234 lines
22 KiB
HTML
234 lines
22 KiB
HTML
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<title>std::nth_element</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_nth_element 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>nth_element</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-algorithm.html"><algorithm></a></code></div>
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<td></td>
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<td></td>
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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> RandomIt <span class="sy1">></span><br />
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<span class="kw4">void</span> nth_element<span class="br0">(</span> RandomIt first, RandomIt nth, RandomIt last <span class="br0">)</span><span class="sy4">;</span></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> RandomIt <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">void</span> nth_element<span class="br0">(</span> RandomIt first, RandomIt nth, RandomIt last <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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<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> RandomIt <span class="sy1">></span><br /></span>
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<p><span class="kw4">void</span> nth_element<span class="br0">(</span> ExecutionPolicy<span class="sy3">&&</span> policy,<br /></p>
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RandomIt first, RandomIt nth, RandomIt 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> RandomIt, <span class="kw1">class</span> Compare <span class="sy1">></span><br /></span>
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<p><span class="kw4">void</span> nth_element<span class="br0">(</span> RandomIt first, RandomIt nth, RandomIt last,<br /></p>
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Compare comp <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-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> RandomIt, <span class="kw1">class</span> Compare <span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> <span class="kw4">void</span> nth_element<span class="br0">(</span> RandomIt first, RandomIt nth, RandomIt last,<br /></p>
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Compare comp <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</tbody>
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<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> RandomIt, <span class="kw1">class</span> Compare <span class="sy1">></span><br /></span>
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<p><span class="kw4">void</span> nth_element<span class="br0">(</span> ExecutionPolicy<span class="sy3">&&</span> policy,<br />
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RandomIt first, RandomIt nth, RandomIt last,<br /></p>
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Compare comp <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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<p><code>nth_element</code> 是部分排序算法，它重排 <code>[first, last)</code> 中元素，使得：</p>
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<ul>
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<li><code>nth</code> 所指向的元素被更改为假如 <code>[first, last)</code> 已排序则该位置会出现的元素。</li>
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<li>这个新的 <code>nth</code> 元素前的所有元素小于或等于新的 <code>nth</code> 元素后的所有元素。</li>
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</ul>
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<p>更正式而言， <code>nth_element</code> 以升序部分排序范围 <code>[first, last)</code> ，使得对于任何范围 <code>[first, nth)</code> 中的 <code>i</code> 和任何范围 <code>[nth, last)</code> 中的 <code>j</code> ，都满足条件 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span><span class="sy2">*</span>j <span class="sy1"><</span> <span class="sy2">*</span>i<span class="br0">)</span></span></span> （对于 <span class="t-v">(1-2)</span> ，对 <span class="t-v">(3-4)</span> 则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">comp<span class="br0">(</span><span class="sy2">*</span>j, <span class="sy2">*</span>i<span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">false</span></span></span> ）。置于 <code>nth</code> 位置的元素则准确地是假如完全排序范围则应出现于此位置的元素。</p>
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<p><code>nth</code> 可以是尾迭代器，此情况下函数无效果。</p>
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<div class="t-li1"><span class="t-li">1)</span> 用 <code>operator<</code> 比较元素。</div>
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<div class="t-li1"><span class="t-li">3)</span> 用给定的二元比较函数 <code>comp</code> 比较元素。</div>
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<div class="t-li1"><span class="t-li">2,4)</span> 同 <span class="t-v">(1,3)</span> ，但按照 <code>policy</code> 执行。这些重载仅若 <span class="t-rev-inl t-until-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-is_execution_policy.html"><span class="kw1564">std::<span class="me2">is_execution_policy_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ExecutionPolicy<span class="sy1">>></span></span></span></span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span><span class="t-rev-inl t-since-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-is_execution_policy.html"><span class="kw1564">std::<span class="me2">is_execution_policy_v</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>ExecutionPolicy<span class="sy1">>></span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span> 为 true 才参与重载决议。</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>first, last</td>
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<td>-</td>
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<td>定义待排序范围的随机访问迭代器</td>
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</tr>
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<tr class="t-par">
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<td>nth</td>
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<td>-</td>
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<td>定义排序划分点的随机访问迭代器</td>
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</tr>
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<tr class="t-par">
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<td>policy</td>
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<td>-</td>
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<td>所用的执行策略。细节见<a href="cpp-algorithm-execution_policy_tag_t.html">执行策略</a>。</td>
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</tr>
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<tr class="t-par">
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<td>comp</td>
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<td>-</td>
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<td>比较函数对象（即满足<a href="cpp-named_req-Compare.html"><span style="font-style:italic">比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Compare)</span></a> 概念的对象），若第一参数<i>小于</i>（即<i>先</i>序于）第二参数则返回 ​<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。<br />
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<p>比较函数的签名应等价于如下：</p>
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<p><span class="t-cc"><span class="mw-geshi cpp source-cpp"> <span class="kw4">bool</span> cmp<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>虽然签名不必有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="sy3">&</span></span></span> ，函数也不能修改传递给它的对象，而且必须接受（可为 const 的）类型 <code>Type1</code> 与 <code>Type2</code> 的值，无关乎<a href="cpp-language-value_category.html">值类别</a>（从而不允许 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type1 <span class="sy3">&</span></span></span> <span class="t-rev-inl t-since-cxx11"><span>，亦不允许 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type1</span></span> ，除非 <code>Type1</code> 的移动等价于复制</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>）。<br />
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类型 <span class="t-c"><span class="mw-geshi cpp source-cpp">Type1</span></span> 与 <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">RandomIt</span></span> 类型的对象能在解引用后隐式转换到这两个类型。 ​</p>
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</td>
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</tr>
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<tr class="t-par-hitem">
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<td colspan="3">类型要求</td>
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</tr>
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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<code>RandomIt</code> 必须满足<a href="cpp-named_req-ValueSwappable.html"><span style="font-style:italic">值可交换</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(ValueSwappable)</span></a> 和 <a href="cpp-named_req-RandomAccessIterator.html"><span style="font-style:italic">老式随机访问迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyRandomAccessIterator)</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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解引用 <code>RandomIt</code> 结果的类型必须满足<a href="cpp-named_req-MoveAssignable.html"><span style="font-style:italic">可移动赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveAssignable)</span></a> 和<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。</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>（无）</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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<div class="t-li1"><span class="t-li">1,3)</span> 平均与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-distance.html"><span class="kw673">std::<span class="me2">distance</span></span></a><span class="br0">(</span>first, last<span class="br0">)</span></span></span> 成线性。</div>
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<div class="t-li1"><span class="t-li">2,4)</span> O(N) 次应用谓词和 O(N log N) 次交换，其中 N = last - first 。</div>
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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>典型地使用<a href="https://en.wikipedia.org/wiki/Introselect" class="extiw" title="enwiki:Introselect">内省选择</a>算法，尽管允许其他拥有适合平均情况复杂度的<a href="https://en.wikipedia.org/wiki/Selection_algorithm" class="extiw" title="enwiki:Selection algorithm">选择算法</a>。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<p>参阅 <a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/d9375e490072d1aae73a93949aa158fcd2a27018/libstdc%2B%2B-v3/include/bits/stl_algo.h#L4718">libstdc++</a> 与 <a rel="nofollow" class="external text" href="https://github.com/llvm-mirror/libcxx/blob/a12cb9d211019d99b5875b6d8034617cbc24c2cc/include/algorithm#L5109">libc++</a> 中的实现。</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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<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 <algorithm></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">5</span>, <span class="nu0">6</span>, <span class="nu0">4</span>, <span class="nu0">3</span>, <span class="nu0">2</span>, <span class="nu0">6</span>, <span class="nu0">7</span>, <span class="nu0">9</span>, <span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">nth_element</span><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> v.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">/</span><span class="nu0">2</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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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"The median is "</span> <span class="sy1"><<</span> v<span class="br0">[</span>v.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">/</span><span class="nu0">2</span><span class="br0">]</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">nth_element</span><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">+</span><span class="nu0">1</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, <a href="cpp-utility-functional-greater.html"><span class="kw1037">std::<span class="me2">greater</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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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"The second largest element is "</span> <span class="sy1"><<</span> v<span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span> <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">The median is 5
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The second largest element is 7</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-partial_sort_copy.html"><span class="t-lines"><span>partial_sort_copy</span></span></a></div>
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</div>
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</td>
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<td>对范围内的元素进行复制并部分排序<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-stable_sort.html"><span class="t-lines"><span>stable_sort</span></span></a></div>
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</div>
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</td>
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<td>将范围内的元素排序，同时保持相等的元素之间的顺序<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-sort.html"><span class="t-lines"><span>sort</span></span></a></div>
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</div>
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</td>
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<td>将范围按升序排序<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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