187 lines
12 KiB
HTML
187 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>if 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_if 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">if 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>有条件地执行代码。</p>
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<p>用于仅若某条件为真才执行代码的场合。</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><span class="t-spar">属性说明符序列</span><span class="t-mark">(可选)</span> <code><b>if (</b></code> <span class="t-spar">表达式</span> <code><b>)</b></code> <span class="t-spar">语句真</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><span class="t-spar">属性说明符序列</span><span class="t-mark">(可选)</span> <code><b>if (</b></code> <span class="t-spar">表达式</span> <code><b>)</b></code> <span class="t-spar">语句真</span> <code><b>else</b></code> <span class="t-spar">语句假</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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</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>if</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>任何标量类型<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">语句真</span></td>
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<td>-</td>
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<td>任何<a href="c-language-statements.html">语句</a>（常为复合语句），若 <span class="t-spar">表达式</span> 比较不等于 <span class="t-c"><span class="mw-geshi c source-c">​<span class="nu0">0</span>​</span></span> 则执行</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">语句假</span></td>
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<td>-</td>
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<td>任何<a href="c-language-statements.html">语句</a>（常为复合语句），若 <span class="t-spar">表达式</span> 比较等于 <span class="t-c"><span class="mw-geshi c source-c">​<span class="nu0">0</span>​</span></span> 则执行</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><span class="t-spar">表达式</span> 必须为任何<a href="c-language-types.html#.E7.B1.BB.E5.9E.8B.E7.BB.84.E5.88.AB">标量类型</a>的表达式。</p>
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<p>若 <span class="t-spar">表达式</span> 与整数零比较不相等，则执行 <span class="t-spar">语句真</span> 。</p>
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<p>形式 <span class="t-v">(2)</span> 中，若 <span class="t-spar">表达式</span> 与整数零比较相等，则执行 <span class="t-spar">语句假</span> 。</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，整个 if 语句拥有其自身的块作用域：</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="kw2">enum</span> <span class="br0">{</span>a, b<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> different<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">if</span> <span class="br0">(</span><span class="kw4">sizeof</span><span class="br0">(</span><span class="kw2">enum</span> <span class="br0">{</span>b, a<span class="br0">}</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span>
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<span class="kw1">return</span> a<span class="sy4">;</span> <span class="co1">// a == 1</span>
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<span class="kw1">return</span> b<span class="sy4">;</span> <span class="co1">// C 89中 b == 0 ， C99 中 b == 1</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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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p><code>else</code> 始终与最接近的前接 <code>if</code> 相关联（换言之，若 <span class="t-spar">语句真</span> 也是 if 语句，则内层 if 必须也含有 <code>else</code> 部分）：</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> j <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>i <span class="sy1">></span> <span class="nu0">1</span><span class="br0">)</span>
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<span class="kw1">if</span><span class="br0">(</span>j <span class="sy1">></span> <span class="nu0">2</span><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">"%d > 1 and %d > 2<span class="es1">\n</span>"</span>, i, j<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">else</span> <span class="co1">// 此 else 是 if(j>2) 的一部分，不是 if(i>1) 的部分 </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 > 1 and %d <= 2<span class="es1">\n</span>"</span>, i, j<span class="br0">)</span><span class="sy4">;</span></pre></div>
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</div>
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<p>若通过 <a href="c-language-goto.html">goto</a> 进入 <span class="t-spar">语句真</span> ，则不执行 <span class="t-spar">语句假</span> 。</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-if.html"><tt>if</tt></a>, <a href="c-keyword-else.html"><tt>else</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">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="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>i <span class="sy1">></span> <span class="nu0">2</span><span class="br0">)</span> <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">"first is true<span class="es1">\n</span>"</span><span class="br0">)</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="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"first is false<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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i <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>i <span class="sy1">==</span> <span class="nu0">3</span><span class="br0">)</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"i == 3<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>i <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">3</span><span class="br0">)</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"i != 3 is true<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">else</span> <a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"i != 3 is false<span class="es1">\n</span>"</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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<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">first is false
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i == 3
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i != 3 is false</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>6.8.4.1 The if 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>6.8.4.1 The if statement (p: 148-149)</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>6.8.4.1 The if statement (p: 133-134)</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>3.6.4.1 The if 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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