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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>4.9.2 用Yacc处理二义文法   
</b></font> 
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<font size="3"><font color="#0000FF">%{</font><br>
<font color="#0000FF">
      #include</font> &lt;ctype.h&gt;<br> 
<font color="#0000FF">
      #include</font> &lt;stdio.h&gt;<br> 
<font color="#0000FF">
      #define</font> YYSTYPE double <font color="#008000">/*Yacc栈定义为double类型*/</font><br>
<font color="#0000FF">
      %}</font><br>
<font color="#0000FF">%</font>token NUMBER<br> 
<font color="#0000FF">%</font>left '+' '-'<br> 
<font color="#0000FF">%</font>left '*' '/'<br> 
<font color="#0000FF">%</font>right UMINUS<br> 
<font color="#0000FF">
      %%</font><br>
      lines&nbsp;&nbsp;&nbsp; :lines expr '\n'&nbsp; {printf(&quot;%g\n&quot;,$2);}<br> 
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |lines '\n'<br> 
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |/*<span style="mso-bidi-font-size: 12.0pt; font-family: 宋体; mso-bidi-font-family: Times New Roman; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">ε*/<br> 
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;<br> 
      expr&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; :expr '+' expr {$$=$1+$3;}<br> 
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |expr '-' expr  
      {$$=$1-$3;}<br>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |expr '*' expr  
      {$$=$1*$3;}<br>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |expr '/' expr  
      {$$=$1/$3;}<br>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; |'(' expr %  
      ')'{$$=$2;}<br>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | '-' expr %prec  
      UMINUS {$$=-$2;}<br> 
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | NUMBER<br> 
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;<br> 
<font color="#0000FF">
      %%</font><br>
      </span><span style="mso-bidi-font-size: 12.0pt; font-family: 宋体; mso-bidi-font-family: Times New Roman; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA">yylec(){<br>
      &nbsp;&nbsp; <font color="#0000FF">int</font> c;<br> 
      &nbsp;&nbsp; <font color="#0000FF">while</font> ((c=getchar())==' ');<br> 
      &nbsp;&nbsp; <font color="#0000FF">if</font>((c=='.')||(isdigit(c))){<br>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ungetc(c,stdin);<br> 
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; scanf(&quot;%1f&quot;,&amp;yylval);<br> 
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <font color="#0000FF">return</font> NMUBER;<br> 
      &nbsp;&nbsp;&nbsp;&nbsp; }<br> 
      &nbsp;&nbsp; <font color="#0000FF">return</font> c:</span></font> 
<p><span style="mso-bidi-font-size: 12.0pt; font-family: 宋体; mso-bidi-font-family: Times New Roman; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA"><font size="3">}</font></span>
      </p>
<p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;图4.30&nbsp;&nbsp; 更高级的台式计算器的Yacc源程序 </p>
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<p>因为图4.30的Yacc说明的文法是二义的,LALR算法将产生有冲突的分析动作。Yacc会报告产生地分析动作的冲突数。项目集和分析动作冲突的描述可以在调用Yacc时加-V选择得到．这个选择产生一个附加的文件y.output，它包含分析时发现的项目集的核，由LALR算法产生的分析动作冲突的描述，以及LR分析表的可读表示，它给出分析动作冲突是怎样解决的。当Yacc报告它发现分析动作冲突时，明智的做法是建立和查阅文件y.output，以明白为什么会出现分析动作的冲突和它们是否已经正确解决。
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