159 lines
12 KiB
HTML
159 lines
12 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>while 循环</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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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_language_while 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">while 循环</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-language.html">language</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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<p>重复执行 <span class="t-spar">语句</span> ，直到 <span class="t-spar">表达式</span> 的值变得比较等于零。此测试在每次迭代前发生。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<table class="t-sdsc-begin">
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td class="t-sdsc-nopad"><span class="t-spar">属性说明符序列</span><span class="t-mark">(可选)</span> <code><b>while (</b></code> <span class="t-spar">表达式</span> <code><b>)</b></code> <span class="t-spar">语句</span></td>
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<td class="t-sdsc-nopad"></td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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</table>
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<table class="t-par-begin">
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<tr class="t-par">
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<td><span class="t-spar">表达式</span></td>
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<td>-</td>
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<td>任何<a href="c-language-types.html#.E7.B1.BB.E5.9E.8B.E7.BB.84.E5.88.AB">标量类型</a><a href="c-language-expressions.html">表达式</a>。在每次迭代前求值此表达式，而且若它与零比较相等，则退出循环。</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">语句</span></td>
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<td>-</td>
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<td>任何<a href="c-language-statements.html">语句</a>，常为一条复合语句，它被用作循环体</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">属性说明符序列</span></td>
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<td>-</td>
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<td><span class="t-mark-rev t-since-c23">(C23)</span>可选的<a href="c-language-attributes.html">属性</a>列表，应用到循环语句</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p><code>while</code> 导致重复执行 <span class="t-spar">语句</span> （亦称为<i>循环体</i>），直至 <span class="t-spar">表达式</span> （亦称为<i>控制表达式</i>）与零比较相等。不管是正常进入循环体还是以 <a href="c-language-goto.html">goto</a> 进入 <span class="t-spar">语句</span> 内部，都会发生重复。</p>
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<p><span class="t-spar">表达式</span>的求值在每次执行 <span class="t-spar">语句</span> 前发生（除非用 goto 进入）。若需要在每次循环体后求值控制表达式，可以用 <a href="c-language-do.html">do-while 循环</a>。</p>
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<p>若循环的执行需要在某些点终止，则能以 <a href="c-language-break.html">break 语句</a>为作终止语句。</p>
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<p>若循环的执行需要从循环体的结尾继续，则能以 <a href="c-language-continue.html">continue 语句</a>为快捷方式。</p>
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<p>若无限循环在其 <span class="t-spar">语句</span> 或 <span class="t-spar">表达式</span> 的任何部分无可观测行为（ I/O 、 volatile 访问、原子或同步操作），则拥有这种循环的程序有未定义行为。这允许编译器优化掉整个不可观测循环，而无需证明他们会结束。仅有的例外是 <span class="t-spar">表达式</span> 为常量表达式的循环： <code>while(true)</code> 始终是无限循环。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-c99">
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<td>
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<p>同所有选择和迭代语句， while 语句建立<a href="c-language-scope.html">块作用域</a>：任何于 <span class="t-spar">表达式</span> 中引入的标识符在语句后离开作用域。</p>
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</td>
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<td><span class="t-mark-rev t-since-c99">(C99 起)</span></td>
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</tr>
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</table>
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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>常以布尔和指针表达式为循环控制表达式。布尔值 <code>false</code> 和任何指针类型的空指针值与零比较相等。</p>
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<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="c-keyword-while.html"><tt>while</tt></a></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="c source-c">
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<pre class="de1"><span class="co2">#include <stdio.h></span>
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<span class="co2">#include <stdlib.h></span>
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<span class="co2">#include <string.h></span>
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<span class="kw2">enum</span> <span class="br0">{</span> SIZE <span class="sy1">=</span> <span class="nu0">8</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="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co1">// 平凡示例</span>
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<span class="kw4">int</span> array<span class="br0">[</span>SIZE<span class="br0">]</span>, n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">while</span><span class="br0">(</span>n <span class="sy1"><</span> SIZE<span class="br0">)</span> array<span class="br0">[</span>n<span class="sy2">++</span><span class="br0">]</span> <span class="sy1">=</span> <a href="c-numeric-random-rand.html"><span class="kw741">rand</span></a><span class="br0">(</span><span class="br0">)</span> <span class="sy2">%</span> <span class="nu0">2</span><span class="sy4">;</span>
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<a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"Array filled!"</span><span class="br0">)</span><span class="sy4">;</span>
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n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">while</span><span class="br0">(</span>n <span class="sy1"><</span> SIZE<span class="br0">)</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%d "</span>, array<span class="br0">[</span>n<span class="sy2">++</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 经典的 strcpy() 实现</span>
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<span class="co1">// （从 src 复制空终止字符串到 dst ）</span>
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<span class="kw4">char</span> src<span class="br0">[</span><span class="br0">]</span><span class="sy1">=</span><span class="st0">"Hello, world"</span>, dst<span class="br0">[</span><span class="kw4">sizeof</span> src<span class="br0">]</span>, <span class="sy2">*</span>p<span class="sy1">=</span>dst, <span class="sy2">*</span>q<span class="sy1">=</span>src<span class="sy4">;</span>
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<span class="kw1">while</span><span class="br0">(</span><span class="sy2">*</span>p<span class="sy2">++</span> <span class="sy1">=</span> <span class="sy2">*</span>q<span class="sy2">++</span><span class="br0">)</span>
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<span class="sy4">;</span> <span class="co1">// 空语句</span>
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<a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span>dst<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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<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">Array filled!
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1 0 1 1 1 1 0 0
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Hello, world</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
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<div class="t-ref-std-17">
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<ul>
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<li>C17 标准（ISO/IEC 9899:2018）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>6.8.5.1 The while statement (p: 109)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-11">
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<ul>
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<li>C11 标准（ISO/IEC 9899:2011）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>6.8.5.1 The while statement (p: 151)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-c99">
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<ul>
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<li>C99 标准（ISO/IEC 9899:1999）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>6.8.5.1 The while statement (p: 136)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-ref-std-c89">
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<ul>
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<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>3.6.5.1 The while statement</li>
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</ul>
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</dd>
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</dl>
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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"></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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