Lex yacc Shift/Reduce error - c

Question

I am learning lex and yacc programming and this yacc program to validate and evaluate arithmetic expression in giving me 25shift/reduce conflicts. After reading other stackoverflow explanations, I understand the the issue is precedence, however, I am not sure how to resolve using error handling. I was wondering if someone could help me out.
I have tried an Association handler like this:
%token VARNAME
%token DIGIT
%token EQ
%token ADD SUB MULT DIV
%token LPAREN RPAREN
%token END
%left ADD SUB MULT DIV LPAREN
%right RPAREN END
%nonassoc EQ VARNAME DIGIT
But, it doesn't work and I'm confused. This is my 1.y file:
%{
#include <stdio.h>
#include <stdlib.h>
int main (int argc, char* argv[])
{
FILE *fp;
extern FILE *yyin;
extern int yylex();
extern int yyparse();
extern int yydebug;
yydebug = 1;
if (argc == 2)
{
fp = fopen(argv[1], "r");
if (!fp)
{
fprintf (stderr, "Usage: %s <filename>\n", argv[0]);
}
else
{
yyin = fp;
yyparse();
fprintf (stdout, "***PARSE COMPLETE***\n");
}
}
else
fprintf (stderr, "Usage: %s <filename>\n", argv[0]);
}
void yyerror (const char *err)
{
fprintf (stderr, "Error: %s\n", err);
}
%}
%token VARNAME
%token DIGIT
%token EQ
%token ADD SUB MULT DIV
%token LPAREN RPAREN
%token END
%%
statement:
END
|
expression END
{printf ("DONE!\n");};
|
error
{ printf(" INVALID EXPRESSION\n"); }
;
expression:
VARNAME
{printf("PARSED A VARIABLE!\n");};
|
DIGIT
{printf("PARSED A DIGIT!\n");};
|
expression ADD expression
{printf("PARSED A PLUS SIGN!\n");};
|
expression SUB expression
{printf("PARSED A MINUS SIGN!\n");};
|
expression MULT expression
{printf("PARSED A MULTPLY SIGN!\n");};
|
expression DIV expression
{printf("PARSED A DIVIDE SIGN!\n");};
|
expression EQ expression
{printf("PARSED A EQUALS SIGN!\n");};
|
LPAREN expression RPAREN
{printf("PARSED A PARENTHESIS!\n");};
;
%%
I have tried an Association handler like this:
%token VARNAME
%token DIGIT
%token EQ
%token ADD SUB MULT DIV
%token LPAREN RPAREN
%token END
%left ADD SUB MULT DIV LPAREN
%right RPAREN END
%nonassoc EQ VARNAME DIGIT
Is this association correct?

Answer 1

The issues are fairly well covered in the GNU bison manual.
As #Jonathan Leffler says:
You need to specify the precedence and associativity of the operators. Look up %left, %right and %nonassoc. Or write the grammar in a different manner altogether. For example, the grammar in the C standard doesn't need the precedence levels or associativity.
The %token items look plausible. I think %left does not need LPAREN; nor does %right need RPAREN. I'm not sure whether you need %nonassoc at all. I'm assume that the 'code'( at the start and the )'code' at the end are artefacts of trying to format a comment. Normally, you want ADD and SUB at a lower priority than MULT and DIV.
In the arithmetic example these are usually resolved by writing the grammar as:
expression : term
| term ADD expression
| term SUB expression
;
term : factor
| factor MULT term
| factor DIV term
;
factor : VARNAME
| DIGIT
| LPAREN expression RPAREN
;
Now you have both the associativity and the precedence of the operators defined in an unambiguous grammar.