[lex-yacc] Simple Expression evalution using lex & yacc

CAL1.l----------------------------------------------------------------------

%{

#include <stdlib.h>

#include <stdio.h>

#include "y.tab.h"

void yyerror(char*);



%}



%%

[ \t]+ ;

[0-9]+\.[0-9]+ {yylval.fval=atof(yytext);return FLOAT;}

[0-9]+ {yylval.ival=atoi(yytext);return INTEGER;}

[-+*/] {return *yytext;}

"(" {return *yytext;}

")" {return *yytext;}

\n {return *yytext;}

. {printf("invalid character: %c",*yytext);}







CAL1.Y--------------------------------------------------------------------------------------------------

%{

#include <stdlib.h>

#include <stdio.h>

int yylex(void);



%}

%union  {int ival; float fval;}



%token <ival> INTEGER

%token <fval> FLOAT

%type <fval> expr

%type <fval> mulex

%type <fval> term

%%



program:

       line program

       | line

line:

       expr '\n' { printf("%f\n",$1); }

       | '\n'

expr:

       expr '+' mulex { $$ = $1 + $3; }

       | expr '-' mulex { $$ = $1 - $3; }

       | mulex { $$ = $1; }

mulex:

       mulex '*' term { $$ = $1 * $3; }

       | mulex '/' term { $$ = $1 / $3; }

       | term { $$ = $1; }

term:

       '(' expr ')' { $$ = $2; }

       | INTEGER { $$ = $1; }

       | FLOAT {$$ = $1;}

%%

void yyerror(char *s)

{

fprintf(stderr,"%s\n",s);

return;

}



int main(void)

{

yyparse();

return 0;

}

How to run it?

$lex cal.l 

$yacc -d cal.y

$gcc -o cal lex.yy.c y.tab.c -ll

$./cal