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<h2 align="center"><a name="_top"></a><font face="楷体_GB2312">9.3 条件编译</font></h2>
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<td width="50%" align="center"><a href="c93.htm#c931.html#c931">条件编译</a></td>
<td width="50%" align="center"><a href="c93.htm#c932.html#c932">练习题</a></td>
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<h3><a name="c931"></a>1.条件编译</h3>
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<p>在这一课里, 我们将学习:</p>
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<td width="139">#ifdef 标识符<br>
语句序列<br>
#endif</td>
<td width="152">#ifndef 标识符<br>
语句序列<br>
#endif</td>
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<td width="139">#ifdef 标识符<br>
语句序列 1<br>
#else<br>
语句序列 2<br>
#endif</td>
<td width="152">#ifdef 表达式<br>
语句序列 1<br>
#else<br>
语句序列 2<br>
#endif</td>
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href="javascript:showwin('c93_11.htm',null,'toolbar=0,location=0,directories=0,status=0,menubar=0,scrollbars=1,resizable=0,copyhistory=0,width=600,height=500')">请看它们的解释</a>:<img
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<th width="100%" colspan="4" height="16" bgcolor="#FF9933">主要规则</th>
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<td width="25%" height="77">#if 常量表达式<br>
<br>
检查常量表达式计算值是否为非 0。</td>
<td width="25%" height="77">#ifdef标识符<br>
<br>
检查标识符当前是否预定义过。</td>
<td width="25%" height="77">#else<br>
else_语句序列;<br>
#endif <br>
当前表达式为假时, else_语句序列将被编译。</td>
<td width="25%" height="77">#undef 标识符<br>
如果标识符以前定义过, 那么 #undef 后,
标识符就被编译器认为是未定义的了。</td>
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<h3><a name="c932"></a>2.练习题</h3>
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<p> </p>
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