Flex and Bison Calculator

Question

I'm trying to implement a calculator for nor expressions, such as true nor true nor (false nor false) using Flex and Bison, but I keep getting my error message back. Here is my .l file:

%{
#include <stdlib.h>
#include "y.tab.h"
%}

%% 
("true"|"false")    {return BOOLEAN;}

.|\n    {yyerror();}

%%

int main(void)
{
    yyparse();
    return 0;
}

int yywrap(void)
{
     return 0;
}
int yyerror(void)
{
     printf("Error\n");
}

Here is my .y file:

/* Bison declarations.  */
 %token BOOLEAN
 %left 'nor'

 %% /* The grammar follows.  */
 input:
   /* empty */
 | input line
 ;

 line:
   '\n'
 | exp '\n'  { printf ("%s",$1); }
 ;

 exp:
   BOOLEAN            { $$ = $1;           }
 | exp 'nor' exp      { $$ = !($1 || $3);  }
 | '(' exp ')'        { $$ = $2;           }
 ;
 %%

Does anyone see the problem?

vitaut vitaut · Accepted Answer · 2013-02-28T05:52:45

The reason why you get errors is that your lexer only recognizes one type of token, namely BOOLEAN, but not the newline, parentheses or nor (and you produce an error for everything else). For single letter tokens like parentheses and the newline you can return the character itself as a token type:

\n { return '\n'; }

For nor thought you should introduce a token type like you did for BOOLEAN and add an appropriate rule to the lexer.

Flex and Bison Calculator

2 Answers