238 lines
21 KiB
HTML
238 lines
21 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::ranges::nth_element</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_algorithm_ranges_nth_element skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span>nth_element</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-algorithm.html">algorithm</a>‎ | <a href="cpp-algorithm-ranges.html">ranges</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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<table class="t-dcl-begin">
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<td>
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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>
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<td></td>
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<td></td>
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<div>调用签名</div>
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<td></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> std<span class="sy4">::</span><span class="me2">random_access_iterator</span> I, std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>I<span class="sy1">></span> S,<br /></span>
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<p> <span class="kw1">class</span> Comp <span class="sy1">=</span> ranges<span class="sy4">::</span><span class="me2">less</span>, <span class="kw1">class</span> Proj <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">identity</span> <span class="sy1">></span><br />
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requires std<span class="sy4">::</span><span class="me2">sortable</span><span class="sy1"><</span>I, Comp, Proj<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> I<br /></p>
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nth_element<span class="br0">(</span> I first, I nth, S last, Comp comp <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span>, Proj proj <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span> <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</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> ranges<span class="sy4">::</span><span class="me2">random_access_range</span> R, <span class="kw1">class</span> Comp <span class="sy1">=</span> ranges<span class="sy4">::</span><span class="me2">less</span>,<br /></span>
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<p> <span class="kw1">class</span> Proj <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">identity</span> <span class="sy1">></span><br />
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requires std<span class="sy4">::</span><span class="me2">sortable</span><span class="sy1"><</span>iterator_t<span class="sy1"><</span>R<span class="sy1">></span>, Comp, Proj<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> ranges<span class="sy4">::</span><span class="me2">borrowed_iterator_t</span><span class="sy1"><</span>R<span class="sy1">></span><br /></p>
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nth_element<span class="br0">(</span> R<span class="sy3">&&</span> r, iterator_t<span class="sy1"><</span>R<span class="sy1">></span> nth, Comp comp <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span>, Proj proj <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span> <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-cxx20">(C++20 起)</span></td>
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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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</tr>
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</tbody>
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</table>
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<p>重排 <code>[first, last)</code> 中的元素使得:</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设 <code>[first, last)</code> 按照 <code>comp</code> 与 <code>proj</code> 排序则会在该位置出现的元素。</li>
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<li>所有在此新的 <code><b>nth</b></code> 元素前的元素<i>小于或等于</i>新的 <code>nth</code> 后的元素。即对于每个分别在范围 <code>[first, nth)</code> 与 <code>[nth, last)</code> 中的迭代器 <i>i</i> 与 <i>j</i> ,表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>comp, <a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>proj, <span class="sy2">*</span>j<span class="br0">)</span>, <a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>proj, <span class="sy2">*</span>i<span class="br0">)</span><span class="br0">)</span></span></span> 求值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span>.</li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">nth <span class="sy1">==</span> last</span></span> 则此函数无效果。</li>
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</ul>
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<div class="t-li1"><span class="t-li">1)</span> 用给定的二元比较函数对象 <code>comp</code> 与投影对象 <code>proj</code> 比较元素。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ,但以 <code>r</code> 为范围,如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 为 <code>first</code> 并以 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">end</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 为 <code>last</code> 。</div>
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<p>此页面上描述的仿函数实体是 <i>niebloid</i> ,即:</p>
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<ul>
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<li>调用它们中的任何一者时不可指定显式模板实参。</li>
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<li>它们无一对<a href="cpp-language-adl.html">实参依赖查找</a>可见。</li>
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<li>在<a href="cpp-language-unqualified_lookup.html">通常无限定查找</a>寻找函数调用运算符左侧的名称找到它们之一时,它禁止<a href="cpp-language-adl.html">实参依赖查找</a>。</li>
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</ul>
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<p>实际上,它们能以函数对象,或以某些特殊编译器扩展实现。</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>要重排序的元素范围</td>
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</tr>
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<tr class="t-par">
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<td>r</td>
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<td>-</td>
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<td>要重排序的元素范围</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>定义划分点的迭代器</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>用于比较投影后元素的比较器</td>
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</tr>
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<tr class="t-par">
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<td>proj</td>
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<td>-</td>
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<td>应用到元素的投影</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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<div class="t-li1"><span class="t-li">1)</span> 等于 <code>last</code> 的迭代器。</div>
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<div class="t-li1"><span class="t-li">2)</span> 若 <code>r</code> 为左值或拥有 <a href="cpp-ranges-borrowed_range.html"><code>borrowed_range</code></a> 类型则同 <span class="t-v">(1)</span> 。否则返回 <span class="t-lc">std::ranges::dangling</span> 。</div>
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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>平均与 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">distance</span><span class="br0">(</span>first, last<span class="br0">)</span></span></span> 成线性。</p>
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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>使用的算法常为<a href="https://en.wikipedia.org/wiki/Introselect" class="extiw" title="enwiki:Introselect">内省选择</a>,尽管允许其他拥有适合的平均情况复杂度的<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/microsoft/STL/blob/e745bad3b1d05b5b19ec652d68abb37865ffa454/stl/inc/algorithm#L8429-L8499">MSVC STL</a> 与 <a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/54258e22b0846aaa6bd3265f592feb161eecda75/libstdc%2B%2B-v3/include/bits/ranges_algo.h#L2052-L2080">libstdc++</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 <algorithm></span>
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<span class="co2">#include <array></span>
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<span class="co2">#include <functional></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <ranges></span>
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<span class="co2">#include <string_view></span>
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<span class="kw4">void</span> print<span class="br0">(</span><a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a> rem, std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">input_range</span> <span class="kw4">auto</span> <span class="kw4">const</span><span class="sy3">&</span> a<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><a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> rem<span class="sy4">;</span> <span class="kw4">const</span> <span class="kw4">auto</span> e <span class="sy4">:</span> a<span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> e <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <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>
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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-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> 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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print<span class="br0">(</span><span class="st0">"Before nth_element: "</span>, v<span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">nth_element</span><span class="br0">(</span>v, 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><span class="br0">)</span><span class="sy4">;</span>
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print<span class="br0">(</span><span class="st0">"After nth_element: "</span>, v<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">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">ranges</span><span class="sy4">::</span><span class="me2">nth_element</span><span class="br0">(</span>v, v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> <span class="nu0">1</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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print<span class="br0">(</span><span class="st0">"After nth_element: "</span>, v<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"The largest element is: "</span> <span class="sy1"><<</span> v<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span><span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">literals</span><span class="sy4">;</span>
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<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> names <span class="br0">{</span>
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<span class="st0">"Diva"</span>sv, <span class="st0">"Cornelius"</span>sv, <span class="st0">"Munro"</span>sv, <span class="st0">"Rhod"</span>sv,
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<span class="st0">"Zorg"</span>sv, <span class="st0">"Korben"</span>sv, <span class="st0">"Bender"</span>sv, <span class="st0">"Leeloo"</span>sv,
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<span class="br0">}</span><span class="sy4">;</span>
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print<span class="br0">(</span><span class="st0">"Before nth_element: "</span>, names<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> fifth_element <span class="br0">{</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">next</span><span class="br0">(</span>names.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, <span class="nu0">4</span><span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">nth_element</span><span class="br0">(</span>names, fifth_element<span class="br0">)</span><span class="sy4">;</span>
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print<span class="br0">(</span><span class="st0">"After nth_element: "</span>, names<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"The 5th element is: "</span> <span class="sy1"><<</span> <span class="sy2">*</span>fifth_element <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">Before nth_element: 5 6 4 3 2 6 7 9 3
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After nth_element: 2 3 3 4 5 6 6 7 9
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The median is: 5
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After nth_element: 9 7 6 6 5 4 3 3 2
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The second largest element is: 7
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The largest element is: 9
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Before nth_element: Diva Cornelius Munro Rhod Zorg Korben Bender Leeloo
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After nth_element: Diva Cornelius Bender Korben Leeloo Rhod Munro Zorg
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The 5th element is: Leeloo</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-ranges-max_element.html"><span class="t-lines"><span>ranges::max_element</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回范围中的最大元素<br />
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<span class="t-mark">(niebloid)</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-ranges-min_element.html"><span class="t-lines"><span>ranges::min_element</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回范围中的最小元素<br />
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<span class="t-mark">(niebloid)</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-ranges-partition.html"><span class="t-lines"><span>ranges::partition</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>将范围中的元素分为二组<br />
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<span class="t-mark">(niebloid)</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-ranges-partial_sort.html"><span class="t-lines"><span>ranges::partial_sort</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>排序一个范围的前 N 个元素<br />
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<span class="t-mark">(niebloid)</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-nth_element.html"><span class="t-lines"><span>nth_element</span></span></a></div>
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</div>
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</td>
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<td>将给定的范围部分排序,确保其按给定元素划分<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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