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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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<p>当使用产生式B→B<span class="down"><sub>1</sub></span> sub B<span class="down"><sub>2</sub></span>时，函数shrink使B<span class="down"><sub>2</sub></span>.ps减少30％。当用函数disp计算B.ht时，可以把B<span class="down"><sub>2</sub></span>的盒子向下放置。这里，产生实际排字命令的规则没有表示出来。 
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表5.7中的语法制导定义是L-属性定义，唯一的继承属性是非终结符号B的ps属性。在所有的语义规则中，它仅由产生式左边非终结符号的继承属性来定义或是一个常数。因此这个语法制导定义是L-属性定义。
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<p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S→｛B.ps:=10}<br> 
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B｛S.ht:=B.ht}<br> 
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B→ {B<sub>1</sub>.ps:=B.ps}<br> 
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B<sub>1</sub>  
{B<sub>2</sub>.ps:=B.ps}<br> 
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B<sub>2</sub>｛B.ht:=max(B<sub>1</sub>.ht,B<sub>2</sub>.ht)}<br> 
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B→ {B<sub>1</sub>.ps:=B.ps}<br> 
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B<sub>1</sub><br> 
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;sub  
{B<sub>2</sub>.ps:=shrink(B.ps)}<br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B<sub>2</sub>｛B.ht:=disp(B<sub>1</sub>.ht,B<sub>2</sub>.ht)}<br> 
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B→text｛B.ht:=text.h*B.ps} 
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<p align=center>图5.12 &nbsp&nbsp从表5.7构造的翻译模式</p> 

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图5.12中的翻译模式是通过在产生式中插入与表5.7中语义规则相应的语义动作得到的，当然还要满足上面所述的三个条件。为了可读性好，产生式中每个文法符号都写在不同的行上，右边表示的是动作。于是<br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;S→{B.ps:=10} 
B{S.ht:=B.ht}<br>
写成<br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;S→ ｛B.ps:=10｝<br>
&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;B&nbsp;｛S.ht:=B.ht}<br>
注意，置继承属性B<span class="down"><sub>1</sub></span>.ps和B<span class="down"><sub>2</sub></span>.ps值的动作正好出现在产生式右边B<span class="down"><sub>1</sub></span>和B<span class="down"><sub>2</sub></span>的前面。
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