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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><b>例5.7</b>&nbsp;下面一系列函数调用建立了表达式a－4＋c的语法树，见图5.6。在这个序列中，P<span class="down"><sub>1</sub></span>，p<span class="down"><sub>2</sub></span>，…，p<span class="down"><sub>5</sub></span>是指向结点的指针,entrya和entryc分别是指向符号表中标识符a和c的指针。 
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<p>(1)p<sub>1</sub>:=mkleaf(<b>id</b>，entrya); 
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<p>(2)p<sub>2</sub>:=mkleaf(<b>num</b>,4); 
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<p>(3)p<sub>3</sub>:=mknode('-',p<sub>1</sub>,p<sub>2</sub>); 
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<p>(4)p<sub>4</sub>:=mkleaf(<b>id</b>,entryc); 
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<p>(5)p<sub>5</sub>:=mknode('+',p<sub>3</sub>,p<sub>4</sub>); 
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<p align=center>图5.6 a-4+c的语法树</p>

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<p>这棵语法树是自底向上构造的。函数调用mkleaf(<b>id</b>,entry 
a)和mkleaf(<b>num</b>,4)建立了代表a和4的叶结点，指向这两个结点的指针分别存放在p<span class="down"><sub>1</sub></span>和p<span class="down"><sub>2</sub></span>中。函数调用mknode('-',p<span class="down"><sub>1</sub></span>,p<span class="down"><sub>2</sub></span>)建立内部结点，它以叶结点a和4为子结点。再经过两步，p<span class="down"><sub>5</sub></span>指向根结点。 
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<p>上面只对一个具体表达式进行了讨论，下面将指出，语法树的建立过程是由有关语法制导定义来确定的。 
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