></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../vector/vector.html">std::vector</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">5</span><span class="p">};</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../vector/vector.html">std::vector</a></span><span class="o"><</span><span class="n"><a href="../string/basic_string.html">std::string</a></span><span class="o">></span><span class="w"> </span><span class="n">v2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="s">"aaa"</span><span class="p">,</span><span class="w"> </span><span class="s">"bbb"</span><span class="p">,</span><span class="w"> </span><span class="s">"ccc"</span><span class="p">};</span>
<span class="w"> </span><span class="c1">// (1) : 合計値を求める</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">sum</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::accumulate</span></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"sum : "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">sum</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// (1) : 合計値をlong long型として求める</span>
<span class="w"> </span><span class="c1">// accumulateの第3引数がlong long型のゼロを表す0LLになっていることに注意</span>
<span class="w"> </span><span class="c1">// accumulateの戻り値型は、第3引数の型となるため、変数sum_llの型はlong long</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">sum_ll</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::accumulate</span></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">,</span><span class="w"> </span><span class="mf">0L</span><span class="n">L</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"sum_ll : "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">sum_ll</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// (1) : 文字列のリストを連結する</span>
<span class="w"> </span><span class="n"><a href="../string/basic_string.html">std::string</a></span><span class="w"> </span><span class="n">concatenate</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::accumulate</span></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v2.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v2.end()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../string/basic_string.html">std::string</a></span><span class="p">());</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"concat : "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">concatenate</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// (2) : 任意の二項演算を行う</span>
<span class="w"> </span><span class="c1">// ここでは、初期値を1として、全ての要素を掛け合わせている</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">product</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::accumulate</span></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="p">[](</span><span class="kt">int</span><span class="w"> </span><span class="n">acc</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">acc</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">i</span><span class="p">;</span>
<span class="w"> </span><span class="p">});</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"product : "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">product</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>sum : 15
sum_ll : 15
concat : aaabbbccc
product : 120
</code></pre></p>
<h3>クラスの一部のメンバ変数を集計する</h3>
<p><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../numeric.html"><numeric></a></span>
<span class="cp">#include <a href="../vector.html"><vector></a></span>
<span class="cp">#include <a href="../string.html"><string></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">value</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../string/basic_string.html">std::string</a></span><span class="w"> </span><span class="n">name</span><span class="p">;</span>
<span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../vector/vector.html">std::vector</a></span><span class="o"><</span><span class="n">X</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="s">"AAA"</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="s">"BBB"</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="s">"CCC"</span><span class="p">}</span>
<span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">sum</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::accumulate</span></span><span class="p">(</span><span class="n"><a href="../vector/vector/begin.html">v.begin()</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</a></span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="p">[](</span><span class="kt">int</span><span class="w"> </span><span class="n">acc</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">X</span><span class="o">&</span><span class="w"> </span><span class="n">x</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">acc</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">x</span><span class="p">.</span><span class="n">value</span><span class="p">;</span>
<span class="w"> </span><span class="p">});</span>
<span class="w"> </span><span class="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">sum</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h4>出力</h4>
<p><pre><code>6
</code></pre></p>
<h2>実装例</h2>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// (1)</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">InputIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span>
<span class="n">T</span><span class="w"> </span><span class="n">accumulate</span><span class="p">(</span><span class="n">InputIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">InputIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="n">init</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="p">(</span><span class="n">first</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">last</span><span class="p">)</span>
<span class="w"> </span><span class="n">init</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">init</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="o">*</span><span class="n">first</span><span class="o">++</span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">init</span><span class="p">;</span>
<span class="p">}</span>
<span class="c1">// (2)</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">InputIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">BinaryOperation</span><span class="o">></span>
<span class="n">T</span><span class="w"> </span><span class="n">accumulate</span><span class="p">(</span><span class="n">InputIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">InputIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="n">init</span><span class="p">,</span>
<span class="w"> </span><span class="n">BinaryOperation</span><span class="w"> </span><span class="n">binary_op</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="p">(</span><span class="n">first</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">last</span><span class="p">)</span>
<span class="w"> </span><span class="n">init</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">binary_op</span><span class="p">(</span><span class="n"><a href="../utility/move.html">std::move</a></span><span class="p">(</span><span class="n">init</span><span class="p">),</span><span class="w"> </span><span class="o">*</span><span class="n">first</span><span class="o">++</span><span class="p">);</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">init</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>関連項目</h2>
<ul>
<li><code><a href="reduce.html">std::reduce()</a></code></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0616r0.pdf" target="_blank">P0616R0 De-pessimize legacy <code><numeric></code> algorithms with <code>std::move</code></a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1645r1.html" target="_blank">P1645R1 <code>constexpr</code> for <code><numeric></code> algorithms</a><ul>
<li>C++20で、並列バージョン以外の数値計算アルゴリズムが<code>constexpr</code>対応した</li>
</ul>
</li>
</ul></div>
</div>
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