FLEX/YACC program not behaving as expected : can't grab int value from sequence of ints - parsing

Question

I am trying to build a parser that takes a list of strings in the following format and performs either an addition or multiplication of all of its elements :
prod 5-6_
sum _
sum 5_
sum 5-6-7_
$
Should print the following to the screen :
prod = 30
sum = 0
sum = 5
sum = 18
What I am actually getting as output is this :
prod = 0
sum = 0
sum = 5
sum = 5
My lex file looks like this :
%{
#include <iostream>
#include "y.tab.h"
using namespace std;
extern "C" int yylex();
%}
%option yylineno
digit [0-9]
integer {digit}+
operator "sum"|"prod"
%%
{integer} { return number; }
{operator} { return oper; }
"-" { return '-'; }
"_" { return '_'; }
"$" { return '$'; }
\n { ; }
[\t ]+ { ; }
. { cout << "unknown char" << endl; }
%%
and my yacc file looks like this :
%token oper
%token number
%token '-'
%token '_'
%token '$'
%start valid
%{
#include <iostream>
#include <string>
#include <cstdio>
#include <cstdlib>
using namespace std;
#define YYSTYPE int
extern FILE *yyin;
extern char yytext[];
extern "C" int yylex();
int yyparse();
extern int yyerror(char *);
char op;
%}
%%
valid : expr_seq endfile {}
| {}
;
expr_seq : expr {}
| expr_seq expr {}
;
expr : op sequence nl {if (op == '+') cout << "sum = " ; else cout << "prod = ";}
| op nl {if (op == '+') cout << "sum = 0"; else cout <<"prod = 1";}
;
op : oper { if (yytext[0] == 's') op = '+'; else op = '*';}
;
sequence : number { $$ = atoi(yytext);}
| sequence '-' number { if (op == '+') $$ = $1 + $3; else $$ = $1 * $3;}
;
nl : '_' { cout << endl;}
;
endfile : '$' {}
;
%%
int main(int argc, char *argv[])
{
++argv, --argc;
if(argc > 0) yyin = fopen(argv[0], "r");
else yyin = stdin;
yyparse();
return 0;
}
int yyerror(char * msg)
{
extern int yylineno;
cerr << msg << "on line # " << yylineno << endl;
return 0;
}
My reasoning for the yacc logic is as follows :
a file is valid only if it contains a sequence of expressions followed by the endfile symbol.
a sequence of expressions is a single expression or several expressions.
an expression is either an operator followed by a new line, OR an operator, followed by a list of numbers, followed by a new line symbol.
an operator is either 'sum' or 'prod'
a list of numbers is either a number or several numbers separated by the '-' symbol.
From my perspective this should work, but for some reason it doesn't interpret the sequence of numbers properly after the first element. Any tips would be helpful.
Thanks

Answer 1

You must not use yytext in your yacc actions. yytext is only valid during a scanner action, and the parser often reads ahead to the next token. (In fact, yacc always reads the next token. Bison sometimes doesn't, but it's not always easily predictable.)
You can associate a semantic value with every token (and non-terminal), and you can reference these semantic values using $1, $2, etc. in your yacc actions. You can even associate semantic values of different types to different grammar symbols. And if you use bison -- and you probably are using bison -- you can give grammar symbols names to make it easier to refer to their semantic values.
This is all explained in depth, with examples, in the bison manual.

Answer 2

The solution that worked was simply to change the following lines :
sequence : number { $$ = atoi(yytext);}
| sequence '-' number { if (op == '+') $$ = $1 + $3; else $$ = $1 * $3;}
;
to this :
sequence : number { $$ = atoi(yytext);}
| sequence '-' number { if (op == '+') $$ = $1 + atoi(yytext); else $$ = $1 * atoi(yytext);}
;