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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3．7 </b>对于表3.5中单词,我们给出识别它的lex程序，考察一下此程序</p>
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						<font color=blue>%{ </font><br>
						<font color=green>/*显明常量 <br>
						LT，LE，EQ，NE，GT，GE， <br>
						IF， THEN， ELSE，ID， NUMBER， RELOP的定义*/ </font><br>
						<font color=blue>%}</font> <br>
						<font color="green">/*正规定义*/ </font><br>
						delim &nbsp&nbsp&nbsp&nbsp;[\t\n] <br>
						ws &nbsp&nbsp&nbsp&nbsp;{delim}+ <br>
						<br>
						letter &nbsp&nbsp&nbsp&nbsp;[A－Za-z] <br>
						digit &nbsp&nbsp&nbsp&nbsp;[0－9] <br>
						id &nbsp&nbsp&nbsp&nbsp;letter}({letter}|{digit})* <br>
						 number &nbsp&nbsp&nbsp&nbsp;{digit}+(\.{digit}+)?(E[+\-]?{digit}+)? <br>
						<font color=blue>%% </font><br>
						{ws}&nbsp&nbsp&nbsp&nbsp;{<font color="green">/*没有动作，也不返回*/</font>} <br>
						if &nbsp&nbsp&nbsp&nbsp;{<font color=blue>return</font> (IF);} <br>
						then &nbsp&nbsp&nbsp&nbsp;{<font color=blue>return</font> (THEN);} <br>
						else &nbsp&nbsp&nbsp&nbsp;{ <font color=blue>return</font> (ELSE);} <br>
						{id}&nbsp&nbsp&nbsp&nbsp;{yylval ＝ install_id(); <font color=blue>return</font>(ID);} <br>
						{number}&nbsp&nbsp&nbsp&nbsp;{yylval＝install_num();<font color=blue>return</font>(NUMBER);} <br>
						“＜”&nbsp&nbsp&nbsp&nbsp;{yylval ＝ LT; <font color=blue>return</font> (RELOP);J <br>
						“＜＝”&nbsp&nbsp&nbsp&nbsp;{yylval=LE; <font color=blue>return</font>(RELOP);} <br>
						“＝” &nbsp&nbsp&nbsp&nbsp;{yylval=EQ; <font color=blue>return</font>(RELOP);} <br>
						“＜＞”&nbsp&nbsp&nbsp&nbsp;{yylval ＝ NE; <font color=blue>return</font>(RELOP);) <br>
						“＞”&nbsp&nbsp&nbsp&nbsp;{yylval ＝ GT; <font color=blue>return</font>(RELOP);} <br>
						“＞＝”&nbsp&nbsp&nbsp&nbsp;{yylval ＝ GE; <font color=blue>return</font>(RELOP)} <br>
						<font color=blue>%% </font><br>
						install_id()<br>
                                                   &nbsp&nbsp&nbsp&nbsp;  {
						<font color="green">/*把单词装入符号表并返回指针。 <br>
						yytext指向该单词的第一个字符， <br>
						yyleng给出它的长度*/ <br>
						</font>} <br>
						install_num()<br>
                                                   &nbsp&nbsp&nbsp&nbsp;  {
						<font color="green">/*类似上面过程，但单词是数*/ </font>}
            <P></P><!--p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;图 3.18</p-->
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			<p>在声明节，可以看见由翻译规则仅用的某些显明常量的声明，这些声明由特别的括号<font class="emphasize">%{</font>和<font class="emphasize">%}</font>包围。出现在括号中的任何东西都直接抄写到词法分析器lex.yy.c中，不作为正规定义和翻译规则的一部分。在第三节中的辅助过程也按同样方式处理，程序中有两个过程install_id()和install_num()，它们由翻译规则使用，被逐字抄入lex.yy.c中。 </p>
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