Arithematic Calculator

%{
#include <stdio.h>
#include <math.h>
#include <ctype.h>
#define YYSTYPE double
%}
%token number
%left '-' '+'
%left '*' '/'
%right '^'
%right NEG
%%
input: /* empty string */
    | input exp '\n'{printf("Result %lf\n",$2);}
    | error '\n'{printf("Invalid Expression\n",$2);}
;
exp: exp '+' exp { $$ = $1 + $3; }
    | exp '-' exp { $$ = $1 - $3; }
    | exp '*' exp { $$ = $1 * $3; }
    | exp '/' exp { $$ = $1 / $3; }
    | '-' exp %prec NEG { $$ = -$2; }
    | exp '^' exp { $$ = pow ($1, $3); }
    | '(' exp ')' { $$ = $2; }
    | number { $$ = $1; }
;
%%
yylex()
{
    char c;
    while((c=getchar())==' ');
    if(isdigit(c)){
      ungetc(c,stdin);
       scanf("%lf",&yylval);
   return number;
   }
    return c;
}
int main()
{
    printf("\nEnter an arithmetic expression:\n\n");
    yyparse();
    return 0;
}
int yywrap()
{
    return 1;
}
int yyerror(char *mes) {printf("%s\n", mes);}