350 lines
22 KiB
HTML
350 lines
22 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>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_statements 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">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>语句是带顺序执行的 C 程序段。任何函数体都是一条复合语句，继而为语句或声明的序列：</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="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> <span class="co1">// 复合语句的开始</span>
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<span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// 声明（非语句）</span>
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n <span class="sy1">=</span> n<span class="sy2">+</span><span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// 表达式语句</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"n = %d<span class="es1">\n</span>"</span>, n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 表达式语句</span>
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<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// 返回语句</span>
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<span class="br0">}</span> <span class="co1">// 复合语句之结尾，函数体之结尾</span></pre></div>
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</div>
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<p><br />
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语句有五种类型：</p>
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<div class="t-li1"><span class="t-li">1)</span> 复合语句</div>
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<div class="t-li1"><span class="t-li">2)</span> 表达式语句</div>
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<div class="t-li1"><span class="t-li">3)</span> 选择语句</div>
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<div class="t-li1"><span class="t-li">4)</span> 循环语句</div>
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<div class="t-li1"><span class="t-li">5)</span> 跳转语句</div>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-c23">
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<td>
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<p><a href="c-language-attributes.html">属性说明符序列</a> (<span class="t-spar">属性说明符序列</span>) 能应用到无标号语句，该情况下（除了表达式语句）属性应用到相应的语句。</p>
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</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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</table>
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<h3><span class="mw-headline" id=".E6.A0.87.E5.8F.B7">标号</span></h3>
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<p>任何语句都能<i>有标号</i>，通过在语句自身前提供一个跟随冒号的名称。</p>
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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><span class="t-spar">属性说明符序列</span><span class="t-mark">(可选)</span><span class="t-mark-rev t-since-c23">(C23 起)</span> <span class="t-spar">标识符</span> <code><b>:</b></code></td>
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<td>(1)</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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<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><span class="t-mark-rev t-since-c23">(C23 起)</span> <code><b>case</b></code> <span class="t-spar">常量表达式</span> <code><b>:</b></code></td>
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<td>(2)</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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<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><span class="t-mark-rev t-since-c23">(C23 起)</span> <code><b>default</b></code> <code><b>:</b></code></td>
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<td>(3)</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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<div class="t-li1"><span class="t-li">1)</span> <a href="c-language-goto.html">goto</a> 语句的目标。</div>
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<div class="t-li1"><span class="t-li">2)</span> <a href="c-language-switch.html">switch</a> 语句的 <code>case</code> 标号。</div>
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<div class="t-li1"><span class="t-li">3)</span> <a href="c-language-switch.html">switch</a> 语句的默认标号。</div>
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<p>任何语句（但非声明）可以前附任意数量的<i>标号</i>，每个都声明一个 <span class="t-spar">标识符</span> 为标号名，标号名必须在闭合的函数中唯一（换言之，标号名拥有<a href="c-language-scope.html">函数作用域</a>）。</p>
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<p>标号声明自身没有效果，不会以任何方式变更控制流，或修改跟随其后的语句的行为。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-c23">
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<td>
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<p>标号应当后随语句。</p>
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</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 class="t-rev t-since-c23">
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<td>
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<p>标号可以不带其后随语句出现。若标号单独出现在块中，则表现为如同它后随一条<a href="c-language-statements.html#.E8.A1.A8.E8.BE.BE.E5.BC.8F.E8.AF.AD.E5.8F.A5">空语句</a>。</p>
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<p>可选的 <a href="c-language-attributes.html"><span class="t-spar">属性说明符序列</span></a> 应用到标号。</p>
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</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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</table>
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<h3><span class="mw-headline" id=".E5.A4.8D.E5.90.88.E8.AF.AD.E5.8F.A5">复合语句</span></h3>
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<p>复合语句，或称<i>块</i>，是花括号所包围的语句与声明的序列。</p>
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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>{</b></code> <span class="t-spar">语句</span> <code>|</code> <span class="t-spar">声明</span>...<span class="t-mark">(可选)</span> <code><b>}</b></code></td>
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<td class="t-sdsc-nopad"></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>{</b></code> <span class="t-spar">无标号语句</span> <code>|</code> <span class="t-spar">标号</span> <code>|</code> <span class="t-spar">声明</span>...<span class="t-mark">(可选)</span> <code><b>}</b></code></td>
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<td class="t-sdsc-nopad"></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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<p>复合语句允许将一组声明和语句组合入一个单元，并将其任何在期待单个语句的场所使用（例如在 <a href="c-language-if.html">if</a> 语句或循环语句中）：</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">if</span> <span class="br0">(</span>expr<span class="br0">)</span> <span class="co1">// if 语句的开始</span>
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<span class="br0">{</span> <span class="co1">// 开始块</span>
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<span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// 声明</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>, n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 表达式语句</span>
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<span class="br0">}</span> <span class="co1">// 块结尾， if 语句结尾</span></pre></div>
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</div>
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<p>每个复合语句引入其自身的<a href="c-language-scope.html">块作用域</a>。</p>
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<p>拥有静态<a href="c-language-storage_duration.html">存储期</a>的变量的初始化器，及 VLA 声明符在每次控制流经过这些声明时按顺序执行，就像它们是语句一样：</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="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> <span class="co1">// 块的开始</span>
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<span class="br0">{</span> <span class="co1">// 块的开始</span>
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<a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"hello"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 表达式语句</span>
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<span class="kw4">int</span> n <span class="sy1">=</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"abc<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 声明，打印 "abc\n" ，将 4 存入 n</span>
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<span class="kw4">int</span> a<span class="br0">[</span>n<span class="sy2">*</span><a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"1<span class="es1">\n</span>"</span><span class="br0">)</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 声明，打印 "1\n" ，分配 8*sizeof(int)</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%zu<span class="es1">\n</span>"</span>, <span class="kw4">sizeof</span><span class="br0">(</span>a<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 表达式语句</span>
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<span class="br0">}</span> <span class="co1">// 块结束， n 与 a 的作用域结束</span>
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<span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">7</span><span class="sy4">;</span> <span class="co1">// n 可以重新使用</span>
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<span class="br0">}</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E8.A1.A8.E8.BE.BE.E5.BC.8F.E8.AF.AD.E5.8F.A5">表达式语句</span></h3>
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<p>跟随分号的表达式是一条语句。</p>
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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><span class="t-spar">表达式</span><span class="t-mark">(可选)</span> <code><b>;</b></code></td>
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<td>(1)</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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<tr class="t-sdsc">
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<td><span class="t-spar">属性说明符序列</span> <span class="t-spar">表达式</span> <code><b>;</b></code></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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<p>典型的 C 程序中大多数语句是表达式语句，例如赋值或函数调用。</p>
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<p>无表达式的表达式语句被称作<i>空语句</i>。它通常用于提供空循环体给 <a href="c-language-for.html">for</a> 或 <a href="c-language-while.html">while</a> 循环。它亦能用于在复合语句尾部或声明之前携带一个标号：</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"><a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"hello"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 表达式语句</span>
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<span class="kw4">char</span> <span class="sy2">*</span>s<span class="sy4">;</span>
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<span class="kw1">while</span> <span class="br0">(</span><span class="sy2">*</span>s<span class="sy2">++</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="st0">'<span class="es5">\0</span>'</span><span class="br0">)</span>
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<span class="sy4">;</span> <span class="co1">// 空语句</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-c23">
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<td>
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<p>可选的 <a href="c-language-attributes.html"><span class="t-spar">属性声明符序列</span></a> 应用到表达式。</p>
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<p>后随 <code><b>;</b></code> 的 <span class="t-spar">属性说明符序列</span> 不构成表达式。它替而构成<a href="c-language-declarations.html">属性声明</a>。</p>
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</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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</table>
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<h3><span class="mw-headline" id=".E9.80.89.E6.8B.A9.E8.AF.AD.E5.8F.A5">选择语句</span></h3>
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<p>选择语句根据表达式的值，选择数条语句之一执行。</p>
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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>if</b></code> <code><b>(</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>(1)</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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<tr class="t-sdsc">
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<td><code><b>if</b></code> <code><b>(</b></code> <span class="t-spar">表达式</span> <code><b>)</b></code> <span class="t-spar">语句</span> <code><b>else</b></code> <span class="t-spar">语句</span></td>
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<td>(2)</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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<tr class="t-sdsc">
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<td><code><b>switch</b></code> <code><b>(</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>(3)</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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<div class="t-li1"><span class="t-li">1)</span> <a href="c-language-if.html">if</a> 语句</div>
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<div class="t-li1"><span class="t-li">2)</span> <a href="c-language-if.html">if</a> 语句带 <code>else</code> 子句</div>
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<div class="t-li1"><span class="t-li">3)</span> <a href="c-language-switch.html">switch</a> 语句</div>
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<h3><span class="mw-headline" id=".E5.BE.AA.E7.8E.AF.E8.AF.AD.E5.8F.A5">循环语句</span></h3>
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<p>循环语句重复执行一条语句。</p>
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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>while</b></code> <code><b>(</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>(1)</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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<tr class="t-sdsc">
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<td><code><b>do</b></code> <span class="t-spar">语句</span> <code><b>while</b></code> <code><b>(</b></code> <span class="t-spar">表达式</span> <code><b>)</b></code> <code><b>;</b></code></td>
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<td>(2)</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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<tr class="t-sdsc">
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<td><code><b>for</b></code> <code><b>(</b></code> <span class="t-spar">初始化子句</span> <code><b>;</b></code> <span class="t-spar">表达式</span><span class="t-mark">(可选)</span> <code><b>;</b></code> <span class="t-spar">表达式</span><span class="t-mark">(可选)</span> <code><b>)</b></code> <span class="t-spar">语句</span></td>
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<td>(3)</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>
|
|
</tr>
|
|
</table>
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<div class="t-li1"><span class="t-li">1)</span> <a href="c-language-while.html">while</a> 循环</div>
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<div class="t-li1"><span class="t-li">2)</span> <a href="c-language-do.html">do-while</a> 循环</div>
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<div class="t-li1"><span class="t-li">3)</span> <a href="c-language-for.html">for</a> 循环</div>
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<h3><span class="mw-headline" id=".E8.B7.B3.E8.BD.AC.E8.AF.AD.E5.8F.A5">跳转语句</span></h3>
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|
<p>跳转语句无条件地转移控制流。</p>
|
|
<table class="t-sdsc-begin">
|
|
<tr>
|
|
<td colspan="10" class="t-sdsc-sep"></td>
|
|
</tr>
|
|
<tr class="t-sdsc">
|
|
<td><code><b>break</b></code> <code><b>;</b></code></td>
|
|
<td>(1)</td>
|
|
<td class="t-sdsc-nopad"></td>
|
|
</tr>
|
|
<tr>
|
|
<td colspan="10" class="t-sdsc-sep"></td>
|
|
</tr>
|
|
<tr class="t-sdsc">
|
|
<td><code><b>continue</b></code> <code><b>;</b></code></td>
|
|
<td>(2)</td>
|
|
<td class="t-sdsc-nopad"></td>
|
|
</tr>
|
|
<tr>
|
|
<td colspan="10" class="t-sdsc-sep"></td>
|
|
</tr>
|
|
<tr class="t-sdsc">
|
|
<td><code><b>return</b></code> <span class="t-spar">表达式</span><span class="t-mark">(可选)</span> <code><b>;</b></code></td>
|
|
<td>(3)</td>
|
|
<td class="t-sdsc-nopad"></td>
|
|
</tr>
|
|
<tr>
|
|
<td colspan="10" class="t-sdsc-sep"></td>
|
|
</tr>
|
|
<tr class="t-sdsc">
|
|
<td><code><b>goto</b></code> <span class="t-spar">标识符</span> <code><b>;</b></code></td>
|
|
<td>(4)</td>
|
|
<td class="t-sdsc-nopad"></td>
|
|
</tr>
|
|
<tr>
|
|
<td colspan="10" class="t-sdsc-sep"></td>
|
|
</tr>
|
|
</table>
|
|
<div class="t-li1"><span class="t-li">1)</span> <a href="c-language-break.html">break</a> 语句</div>
|
|
<div class="t-li1"><span class="t-li">2)</span> <a href="c-language-continue.html">continue</a> 语句</div>
|
|
<div class="t-li1"><span class="t-li">3)</span> <a href="c-language-return.html">return</a> 语句带可选的表达式</div>
|
|
<div class="t-li1"><span class="t-li">4)</span> <a href="c-language-goto.html">goto</a> 语句</div>
|
|
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
|
|
<div class="t-ref-std-17">
|
|
<ul>
|
|
<li>C17 标准（ISO/IEC 9899:2018）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>6.8 Statements and blocks (p: 106-112)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
<div class="t-ref-std-11">
|
|
<ul>
|
|
<li>C11 标准（ISO/IEC 9899:2011）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>6.8 Statements and blocks (p: 146-154)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
<div class="t-ref-std-c99">
|
|
<ul>
|
|
<li>C99 标准（ISO/IEC 9899:1999）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>6.8 Statements and blocks (p: 131-139)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
<div class="t-ref-std-c89">
|
|
<ul>
|
|
<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>3.6 STATEMENTS</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin"></table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|