Getting wrong answer for Project Euler #27 - c++

Question

I'm working on Project Euler #27 in C++:
Euler published the remarkable quadratic formula:
n² + n + 41
It turns out that the formula will produce 40 primes for the
consecutive values n = 0 to 39. However, when n = 40, 40² + 40 + 41 =
40(40 + 1) + 41 is divisible by 41, and certainly when n = 41, 41² +
41 + 41 is clearly divisible by 41.
Using computers, the incredible formula n² − 79n + 1601 was
discovered, which produces 80 primes for the consecutive values n = 0
to 79. The product of the coefficients, −79 and 1601, is −126479.
Considering quadratics of the form:
n² + an + b, where |a| < 1000 and |b| < 1000
where |n| is the modulus/absolute value of n
e.g. |11| = 11 and |−4| = 4
Find the product of the coefficients, a and b, for the quadratic
expression that produces the maximum number of primes for consecutive
values of n, starting with n = 0.
I keep getting -60939 when the real answer is -59231. What am I missing?
#include <iostream>
#include "Helper.h"
using namespace std;
int formula(int a, int b, int n) {
return ((n * n) + (a * n) + b);
}
int main() {
int most = 0;
int ansA = 0;
int ansB = 0;
bool end = false;
for(int a = 999; a >= -999; a--) {
for(int b = 999; b >= 2; b--) { //b must be prime
if(Helper::isPrime(b)) {
end = false;
for(int n = 0; !end; n++) {
if(!Helper::isPrime(formula(a, b, n))) {
if(n-1 > most) {
most = n-1;
ansA = a;
ansB = b;
}
end = true;
}
}
}
}
}
cout << ansA << " * " << ansB << " = " << ansA * ansB << " with " << most << " primes." << endl;
return 0;
}
In case it's the problem, here is my isPrime function:
bool Helper::isPrime(int num) {
if(num == 2)
return true;
if(num % 2 == 0 || num == 1 || num == 0)
return false;
int root = (int) sqrt((double)num) + 1;
for(int i = root; i >= 2; i--) {
if (num % i == 0)
return false;
}
return true;
}

Answer 1

You are allowing a to be negative, and your formula returns an int. Does calling Helper::isPrime with a negative number even make sense (in other words, does Helper::isPrime take an unsigned int?)

Answer 2

Here is my java version. Hope it helps:
static int function(int n, int a, int b){
return n*n + a*n + b;
}
static int consequitive_Primes(int a, int b, HashSet<Integer> primes){
int n = 0;
int number = 0;
while(true){
if(!primes.contains(function(n, a, b)))
break;
number++;
n++;
}
return number;
}
static HashSet<Integer> primes (int n){
ArrayList<Integer> primes = new ArrayList<Integer>();
primes.add(3);
for(int i=3; i<n;i+=2){
boolean isPrime = true;
for(Integer k:primes){
if(i%k==0){
isPrime = false;
break;
}
}
if(isPrime) primes.add(i);
}
return new HashSet<Integer>(primes);
}
static long q27(){
HashSet<Integer> primes = primes(1000);
int max = 0;
int max_ab = 0;
for(int a = -999; a<1000;a++){
for(int b = -999; b<1000;b++){
int prime_No = consequitive_Primes(a,b,primes);
if(max<prime_No){
max = prime_No;
max_ab = a*b;
}
}
}
return max_ab;
}