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<div class="identifier-type">function template</div><div class="header"><algorithm></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">nth_element</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">RandomAccessIterator</span><span class="o">></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">nth_element</span><span class="p">(</span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">nth</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">last</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">RandomAccessIterator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">nth_element</span><span class="p">(</span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">nth</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">last</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">RandomAccessIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Compare</span><span class="o">></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">nth_element</span><span class="p">(</span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">nth</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="n">Compare</span><span class="w"> </span><span class="n">comp</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">RandomAccessIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Compare</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">nth_element</span><span class="p">(</span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">nth</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="n">Compare</span><span class="w"> </span><span class="n">comp</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) 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"><a href="../execution/execution/execution_policy.html">ExecutionPolicy</a></span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">RandomAccessIterator</span><span class="o">></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">nth_element</span><span class="p">(</span><span class="n"><a href="../execution/execution/execution_policy.html">ExecutionPolicy</a></span><span class="o">&&</span><span class="w"> </span><span class="n">exec</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">nth</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">last</span><span class="p">);</span><span class="w"> </span><span class="c1">// (3) C++17</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"><a href="../execution/execution/execution_policy.html">ExecutionPolicy</a></span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">RandomAccessIterator</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Compare</span><span class="o">></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">nth_element</span><span class="p">(</span><span class="n"><a href="../execution/execution/execution_policy.html">ExecutionPolicy</a></span><span class="o">&&</span><span class="w"> </span><span class="n">exec</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">nth</span><span class="p">,</span>
<span class="w"> </span><span class="n">RandomAccessIterator</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="n">Compare</span><span class="w"> </span><span class="n">comp</span><span class="p">);</span><span class="w"> </span><span class="c1">// (4) C++17</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>基準となる要素よりも小さい要素が前に来るよう並べ替える。</p>
<p>この関数は<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> の並び替えを行うが、基準位置 <code>nth</code> のみが正しい要素、つまり仮に範囲 <code>[first,last)</code> 全体を並び替えた際に<code>nth</code>に位置すべき要素となる。前半の<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,nth)</code> は関数呼び出し後の位置 <code>nth</code> にある要素よりも小さいことは保証されるが、その<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,nth)</code> 内での要素並び順はなんら保証されない。</p>
<p>ある<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>に対して部分的な並び替えを行う場合は、<code><a href="partial_sort.html">partial_sort()</a></code>を使用する。また範囲全体に対して並び替えを行う場合は、<code><a href="sort.html">sort()</a></code>を使用する。</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。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>RandomAccessIterator</code> は <code>ValueSwappable</code> の要件を満たしている必要がある。<code>*first</code> の型は <code>MoveConstructible</code> と <code>MoveAssignable</code> の要件を満たしている必要がある。</li>
</ul>
<h2>効果</h2>
<p><code>nth_element()</code> を呼び出した後、<code>nth</code> が指している位置の要素は、全ての範囲がソートされた場合の位置にある要素になる。そして、<code>[first,nth)</code> にあるイテレータ <code>i</code> と、<code>[nth,last)</code> にあるイテレータ <code>j</code> について、<code>!(*j < *i)</code> または <code>comp(*j, *i) == false</code> になる。</p>
<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>なし</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>
<ul>
<li>(1), (2) : 平均で線形時間</li>
<li>(3), (4) : <code>N = last - first</code>であるとして、O(N)回だけ比較または<a class="cpprefjp-defined-word" data-desc="boolを返す関数・関数オブジェクト。predicate">述語</a>の適用と、O(NlogN)回だけswap操作を行う</li>
</ul>
<h2>例</h2>
<p><div class="yata" id="37a5b546d4b4c8768ce10ac7e1bd11a24a9e317a"><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="../algorithm.html"><algorithm></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="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">5</span><span class="p">,</span><span class="w"> </span><span class="mi">10</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">7</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">9</span><span class="p">,</span><span class="w"> </span><span class="mi">8</span><span class="p">,</span><span class="w"> </span><span class="mi">6</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">};</span>
<span class="w"> </span><span class="c1">// 4番目に小さい値より小さい値を前に集める</span>
<span class="w"> </span><span class="n"><span style="color:#ff0000">std::nth_element</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/begin.html">v.begin()</a></span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">3</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="n"><a href="for_each.html">std::for_each</a></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="p">[](</span><span class="kt">int</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="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">x</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="p">});</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力例</h3>
<p><pre><code>2
1
4
5
10
9
8
6
7
</code></pre></p>
<h2>関連項目</h2>
<ul>
<li><code><a href="ranges_nth_element.html">ranges::nth_element</a></code></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2163" target="_blank">LWG Issue 2163. <code>nth_element</code> requires inconsistent post-conditions</a><ul>
<li>C++11まで、この関数を呼び出したあとの状態について「<code>!(*i > *j)</code>」と記載していたが、並べ替えには<code>operator<()</code>を使用するので、C++14で「<code>!(*j < *i)</code>」に訂正。</li>
</ul>
</li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2150" target="_blank">LWG Issue 2150. Unclear specification of <code>find_end</code></a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0574r1.html" target="_blank">P0574R1 Algorithm Complexity Constraints and Parallel Overloads</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0879r0.html" target="_blank">P0879R0 Constexpr for <code>swap</code> and <code>swap</code> related functions</a></li>
</ul></div>
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