142 lines
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142 lines
12 KiB
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<title>std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin, std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::cbegin</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_unordered_map_begin skin-cppreference2 action-view cpp-page-template cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::</span>begin, <span style="font-size:0.7em; line-height:130%">std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::</span>cbegin</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-container.html">container</a>‎ | <a href="cpp-container-unordered_map.html">unordered map</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 class="t-page-template" style="">
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<p></p>
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</div>
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<table class="t-dcl-begin">
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<tbody>
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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp">iterator begin<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</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-cxx11">(C++11 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp">const_iterator begin<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</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-cxx11">(C++11 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp">const_iterator cbegin<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</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-cxx11">(C++11 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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<td></td>
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</tr>
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</tbody>
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</table>
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<p>返回指向 <code>unordered_map</code> 首元素的迭代器。</p>
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<p>若 <code>unordered_map</code> 为空，则返回的迭代器将等于 <span class="t-lc"><a href="cpp-container-unordered_map-end.html">end()</a></span> 。</p>
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<div class="t-image">
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<p><img alt="range-begin-end.svg" src="range-begin-end.svg" width="546" height="157" /></p>
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</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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<p>（无）</p>
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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>指向首元素的迭代器。</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>常数。</p>
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<p><br /></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 t-example-std-c++11">
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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 <cmath></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <unordered_map></span>
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<span class="kw1">struct</span> Node <span class="br0">{</span> <span class="kw4">double</span> x, y<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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Node nodes<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">0</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">2</span>, <span class="nu0">0</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">3</span>, <span class="nu0">0</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// mag 是映射 Node 地址到其平面中长度的 map</span>
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<a href="cpp-container-unordered_map.html"><span class="kw1293">std::<span class="me2">unordered_map</span></span></a><span class="sy1"><</span>Node <span class="sy2">*</span>, <span class="kw4">double</span><span class="sy1">></span> mag <span class="sy1">=</span> <span class="br0">{</span>
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<span class="br0">{</span> nodes, <span class="nu0">1</span> <span class="br0">}</span>,
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<span class="br0">{</span> nodes <span class="sy2">+</span> <span class="nu0">1</span>, <span class="nu0">2</span> <span class="br0">}</span>,
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<span class="br0">{</span> nodes <span class="sy2">+</span> <span class="nu0">2</span>, <span class="nu0">3</span> <span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// 从 0 到长度更改每个 y 坐标</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">auto</span> iter <span class="sy1">=</span> mag.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> iter <span class="sy3">!</span><span class="sy1">=</span> mag.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="sy2">++</span>iter<span class="br0">)</span><span class="br0">{</span>
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<span class="kw4">auto</span> cur <span class="sy1">=</span> iter<span class="sy2">-</span><span class="sy1">></span>first<span class="sy4">;</span> <span class="co1">// 指向 Node 的指针</span>
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cur<span class="sy2">-</span><span class="sy1">></span>y <span class="sy1">=</span> mag<span class="br0">[</span>cur<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 亦能使用 cur->y = iter->second;</span>
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<span class="br0">}</span>
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<span class="co1">// 更新并打印每个结点的长度</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">auto</span> iter <span class="sy1">=</span> mag.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> iter <span class="sy3">!</span><span class="sy1">=</span> mag.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="sy2">++</span>iter<span class="br0">)</span><span class="br0">{</span>
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<span class="kw4">auto</span> cur <span class="sy1">=</span> iter<span class="sy2">-</span><span class="sy1">></span>first<span class="sy4">;</span>
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mag<span class="br0">[</span>cur<span class="br0">]</span> <span class="sy1">=</span> <a href="cpp-numeric-math-hypot.html"><span class="kw1331">std::<span class="me2">hypot</span></span></a><span class="br0">(</span>cur<span class="sy2">-</span><span class="sy1">></span>x, cur<span class="sy2">-</span><span class="sy1">></span>y<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 magnitude of ("</span> <span class="sy1"><<</span> cur<span class="sy2">-</span><span class="sy1">></span>x <span class="sy1"><<</span> <span class="st0">", "</span> <span class="sy1"><<</span> cur<span class="sy2">-</span><span class="sy1">></span>y <span class="sy1"><<</span> <span class="st0">") is "</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> iter<span class="sy2">-</span><span class="sy1">></span>second <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="co1">// 以基于范围的 for 循环重复上述内容</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">auto</span> i <span class="sy4">:</span> mag<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">auto</span> cur <span class="sy1">=</span> i.<span class="me1">first</span><span class="sy4">;</span>
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cur<span class="sy2">-</span><span class="sy1">></span>y <span class="sy1">=</span> i.<span class="me1">second</span><span class="sy4">;</span>
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mag<span class="br0">[</span>cur<span class="br0">]</span> <span class="sy1">=</span> <a href="cpp-numeric-math-hypot.html"><span class="kw1331">std::<span class="me2">hypot</span></span></a><span class="br0">(</span>cur<span class="sy2">-</span><span class="sy1">></span>x, cur<span class="sy2">-</span><span class="sy1">></span>y<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 magnitude of ("</span> <span class="sy1"><<</span> cur<span class="sy2">-</span><span class="sy1">></span>x <span class="sy1"><<</span> <span class="st0">", "</span> <span class="sy1"><<</span> cur<span class="sy2">-</span><span class="sy1">></span>y <span class="sy1"><<</span> <span class="st0">") is "</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> mag<span class="br0">[</span>cur<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="co1">// 注意与上述 std::cout << iter->second << '\n'; 相反，</span>
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<span class="co1">// std::cout << i.second << '\n'; 将不会打印更新的长度</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>可能的输出：</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 magnitude of (3, 3) is 4.24264
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The magnitude of (1, 1) is 1.41421
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The magnitude of (2, 2) is 2.82843
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The magnitude of (3, 4.24264) is 5.19615
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The magnitude of (1, 1.41421) is 1.73205
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The magnitude of (2, 2.82843) is 3.4641</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-container-unordered_map-end.html"><span class="t-lines"><span>end</span> <span>cend</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>返回指向末尾的迭代器<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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<table class="t-dcl-begin">
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<tbody></tbody>
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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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