Shift/reduce conflicts in yacc - c

Question

I am learning lex and yacc programming and this yacc program to validate and evaluate arithmetic expression in giving me 10 shift/reduce conflicts. Can you point out whats wrong with this program
This is 611.y
%{
#include<stdio.h>
int flag=1;
%}
%token id num
%left '(' ')'
%left '+' '-'
%left '/' '*'
%nonassoc UMINUS
%%
stmt:
expression { printf("\n valid exprn");}
;
expression:
'(' expression ')'
| '(' expression {printf("\n Syntax error: Missing right paranthesis");}
| expression '+' expression {printf("\nplus recog!");$$=$1+$3;printf("\n %d",$$);}
| expression '+' { printf ("\n Syntax error: Right operand is missing ");}
| expression '-' expression {printf("\nminus recog!");$$=$1-$3;printf("\n %d",$$);}
| expression '-' { printf ("\n Syntax error: Right operand is missing ");}
| expression '*' expression {printf("\nMul recog!");$$=$1*$3;printf("\n %d",$$);}
| expression '*' { printf ("\n Syntax error: Right operand is missing ");}
| expression '/' expression {printf("\ndivision recog!");if($3==0) printf("\ndivision cant be done, as divisor is zero.");else {$$=$1+$3;printf("\n %d",$$);}}
| expression '/' { printf ("\n Syntax error: Right operand is missing ");}
| expression '%' expression
| expression '%' { printf ("\n Syntax error: Right operand is missing ");}
| id
| num
;
%%
main()
{
printf(" Enter an arithmetic expression\n");
yyparse();
}
yyerror()
{
printf(" Invalid arithmetic Expression\n");
exit(1);
}
This is 611.l
%{
#include "y.tab.h"
#include<stdio.h>
#include<ctype.h>
extern int yylval;
int val;
%}
%%
[a-zA-Z][a-zA-Z0-9]* {printf("\n enter the value of variable %s:",yytext);scanf("%d",&val);yylval=val;return id;}
[0-9]+[.]?[0-9]* {yylval=atoi(yytext);return num;}
[ \t] ;
\n {return 0;}
. {return yytext[0];}
%%
int yywrap()
{
return 1;
}
When I complie the code like this
lex 611.l
yacc -d 611.y
It gives me
yacc:10 shift/reduce conflicts.
Please help me out here.

Answer 1

Two things are wrong:
Precedence of '%' is missing, add it to '/' '*'
The '(' expression error handler is ambiguous (in an expression (4*(2+3)+5*7 there are many ways to insert a missing parenthesis) and is in fact in conflict with the normal '(' expression ')' rule. It is non-trivial to make such a handler work. I would recommend to remove it and rely on built-in yacc error handler.
Simple error handling can be implemented like this:
stmt:
expression { printf("\n valid exprn");}
| error { printf(" Invalid arithmetic Expression\n"); }
;
expression:
'(' expression ')'
| '(' error ')' { printf(" Invalid arithmetic Expression\n"); }
| ... /* all the rest */
You won't need all other error handlers too.