CODEWITHSUNDEEP
c++project
Shane
Shane

Reputation: 103

Project Euler: Summation of primes... Why won't this work?

I keep getting the wrong answer of 1179908154. At first I blamed it on my summation variable being type int, rather than long. I gave it long type but I get the same answer. Thoughts?

// Project Euler

// Problem 10

#include <iostream>
#include <cmath>
using namespace std;

void main() 
{

int p = 3;
long sum = 2;
bool isPrime;
for (p; p < 2000000; p++)
{
    isPrime = true;

    for (int i = 2; i <= sqrt(static_cast<double>(p)); i++) // cast into double for sqrt function
    {
        if (p % i == 0)
        {
            isPrime = false;
            break;

        }
    }
    if (isPrime == true)
    {
        cout << p << endl; // show each prime
        sum += p; // add prime to sum 
    }

}
cout << sum << endl; // show sum 

system("pause");

}

Upvotes: 0

Views: 810

Answers (2)

Eduardo Almeida
Eduardo Almeida

Reputation: 171

Maybe on your platform the long is not enough to hold the value too. Try a long long instead.

Upvotes: 2

SparkleStilettos
SparkleStilettos

Reputation: 1

When putting the bounds on your for loop, you should check numbers up until sqrt(p) + 1. You can get floating point errors when calculating the square root (it might underestimate it slightly), so it's possible that some potential factors are not checked in the loop.

Upvotes: 0

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