Bison/flex help (I can't resolve this parse error)

Codeless · 03-01-2011, 12:25 PM

I don't know what I'm doing wrong, perhapes I don't know what i'm doing... I could really use some help. This is my first compiler and I'm trying to design something very simple.

here's my flex file

Code:

%{
#include <stdlib.h>
#include "compile.h"
%}

%%
[ \t]+
"+"	{return PLUS;}
"-"	{return MINUS;}
"*"	{return TIMES;}
"/"	{return DIVIDE;}
"("	{return LEFT;}
")"	{return RIGHT;}
"\n"	{return END;}
[0-9]+	{yylval=atof(yytext); return NUMBER;}


%%
yywrap() {
  return 1;
}

and here's my bison file

Code:

%{
#define YYSTYPE double	
#include <stdio.h>
#include <stdlib.h>
#include "compile.h"	
%}

%token NUMBER
%token PLUS
%token MINUS
%token TIMES
%token DIVIDE
%token LEFT
%token RIGHT
%token END

%left PLUS 
%left MINUS
%left TIMES
%left DIVIDE

%start Input
%%

Input:	
	|	Input	Line 
;

Line:	'\n'	
	| Expression '\n'  { printf ("\t%.10g\n", $1); }	
;

Expression:
	NUMBER { $$=$1; }
	|	Expression PLUS Expression { $$=$1+$3; printf("test");}	
	|	Expression MINUS Expression { $$=$1-$3; }
	|	Expression TIMES Expression { $$=$1*$3; }
	|	Expression DIVIDE Expression { $$=$1/$3; if ($1 = 0) printf("testagain");}
	|	LEFT Expression RIGHT { $$=$2; }
;

%%

int yyerror(char *errMessage) {
	printf("Trouble: %s\n",errMessage);
}

main() {
	yyparse(); 
}

So you can see I'm trying to impliment a very simple calculator, I go through the cmds

flex lang.lex
bison -d -o compile.c land.y
gcc -o compile compile.c lex.yy.c
compile < test.txt (which is say "5+6") and I get a parse error

whats going on?

theNbomr · 03-01-2011, 02:20 PM

Does your input data file include a newline? Perhaps the parser is reaching EOF before seeing the require newline. It is hard to replicate your scenario without 'compile.h'.

--- rod.

Dr_P_Ross · 03-01-2011, 05:15 PM

I think there are two things going wrong.

First, your flex file returns END for '\n', but you have explicit '\n' in your bison file. In your bison file, put END instead of '\n' in both clauses for Line.

Second, compile.h is generated by your call of bison. Have a look at it, you'll see that it only defines YYSTYPE if YYSTYPE is undeclared. So in your flex file, you need to define YYSTYPE as double in the declarations. The it should all work.