lex and yacc : a simple calculator with syntax error - yacc

Question

a simple calculator support only + - * / and integer. I use GNU/Linux.
hoc1.l:
%{
#include "y.tab.h"
extern int yylval;
%}
%%
[ \t] { ; }
[0-9]+ { sscanf(yytext, "%d", &yylval); printf("\nget %d\n", yylval); return NUMBER; }
\n {return 0;}
%%
int yywrap(void) {
return 1;
}
hoc1.y
%{
#include<stdio.h>
#define YYSTYPE int
%}
%token NUMBER
%left '+' '-'
%left '*' '/'
%%
list:
| list '\n'
| list expr '\n' {printf("\t%d\n",$2);}
;
expr: NUMBER { $$ = $1; }
| expr '+' expr {$$ = $1+$3;}
| expr '-' expr {$$ = $1-$3;}
| expr '*' expr {$$ = $1*$3;}
| expr '/' expr {$$ = $1/$3;}
;
%%
int main(void)
{
yyparse();
return 0;
}
int yyerror(char *s) {
fprintf(stderr, "*%s*\n", s);
return 0;
}
runtime-error:
% ./hoc
8+9
get 8
+
get 9
*syntax error*
why and how to sovle it, thx!

Answer 1

You forgot to include your operators in your lex file, and you should return nonzero on a successful token read: returning 0 intuitively means there was no match by yylex. Remove the line in your lex file handling the newline character and replace it with the following:
[-+*/\n] { return *yytext; }
. { yyerror("unrecognized character"); return 0; }
Now it should work. Returning *yytext allows your yacc grammar to parse an expression successfully, e.g. if you get a '+', return it to allow the grammar to parse properly.