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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>4.9.1 书写Yacc的源程序   
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<p>Yacc源程序由三部分组成<br> 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;声明<br> 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;％％<br> 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;翻译规则<br> 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;％％<br> 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;C写的支持例程 
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<p><font class="example">例4.23 </font>为了说明怎样准备Yacc源程序，让我们构造一个简单的台式计算器，它读入一个算术表达式，计算它，并打印它的值。建立台式计算器从下面算术表达式的文法开始。<br> 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;E→E+T|T<br> 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;T→T*F|F<br> 
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;F→(E)|digit 
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<p>记号digit是0-9的单个数字。根据这个文法书写的台式计算器的Yacc源程序见图4.29。 
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<font color="#0000FF"> 
	%{</font><br> 
<font color="#0000FF"> 
        #include</font> &lt;ctype.h><br>   
<font color="#0000FF">  
        %}</font><br> 
<font color="#0000FF">%</font>token DIGIT<br>   
<font color="#0000FF">  
        %%</font><br> 
        line	: expr '\n'&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; {printf("%d\n",$1);}<br> 
        &nbsp;&nbsp;&nbsp;&nbsp; ;<br>  
        expr	: expr '+' term&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; {$$=$1+$3;}<br>  
        &nbsp;&nbsp;&nbsp;&nbsp; | term<br>   
        &nbsp;&nbsp;&nbsp;&nbsp; ;<br>  
        term	: term '*' factor&nbsp;&nbsp;&nbsp;&nbsp; {$$=$1*$3;}<br>  
        &nbsp;&nbsp;&nbsp;&nbsp; | factor<br>   
        &nbsp;&nbsp;&nbsp;&nbsp; ;&nbsp;<br>  
        factor	: '(' expr ')'&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; {$$=$2;}<br>  
        &nbsp;&nbsp;&nbsp;&nbsp; | DIGIT<br>   
        &nbsp;&nbsp;&nbsp;&nbsp; ;<br>  
<font color="#0000FF"> 
        %%</font><br> 
        yylec(){<br> 
        &nbsp;&nbsp;&nbsp;&nbsp; <font color="#0000FF">int</font> c;<br>   
        &nbsp;&nbsp;&nbsp;&nbsp; c=getchar();<br>  
        &nbsp;&nbsp;&nbsp;&nbsp; <font color="#0000FF">if</font>(isdigit(c)) {<br>   
        &nbsp;&nbsp;&nbsp;&nbsp; yylval = c - '0';<br>   
        &nbsp;&nbsp;&nbsp;&nbsp; <font color="#0000FF">return</font> DIGIT;<br>   
        &nbsp;&nbsp;&nbsp;&nbsp; }<br>  
<font color="#0000FF"> 
	return</font> c;<br>   
        } 
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        <p >&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
        图4.29&nbsp;&nbsp; 简单台式机计算器的yacc源程序 
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