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<font class="title2">9.2.4.1 代码外提</font><br>


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减少循环中代码总数的一个重要办法是<font class = "definition2">代码外提</font>。这种变换是把循环不变计算（即产生的结果独立于循环执行次数的表达式）放到循环的前面。(注意：这里隐含着循环只存在一个入口)
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例如：下面是一个while语句：
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&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp				<font color="#0000FF">while</font>( i <= limit - 2 )<br>
&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp		循环体<br>
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假设while的循环体不改变limit的值，那么limit-2是循环不变计算。代码外提的结果是：
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&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp				t = limit - 2;<br>
&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp				<font color="#0000FF">while</font> ( i<=t )<br>
&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp		循环体
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当然while的循环体也不改变t的值。
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