📄 5.5.1_1.htm

📁 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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<p>为了使其它的左递归翻译模式也适于预测分析，我们将进行更一般化的讨论。譬如，假设我们有下面的翻译模式:
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<p>&nbsp;&nbsp;&nbsp;&nbsp; A→A<span class="down"><sub>1</sub></span>Y｛A.a:=g(A<span class="down"><sub>1</sub></span>.a,Y.y)} &nbsp&nbsp&nbsp&nbsp   
（5.2）<br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A→X&nbsp; ｛A.a:=f(X.x)}<br>   
&nbsp;&nbsp;&nbsp;
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<p>对（5.2）应用第四章中消除左递归的算法，可将其转换成下面的文法<br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A→X R &nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp;&nbsp;&nbsp;（5．3）<br>   
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;R→Y R|ε   
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<p>再考虑语义动作，翻译模式变为：<br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A→X&nbsp;｛R.i:=f(X.x)}<br>   
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp&nbsp&nbsp;R
&nbsp;｛A.a:=R.s}<br>   
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;R→Y&nbsp;｛R<span class="down"><sub>1</sub></span>.i:=g（R.i,Y.y）}    
&nbsp&nbsp&nbsp&nbsp（5．4）<br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp&nbsp;
 R<span class="down"><sub>1</sub></span>&nbsp;｛R.s:=R<span class="down"><sub>1</sub></span>.s}<br>   
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;R→ε｛R.s:=R.i}   
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<p>经过转换的翻译模式与图5.14中一样使用R的继承属性i和综合属性s。要看清楚为什么（5.2）和（5.4）的结果是一样的，我们考虑图5.16中两棵带注释的分析树。图5.16（a）中A.a的值是根据（5.2）由底向上来计算的。图5.16（b）中包含了根据（5.4）从上到下对R.i的计算。而且最下面的R.i值不变地被传递到上面作为R.s的值，并作为根结点A的A.a值（R.s在图5.16（b)中没有被表示出来）。
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