📄 使用lex&yacc设计一个计算器的例子 - fohoo的专栏 - csdnblog.htm
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<DIV class=postTitle><A
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width=15 border=0> 使用LEX&YACC设计一个计算器的例子</A> </DIV>
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<P><STRONG>calc.l</STRONG></P>
<P>%{<BR>#include <math.h><BR>#include <stdlib.h><BR>#include
<stdio.h><BR>#include "calc.tab.h"<BR>%}</P>
<P>%option noyywrap</P>
<P>%%<BR>([0-9])+|([0-9])*"."([0-9])+ { yylval = atof( yytext
); /*printf("yytext:%s, yylval:%f\n", yytext, yylval);*/ return NUM; }<BR>[
\t]+<BR>"\n"
{ return '\n';
}<BR>.
{ return *yytext; }</P>
<P>%%<BR></P>
<P><STRONG> calc.y</STRONG></P>
<P>%{<BR> #define YYSTYPE double<BR> #include
<math.h><BR> #include <stdio.h><BR> int yylex
(void);<BR> void yyerror (char const *);<BR>%}</P>
<P>/* Bison declarations. */<BR>%token NUM<BR>%left '-' '+'<BR>%left '*'
'/'<BR>%left NEG /* negation--unary minus */<BR>%right
'^' /* exponentiation */</P>
<P>%% /* The grammar follows. */<BR>input: /* empty
*/<BR> | input line<BR>;</P>
<P>line: '\n'<BR> |
exp '\n' { printf ("%f\n", $1); }<BR>;</P>
<P>exp:
NUM
{ $$ = $1; /*printf("%f =
%f\n", $$,
$1);*/
}<BR> | exp '+'
exp { $$ = $1 +
$3; /*printf("%f = %f + %f\n", $$, $1,
$3);*/ }<BR> | exp
'-' exp { $$ = $1 -
$3; /*printf("%f = %f - %f\n", $$, $1,
$3);*/ }<BR> | exp
'*' exp { $$ = $1 *
$3; /*printf("%f = %f * %f\n", $$, $1,
$3);*/ }<BR> | exp
'/' exp { $$ = $1 /
$3; /*printf("%f = %f / %f\n", $$, $1,
$3);*/ }<BR> | '-'
exp %prec NEG { $$ =
-$2; /*printf("%f =
-%f\n",
$2);*/
}<BR> | exp '^'
exp { $$ = pow ($1, $3); /*printf("%f
= pow(%f, %f)\n", $$, $1, $3);*/ }<BR> | '('
exp ')' { $$ =
$2; /*printf("%f =
(%f)\n", $$,
$2);*/
}<BR>;<BR>%%</P>
<P><BR>然后使用<A href="http://gnuwin32.sourceforge.net/packages/flex.htm">FLEX</A>,
<A href="http://gnuwin32.sourceforge.net/packages/bison.htm">BISON</A>生成C源程序</P>
<P>flex calc.l -->lex.yy.c<BR>bison -d calc.y
-->calc.tab.c(-d生成头文件calc.tab.h给calc.l使用)</P>
<P>然后在calc.tab.c加入:</P>
<P>int main()<BR>{<BR> return yyparse(); //调用语法分析yyparse<BR>}</P>
<P>void yyerror(char const *s)<BR>{<BR> fprintf(stderr, "%s\n", s);
//语法分析出错显示<BR>}</P>
<P>使用VC编译生成calc.exe运行。</P><BR><BR>
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