yacc: Distinguish Integers from Floating Point Numbers

Question

I'm supposed to write a program that does 2 + 2 = 4 and 2.2 + 2 = 4.2.

I've already done it so that it treats everything as a floating point, but that's "wrong". I have to distinguish them. Here's what I have so far:

%{
#include <stdio.h>
#include <ctype.h>
%}

%token <dval> FLOAT
%token <ival> INTEGER

%union
{
   float dval;
   int ival;
}

%type <dval> command exp term factor

%%

command : exp           {printf("%f\n",$1);}
    ;

exp : exp '+' term      {$$ = $1 + $3;}
    | exp '-' term      {$$ = $1 - $3;}
    | term          {$$ = $1;}
    ;

term    : term '*' factor   {$$ = $1 * $3;}
    | factor        {$$ = $1;}
    ;

factor : '(' exp ')'        {$$ = $2;}
    | FLOAT         {$$ = $1;}
    | INTEGER       {$$ = $1;}
    ;

%% 

int main()
{ 
  return yyparse();
}

int yylex()
{
   int c;
   while( (c=getchar()) == ' ');
   if( isdigit(c) )
   {
      ungetc(c, stdin);
      float f1;
      scanf("%f", &f1);
      int i1 = (int) f1;
      if(f1 == 0)
      {
         yylval.ival = 0;
     return INTEGER;
      }
      else if( (((float) i1) / f1 ) == 1)
      {
     yylval.ival = i1;
         return INTEGER;
      }
      else
      {
     yylval.dval = f1;
     return FLOAT;
      }
      //scanf("%f",&yylval.dval);
      //return(NUMBER);
   }
   if(c == '\n') return 0;
   return c;
}

int yyerror(char *s)
{
   fprintf(stderr,"%s\n",s);
   return 0;
}

The problem I have is that each expression can only have 1 type. Right now everything is basically float, so while the operations are right, this isn't the right solution.

I thought about defining more expressions, basically having factor_int and factor_float, and then replacing everything in it, but that seems really wrong. I have no idea how to get this done though, and the tutorials I've seen haven't really helped me.

Answer 1

Basically you can do something like this:

%{
#include <stdio.h>
#include <ctype.h>

struct number
{
  union
  {
    int ival;
    float fval;
  }
  char type;
}

char INT_TYPE = 1;
char FLOAT_TYPE = 2;

%}

%union
{
   struct number value;
}

%token <value> FLOAT INTEGER command exp term factor

int yylex()
{
   ...
   if(f1 == 0)
   {
     yylval.value.type = INT_TYPE;
     yylval.value.ival = 0
   }
   ...
}

and so on..

in this way you can check operands when reducing rules being sure to generate new correct types.. for example:

exp : exp '+' term {
   if ($1.type == INT_TYPE && $3.type == INT_TYPE)
   {
      $$.type = INT_TYPE;
      $$.ival = $1.ival + $3.ival;
   }
   else if ($1.type == INT_TYPE && $3.type == FLOAT_TYPE)
   {
      // this is a sort of implicit conversion to float
      $$.type = FLOAT_TYPE; 
      $$.fval = $1.ival + $3.fval;
   }
   // and so on

}

PS. I did something similar with Flex+Bison, I don't know if everything is supported as well as in Lex+Yacc but I think so..