190 lines
15 KiB
HTML
190 lines
15 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>for 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_for 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">for 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>执行循环。</p>
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<p>用作 <a href="c-language-while.html">while 循环</a>的简短等价版本。</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-rev t-since-c23">(C23 起)</span><span class="t-mark">(可选)</span> <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">条件表达式</span> <code><b>;</b></code> <span class="t-spar">迭代表达式</span> <code><b>)</b></code> <span class="t-spar">循环语句</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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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p>表现如下：</p>
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<ul>
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<li><span class="t-spar">初始化子句</span> 可以为表达式<span class="t-rev-inl t-since-c99"><span>或声明</span> <span><span class="t-mark-rev t-since-c99">(C99 起)</span></span></span></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>作为表达式的 <span class="t-spar">初始化子句</span> 在首次求值 <span class="t-spar">条件表达式</span> 前被求值一次，并舍弃其结果。</li>
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</ul>
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</dd>
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</dl>
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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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<dl>
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<dd>
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<ul>
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<li>作为声明的 <span class="t-spar">初始化子句</span> 在整个循环体内都处于作用域内，包括 <span class="t-spar">初始化子句</span> 的剩余部分、整个<span class="t-spar">条件表达式</span>、整个 <span class="t-spar">迭代表达式</span> 及整个<span class="t-spar">循环语句</span>。声明于此声明的变量只允许有 <code>auto</code> 和 <code>register</code> <a href="c-language-storage_duration.html">存储类说明符</a>。</li>
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</ul>
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</dd>
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</dl>
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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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<ul>
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<li><span class="t-spar">条件表达式</span> 在循环体前求值。若表达式的结果是零，则循环立即退出。</li>
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<li><span class="t-spar">迭代表达式</span> 在循环体后求值，并舍弃其结果。求值 <span class="t-spar">迭代表达式</span> 后，将控制转移到 <span class="t-spar">条件表达式</span>。</li>
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</ul>
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<p><span class="t-spar">初始化子句</span> 、 <span class="t-spar">条件表达式</span> 和 <span class="t-spar">迭代表达式</span> 都是可选的。若忽略 <span class="t-spar">条件表达式</span> ，则它被替换成非零的整数常量，这使得循环不终止：</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">for</span><span class="br0">(</span><span class="sy4">;;</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">"endless loop!"</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><span class="t-spar">循环语句</span> 不是可选的，但它可以是空语句：</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">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="nu0">10</span><span class="sy4">;</span> <span class="sy2">++</span>n, <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="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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<p>若需要在某些点终止循环的执行，则可在 <span class="t-spar">循环语句</span> 中的任何位置使用 <a href="c-language-break.html">break 语句</a>。</p>
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<p><span class="t-spar">循环语句</span> 内任何位置使用的 <a href="c-language-continue.html">continue 语句</a>会将控制转移到<span class="t-spar">迭代表达式</span>。</p>
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<p>若无限循环在其 <span class="t-spar">条件表达式</span> 、 <span class="t-spar">迭代表达式</span> 或 <span class="t-spar">循环语句</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">条件表达式</span> 被忽略或为常量表达式的循环： <code>for(;;)</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， for 语句建立<a href="c-language-scope.html">块作用域</a>：任何于 <span class="t-spar">初始化子句</span> 、 <span class="t-spar">条件表达式</span> 或 <span class="t-spar">迭代表达式</span> 中引入的标识符在 <span class="t-spar">循环语句</span> 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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<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><span class="t-spar">属性说明符序列</span> 是<a href="c-language-attributes.html">属性</a>的可选序列，应用到 <code>for</code> 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=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="c-keyword-for.html"><tt>for</tt></a></p>
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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>用作 <span class="t-spar">循环语句</span> 的表达式语句建立其自身的，异于 <span class="t-spar">初始化子句</span> 作用域的块作用域，不同于 C++ 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">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="sy4">;</span> <span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">long</span> i <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// C 中合法， C++ 中非法</span>
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<span class="co1">// ...</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可以用 <a href="c-language-goto.html">goto</a> 进入循环体，以此方式进入循环时，不执行 <span class="t-spar">初始化子句</span> 和 <span class="t-spar">条件表达式</span> 。（若控制抵达循环体尾，则可能出现包括执行 <span class="t-spar">条件表达式</span> 的重复。）</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="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="kw4">int</span> array<span class="br0">[</span>SIZE<span class="br0">]</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><a href="c-types-size_t.html"><span class="kw100">size_t</span></a> i <span class="sy1">=</span> <span class="nu0">0</span> <span class="sy4">;</span> i <span class="sy1"><</span> SIZE<span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span>
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array <span class="br0">[</span>i<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-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"Array filled!<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><a href="c-types-size_t.html"><span class="kw100">size_t</span></a> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><</span> SIZE<span class="sy4">;</span> <span class="sy2">++</span>i<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 "</span>, array<span class="br0">[</span>i<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="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">Array filled!
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1 0 1 1 1 1 0 0</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>6.8.5.3 The for statement (p: 110)</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>6.8.5.3 The for 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>6.8.5.3 The for 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>3.6.5.3 The for 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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