Arithmetic Expression Evaluation with lex, yacc

arith..y

%{
/* Definition section */
  #include<stdio.h>
  int yylex();
  void yyerror();
%}

%token NUMBER ID
// setting the precedence
// and associativity of operators
%left '+' '-'
%left '*' '/'

/* Rule Section */
%%
E : T
{

printf("Result = %d\n", $$);
return 0;

}

T : T '+' T { $$ = $1 + $3; }
| T '-' T { $$ = $1 - $3; }
| T '*' T { $$ = $1 * $3; }
| T '/' T { $$ = $1 / $3; }
| '-' NUMBER { $$ = -$2; }
| '-' ID { $$ = -$2; }
| '(' T ')' { $$ = $2; }
| NUMBER { $$ = $1; }
| ID { $$ = $1; };
%%

int main()
{
	printf("Enter the expression\n");
	yyparse();
}

/* For printing error messages */
void yyerror(char* s)
{
	printf("\nExpression is invalid\n");
}

arith.l

%{
	/* Definition section*/
	#include "y.tab.h"
	extern int yylval;
%}
%%
[0-9]+ {
          yylval=atoi(yytext);
          return NUMBER;

       }

[a-zA-Z]+ { return ID; }
[\t] ;

\n { return 0; }
. { return yytext[0]; }
%%
int yywrap()
{
 return 1;
}