236 lines
20 KiB
HTML
236 lines
20 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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<title>std::binary_search</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_binary_search 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::</span>binary_search</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></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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</tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<td></td>
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<td rowspan="142">(1)</td>
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<td></td>
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<tr class="t-dcl t-until-cxx20">
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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> ForwardIt, <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="kw4">bool</span> binary_search<span class="br0">(</span> ForwardIt first, ForwardIt last, <span class="kw4">const</span> T<span class="sy3">&</span> value <span class="br0">)</span><span class="sy4">;</span></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-until-cxx20">(C++20 前)</span></td>
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</tr>
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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> <span class="kw1">class</span> ForwardIt, <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> binary_search<span class="br0">(</span> ForwardIt first, ForwardIt last, <span class="kw4">const</span> T<span class="sy3">&</span> value <span class="br0">)</span><span class="sy4">;</span></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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</tbody>
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<td></td>
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<td rowspan="142">(2)</td>
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<td></td>
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<tr class="t-dcl t-until-cxx20">
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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> ForwardIt, <span class="kw1">class</span> T, <span class="kw1">class</span> Compare <span class="sy1">></span><br />
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<span class="kw4">bool</span> binary_search<span class="br0">(</span> ForwardIt first, ForwardIt last, <span class="kw4">const</span> T<span class="sy3">&</span> value, Compare comp <span class="br0">)</span><span class="sy4">;</span></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-until-cxx20">(C++20 前)</span></td>
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</tr>
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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> <span class="kw1">class</span> ForwardIt, <span class="kw1">class</span> T, <span class="kw1">class</span> Compare <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> binary_search<span class="br0">(</span> ForwardIt first, ForwardIt last, <span class="kw4">const</span> T<span class="sy3">&</span> value, Compare comp <span class="br0">)</span><span class="sy4">;</span></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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</tbody>
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<tbody>
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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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<td></td>
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</tr>
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</tbody>
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</table>
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<p>检查等价于 <code>value</code> 的元素是否出现于范围 <code>[first, last)</code> 中。</p>
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<p>对于要成功的 <code>std::binary_search</code> ，范围 <code>[first, last)</code> 必须至少相对于 <code>value</code> 部分有序，即它必须满足下列所有要求：</p>
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<ul>
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<li>已相对 <span class="t-c"><span class="mw-geshi cpp source-cpp">element <span class="sy1"><</span> value</span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">comp<span class="br0">(</span>element, value<span class="br0">)</span></span></span> 划分</li>
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<li>已相对 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span>value <span class="sy1"><</span> element<span class="br0">)</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span>comp<span class="br0">(</span>value, element<span class="br0">)</span></span></span> 划分（即所有令此表达式为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 的元素必须前趋所有令此表达式为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 的元素）</li>
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<li>对于所有元素，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">element <span class="sy1"><</span> value</span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">comp<span class="br0">(</span>element, value<span class="br0">)</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span>value <span class="sy1"><</span> element<span class="br0">)</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span>comp<span class="br0">(</span>value, element<span class="br0">)</span></span></span> 亦为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span></li>
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</ul>
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<p>完全排序的范围满足这些判别标准。</p>
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<p>第一版本用 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><</span></span></span> 比较元素，第二版本用给定的比较函数 <code>comp</code> 。</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>要检验的元素范围</td>
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</tr>
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<tr class="t-par">
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<td>value</td>
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<td>-</td>
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<td>要与元素比较的值</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>若第一参数<i>小于</i>（即先序于）第二个则返回 ​<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 的二元谓词。<br />
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<p>谓词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> pred<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> 必须使得 <span class="t-c"><span class="mw-geshi cpp source-cpp">ForwardIt</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">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">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">Type2</span></span> 。 ​</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>ForwardIt</code> 必须满足<a href="cpp-named_req-ForwardIterator.html"><span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</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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<code>Compare</code> 必须满足<a href="cpp-named_req-BinaryPredicate.html"><span style="font-style:italic">二元谓词</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(BinaryPredicate)</span></a> 的要求。不要求满足 <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> 。</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>若找到等于 <code>value</code> 的元素则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</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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<p>进行的比较次数与 <code>first</code> 和 <code>last</code> 间的距离成对数（至多 <span class="texhtml" style="white-space: nowrap;">log<span class="t-su t-su-b"><br />
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2</span>(last - first) + O(1)</span> 次比较）。然而，对于非<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> ，迭代次自增次数为线性。</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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<tr>
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<th>版本一</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> ForwardIt, <span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">bool</span> binary_search<span class="br0">(</span>ForwardIt first, ForwardIt last, <span class="kw4">const</span> T<span class="sy3">&</span> value<span class="br0">)</span>
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<span class="br0">{</span>
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first <span class="sy1">=</span> <a href="cpp-algorithm-lower_bound.html"><span class="kw1623">std::<span class="me2">lower_bound</span></span></a><span class="br0">(</span>first, last, value<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="br0">(</span><span class="sy3">!</span><span class="br0">(</span>first <span class="sy1">==</span> last<span class="br0">)</span> <span class="sy3">&&</span> <span class="sy3">!</span><span class="br0">(</span>value <span class="sy1"><</span> <span class="sy2">*</span>first<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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</td>
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</tr>
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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> ForwardIt, <span class="kw1">class</span> T, <span class="kw1">class</span> Compare<span class="sy1">></span>
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<span class="kw4">bool</span> binary_search<span class="br0">(</span>ForwardIt first, ForwardIt last, <span class="kw4">const</span> T<span class="sy3">&</span> value, Compare comp<span class="br0">)</span>
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<span class="br0">{</span>
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first <span class="sy1">=</span> <a href="cpp-algorithm-lower_bound.html"><span class="kw1623">std::<span class="me2">lower_bound</span></span></a><span class="br0">(</span>first, last, value, comp<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="br0">(</span><span class="sy3">!</span><span class="br0">(</span>first <span class="sy1">==</span> last<span class="br0">)</span> <span class="sy3">&&</span> <span class="sy3">!</span><span class="br0">(</span>comp<span class="br0">(</span>value, <span class="sy2">*</span>first<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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</td>
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</tr>
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</table>
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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 <algorithm></span>
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<span class="co2">#include <vector></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> haystack <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">3</span>, <span class="nu0">4</span>, <span class="nu0">5</span>, <span class="nu0">9</span><span class="br0">}</span><span class="sy4">;</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> needles <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> needle <span class="sy4">:</span> needles<span class="br0">)</span> <span class="br0">{</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">"Searching for "</span> <span class="sy1"><<</span> needle <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>std<span class="sy4">::</span><span class="me2">binary_search</span><span class="br0">(</span>haystack.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, haystack.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, needle<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</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">"Found "</span> <span class="sy1"><<</span> needle <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> <span class="kw1">else</span> <span class="br0">{</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">"no dice!<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</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">Searching for 1
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Found 1
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Searching for 2
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no dice!
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Searching for 3
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Found 3</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue270">LWG 270</a></td>
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<td>C++98</td>
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<td>曾要求 Compare 为严格弱序</td>
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<td>只需要划分；容许异相比较</td>
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</tr>
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</table>
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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-equal_range.html"><span class="t-lines"><span>equal_range</span></span></a></div>
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</div>
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</td>
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<td>返回匹配特定键值的元素范围<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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