lex／yacc实现计算器

cal.y

%{
#include 
#include "lex.yy.c"
#define YYSTYPE int  
int yyparse(void);
%}
%token INTEGER PLUS MINUS TIMES DIVIDE LP RP
%%
command : exp {printf("%d/n",$1);}

exp: exp PLUS term {$$ = $1 + $3;}
    |exp MINUS term {$$ = $1 - $3;}
    |term {$$ = $1;}
    ;
term : term TIMES factor {$$ = $1 * $3;}
    |term DIVIDE factor {$$ = $1/$3;}
    |factor {$$ = $1;}
    ;
factor : INTEGER {$$ = $1;}
    | LP exp RP {$$ = $2;}
    ;
%%
int main()
{
    return yyparse();
}
void yyerror(char* s)
{
    fprintf(stderr,"%s",s);
}
int yywrap()
{
    return 1;
 }

cal.l

%{
#include  
#include "y.tab.h"  
extern int yylval;  
%}  
numbers ([0-9])+  
plus "+"  
minus "-"  
times "*"  
divide "/"  
lp "("  
rp ")"  
delim [ /n/t]  
ws {delim}*  
%%  
{numbers} {sscanf(yytext, "%d", &yylval); return INTEGER;}  
{plus} {return PLUS;}  
{minus} {return MINUS;}  
{times} {return TIMES;}  
{divide} {return DIVIDE;}  
{lp} {return LP;}  
{rp} {return RP;}  
{ws}       ;   
. {printf("Error");exit(1);}    
%% 

使用方式：

yacc -d cal.y 
lex cal.l
g++ -o cal y.tab.c 

运行./cal 然后输入3+4 ctrl+D就可以看到结果了