243 lines
17 KiB
HTML
243 lines
17 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>switch 语句</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_switch 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">switch 语句</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>按照整数参数的值执行代码。</p>
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<p>在需要按照整数值执行多个分支中的一个或数个之处使用。</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 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>switch (</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">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 <code>switch</code> 语句</td>
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</tr>
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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">整数类型</a>（ char 、有符号或无符号整数，或枚举）<a href="c-language-expressions.html">表达式</a></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>（典型为复合语句）。允许在 <span class="t-spar">语句</span> 中有 <code><b>case:</b></code> 和 <code><b>default:</b></code> 标号，而 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw1">break</span><span class="sy4">;</span></span></span> 语句拥有特殊含义。</td>
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</tr>
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</table>
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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><code><b>case</b></code> <span class="t-spar">常量表达式</span> <code><b>:</b></code> <span class="t-spar">语句</span></td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-until-c23">(C23 前)</span></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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<tr class="t-sdsc">
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<td><span class="t-spar">属性说明符序列</span><span class="t-mark">(可选)</span> <code><b>case</b></code> <span class="t-spar">常量表达式</span> <code><b>:</b></code> <span class="t-spar">语句</span><span class="t-mark">(可选)</span></td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-c23">(C23 起)</span></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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<tr class="t-sdsc">
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<td><code><b>default</b></code> <code><b>:</b></code> <span class="t-spar">语句</span></td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-until-c23">(C23 前)</span></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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<tr class="t-sdsc">
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<td><span class="t-spar">属性说明符序列</span><span class="t-mark">(可选)</span> <code><b>default</b></code> <code><b>:</b></code> <span class="t-spar">语句</span><span class="t-mark">(可选)</span></td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-c23">(C23 起)</span></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">常量表达式</span></td>
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<td>-</td>
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<td>任何整数<a href="c-language-constant_expression.html">常量表达式</a></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，应用到标号</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>switch 语句体可拥有任意数量的 <code>case:</code> 标号，只要所有 <span class="t-spar">表达式</span> 均为独有（在<a href="c-language-conversion.html">转换</a>到 <span class="t-spar">表达式</span> 的<a href="c-language-conversion.html#.E6.95.B4.E6.95.B0.E6.8F.90.E5.8D.87">提升后类型</a>后）。至多可以存在一个 <code>default:</code> 标号（尽管嵌套的 switch 语句可使用其自身的 <code>default:</code> 标号，或拥有常量等于外围 switch 所用的 <code>case:</code> 的标号）。</p>
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<p>若 <span class="t-spar">表达式</span> 求值为等于一个 <span class="t-spar">常量表达式</span> 在转换到 <span class="t-spar">表达式</span> 的提升类型后的值，则转移控制到标号为该 <span class="t-spar">常量表达式</span> 的语句。</p>
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<p>若 <span class="t-spar">表达式</span> 求值为不匹配任何 <code>case:</code> 标号的值，而存在 <code>default:</code> 标号，则转移控制到标号为 <code>default:</code> 的语句。</p>
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<p>若 <span class="t-spar">表达式</span> 求值为不匹配任何 <code>case:</code> 标号的值，且不存在 <code>default:</code> 标号，则不执行 switch 体的任何部分。</p>
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<p>在 <span class="t-spar">语句</span> 中的任何位置遇到 <a href="c-language-break.html">break</a> 语句时，跳出语句体：</p>
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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="kw1">switch</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">case</span> <span class="nu0">1</span> <span class="sy4">:</span> <a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"1"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印“ 1 ”</span>
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<span class="kw1">case</span> <span class="nu0">2</span> <span class="sy4">:</span> <a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"2"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 然后打印“ 2 ”（“继续”）</span>
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<span class="br0">}</span></pre></div>
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</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="kw1">switch</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">case</span> <span class="nu0">1</span> <span class="sy4">:</span> <a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"1"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 打印“ 1 ”</span>
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<span class="kw1">break</span><span class="sy4">;</span> <span class="co1">// 并退出 switch</span>
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<span class="kw1">case</span> <span class="nu0">2</span> <span class="sy4">:</span> <a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"2"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">break</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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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， switch 语句建立<a href="c-language-scope.html">块作用域</a>：任何 <span class="t-spar">表达式</span> 的标识符在 <span class="t-spar">语句</span> 后离开作用域。</p>
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<p>若 <code>case:</code> 或 <code>default:</code> 标号在 VLA 或另一拥有可变修改类型的标识符的作用域内，则整个 switch 语句都必须在其作用域内（换言之， VLA 必须在整个 switch 前或最后的标号后声明）：</p>
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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="kw1">switch</span> <span class="br0">(</span>expr<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">4</span><span class="sy4">;</span> <span class="co1">// 非 VLA ；声明于此 OK</span>
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f<span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 决不调用</span>
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<span class="co1">// int a[i]; // 错误： VLA 不能声明于此</span>
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<span class="kw1">case</span> <span class="nu0">0</span><span class="sy4">:</span>
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i <span class="sy1">=</span> <span class="nu0">17</span><span class="sy4">;</span>
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<span class="kw1">default</span><span class="sy4">:;</span>
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<span class="kw4">int</span> a<span class="br0">[</span>i<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 在此声明 VLA OK</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%d<span class="es1">\n</span>"</span>, i<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 若 expr == 0 则打印 17 ，否则打印不确定值</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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<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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<p><br /></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-switch.html"><tt>switch</tt></a>, <a href="c-keyword-case.html"><tt>case</tt></a>, <a href="c-keyword-default.html"><tt>default</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="kw4">void</span> func<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"func(%d): "</span>, x<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">switch</span><span class="br0">(</span>x<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">case</span> <span class="nu0">1</span><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"case 1, "</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">case</span> <span class="nu0">2</span><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"case 2, "</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">case</span> <span class="nu0">3</span><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"case 3.<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
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<span class="kw1">case</span> <span class="nu0">4</span><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"case 4, "</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">case</span> <span class="nu0">5</span><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"case 5, "</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">default</span><span class="sy4">:</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"default.<span class="es1">\n</span>"</span><span class="br0">)</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="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="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> i <span class="sy1"><</span> <span class="nu0">9</span><span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span> func<span class="br0">(</span>i<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">func(1): case 1, case 2, case 3.
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func(2): case 2, case 3.
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func(3): case 3.
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func(4): case 4, case 5, default.
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func(5): case 5, default.
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func(6): default.
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func(7): default.
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func(8): default.</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>6.8.4.2 The switch statement (p: 108-109)</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>6.8.4.2 The switch statement (p: 149-150)</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>6.8.4.2 The switch statement (p: 134-135)</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>3.6.4.2 The switch 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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