Design and Implement a Calculator using ‘YACC’ and ‘LEX’

Calc.y

%{
#include <stdlib.h>
#include <stdio.h>
int yylex(void);
#include "y.tab.h"
%}

%token INTEGER

%%
program:
line program
| line

line:
expr '\n' { printf("%d\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; }

%%
void yyerror(char *s)
{
fprintf(stderr,"%s\n",s);
return;
}
yywrap()
{
  return(1);
}

int main(void)
{
/*yydebug=1;*/
yyparse();
return 0;
}

Lexx.l

%{
#include <stdlib.h>
#include <stdio.h>
#include "y.tab.h"
void yyerror(char*);
extern int yylval;
%}


%%
[ \t]+ ;
[0-9]+ {yylval = atoi(yytext);
return INTEGER;}
[-+*/] {return *yytext;}
"(" {return *yytext;}
")" {return *yytext;}
\n {return *yytext;}
. {char msg[25];
sprintf(msg,"%s <%s>","invalid character",yytext);
yyerror(msg);}


Output :-

[S@localhost Desktop]$ flex lexx.l
[S@localhost Desktop]$ yacc -d calc.y
[S@localhost Desktop]$ gcc y.tab.c lex.yy.c
[S@localhost Desktop]$ ./a.out

5+6
11
8+9-7
10
6++9
syntax error
[S@localhost Desktop]$