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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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<table><tr><td>&nbsp&nbsp&nbsp&nbsp</td>
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<p> 
如果存在一个直接推导序列：<br> 
<center>α<sub>0</sub> <img src="IMG/equal.gif" width="19" height="13"></img> α<sub>1</sub> 
<img src="IMG/equal.gif" width="19" height="13"></img> α<sub>2</sub> 
<img src="IMG/equal.gif" width="19" height="13"></img> … 
<img src="IMG/equal.gif" width="19" height="13"></img> α<sub>n</sub>(n>0) </center> 
则我们称这个序列是一个从α<sub>0</sub>至α<sub>n</sub>的<font class="definition2">长度为n的推导</font>(简称推导)。或说α<sub>0</sub>可推导出α<sub>n</sub>，或α<sub>n</sub>可归约到α<sub>0</sub>。记作： 
<center> α<sub>0</sub><img src="IMG/equaladd.gif"  width="20" height="19">α<sub>n</sub> </center> 
即α<sub>0</sub><img src="IMG/equaladd.gif"  width="20" height="19">α<sub>n</sub>表示：从α<sub>0</sub>出发，经一步或若干步，可推导出α<sub>n</sub>。而用: 
<center> α<sub>0</sub><img src="IMG/equalstar.gif"  width="20" height="23">α<sub>n</sub> </center> 
表示：从α<sub>0</sub>出发，经0步或若干步，可推导出α<sub>n</sub>。换言之，α<sub>0</sub><img src="IMG/equalstar.gif"  width="20" height="23">α<sub>n</sub>意味着，或者α<sub>0</sub>=α<sub>n</sub>或者α<sub>0</sub><img src="IMG/equaladd.gif"  width="20" height="19">α<sub>n</sub>。<br> 
例如可从前面所得到的直接推导序列<br> 
<center> E <img src="IMG/equal.gif" width="19" height="13"></img> E+T 
<img src="IMG/equal.gif" width="19" height="13"></img> … 
<img src="IMG/equal.gif" width="19" height="13"></img> a+a </center> 
中看到：<br>  
<center> E<img src="IMG/equaladd.gif"  width="20" height="19">E+T </center> 
是长度为1的推导。而 <br> 
<center> E<img src="IMG/equaladd.gif"  width="20" height="19">T+T </center> 
是长度为2的推导。类似于此，<br> 
<center> E<img src="IMG/equaladd.gif"  width="20" height="19">a+a </center> 
是长度为6的推导。      
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<p> 
在图2.2中列出了上述推导中的每一个直接推导的详细情况。 <br><br> 
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<td width=25% align=center>αAβ</td> 
<td width=25% align=center>αγβ</td> 
<td width=26% align=center>所用规则(产生式)</td> 
<td width=12% align=center>α</td> 
<td width=12% align=center>β</td> 
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<td align=center>E</td> 
<td>&nbsp <img src="IMG/equal.gif" width="19" height="13"></img> E+T</td> 
<td>&nbspE->E+T</td> 
<td align=center>ε</td> 
<td align=center>ε</td> 
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<td align=center>E+T</td> 
<td>&nbsp <img src="IMG/equal.gif" width="19" height="13"></img> T+T</td> 
<td>&nbspE->T</td> 
<td align=center>ε</td> 
<td align=center>+T</td> 
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<td align=center>T+T</td> 
<td>&nbsp <img src="IMG/equal.gif" width="19" height="13"></img> F+T</td> 
<td>&nbspT->F</td> 
<td align=center>ε</td> 
<td align=center>+T</td> 
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<td align=center>F+T</td> 
<td>&nbsp <img src="IMG/equal.gif" width="19" height="13"></img> a+F</td> 
<td>&nbspF-&gt;a</td> 
<td align=center>ε</td> 
<td align=center>+T</td> 
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<td align=center>a+T</td> 
<td>&nbsp <img src="IMG/equal.gif" width="19" height="13"></img> a+F</td> 
<td>&nbspT->F</td> 
<td align=center>a+</td> 
<td align=center>ε</td> 
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<td align=center>a+F</td> 
<td>&nbsp <img src="IMG/equal.gif" width="19" height="13"></img> a+a</td> 
<td>&nbspF-&gt;a</td> 
<td align=center>a+</td> 
<td align=center>ε</td> 
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<center>图2.2 直接推导的例子</center> 

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