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📁 建立《编译原理网络课程》的目的不仅使学生掌握构造编译程序的原理和技术
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<font class="title2"><b>7.5 CASE语句</b></font>  
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许多程序设计语言中含有“switch”语句或“case”语句。假定switch语句的语法如下: <BR>&nbsp;&nbsp;&nbsp; <B>&nbsp;&nbsp;&nbsp;&nbsp;<font color="#0000FF">switch</font></B>  
expression <BR>&nbsp;&nbsp;&nbsp; <B>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<font color="#0000FF">begin</font> </B><BR>&nbsp;&nbsp;&nbsp; <B>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<font color="#0000FF">case</font></B>  
 value<SPAN class=down><sub>1</sub></SPAN>： statement<SPAN class=down><sub>1</sub></SPAN> <BR>&nbsp;&nbsp;&nbsp;   
<B>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<font color="#0000FF">case</font></B>  
value<SPAN class=down><sub>2</sub></SPAN>： statement<SPAN   
class=down><sub>2</sub></SPAN> <BR>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;......................... <BR>&nbsp;&nbsp;&nbsp;   
<B>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<font color="#0000FF">case</font></B> value<SPAN class=down><sub>n-1</sub></SPAN> statement<SPAN   
class=down><sub>n-1</sub></SPAN> <BR>&nbsp;&nbsp;&nbsp; <B>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<font color="#0000FF">defalt</font></B>：statement<SPAN  
class=down><sub>n</sub></SPAN> <BR>&nbsp;&nbsp;&nbsp; <B>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<font color="#0000FF">end</font></B>  
<BR>&nbsp;&nbsp;&nbsp;&nbsp;这里expression是一个表达式，称为选择器，它将被计算出一个值。表达式后面跟随的value<SPAN 
class=down><sub>1</sub></SPAN>,value<SPAN class=down><sub>2</sub></SPAN>,…，value<SPAN 
class=down><sub>n-1</sub></SPAN>是表达式可能取的值，default称为"缺席值" 它用来在expression的值不等于任何value<sub>i</sub>时来匹配expression。对于switch的语句的预期翻译可表示如下： <BR>&nbsp;&nbsp;&nbsp; 1．对表达式求值；   
<BR>&nbsp;&nbsp;&nbsp; 2．在列出的value<SPAN class=down><sub>1</sub></SPAN>,value<SPAN  
class=down><sub>2</sub></SPAN>,…，value<SPAN 
class=down><sub>n-1</sub></SPAN>这些值中寻找与表达式的值相等的值。如果没有这样的值存在，则让“缺席值”与表达式匹配； <BR>&nbsp;&nbsp;&nbsp;   
3．执行在（2）中寻找到的值相联系的话句（某个statement）。 <BR>&nbsp;&nbsp;&nbsp; 这样，switch语句的目标代码结构可作如下安排： <BR>&nbsp;&nbsp;&nbsp;   
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;对expression求值并置于t的有关代码 <BR>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;goto test <BR>&nbsp;&nbsp;&nbsp;   
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;L<SPAN class=down><sub>1</sub></SPAN>:&nbsp;&nbsp; 有关statement<SPAN class=down><sub>1</sub></SPAN>的代码  
<BR>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;goto next <BR>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;L<SPAN   
class=down><sub>2</sub></SPAN>：&nbsp;&nbsp;有关statement<SPAN class=down><sub>2</sub></SPAN>的代码 <BR>&nbsp;&nbsp;&nbsp;   
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;goto next <BR>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;L<SPAN class=down><sub>n-1</sub></SPAN>：&nbsp;有关statement<SPAN  
class=down><sub>n-1</sub></SPAN>的代码 <BR>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;goto next <BR>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;L<SPAN   
class=down><sub>n</sub></SPAN>:&nbsp;&nbsp; 有关statement<sub>n</sub>的代码 <BR>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;goto next <BR>&nbsp;&nbsp;&nbsp;   
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;test：if t＝value<SPAN class=down><sub>1</sub></SPAN> goto L<SPAN class=down><sub>1</sub></SPAN>  
<BR>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if t＝value<SPAN class=down><sub>2</sub></SPAN> goto L<SPAN   
class=down><sub>2</sub></SPAN> <br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
.................<br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if t＝value<SPAN class=down><sub>n-1</sub>   
</SPAN>goto L<SPAN class=down><sub>n-1</sub></SPAN><br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;goto L<SPAN   
class=down><sub>n</sub></SPAN> <BR>&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;next： <BR>&nbsp;&nbsp;&nbsp;   
&nbsp;Pascal语言中的case语句与上述switch语句在形式上类似。
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