Question

Am trying to find all prime numbers between two given numbers and sum the primes up.

I have this loop that does the prime number detection correctly.

However, for some reason I don’t know how to sum all the primes.

int a,b,i,j,sum=0;

do
{   cout << "Enter a number: ";
    cin >> a;
    if (a < 4 || a > 1000000) 
    {   cout << "Input must be between 4 and 1000000 inclusive." << endl;
    }
}while (a < 4 || a > 1000000);

do
{   cout << "Enter a second number: ";
    cin >> b;
    if (b < 4 || b > 1000000) 
    {   cout << "Input must be between 4 and 1000000 inclusive." << endl;
    }
}while (b < 4 || b > 1000000);

if (a > b)
{   int hold;
    hold = b;
    b = a;
    a = hold;
}

cout << "The prime numbers between " << a << " and " << b << " inclusive are: " << endl;
//int sum;
for (i = a; i <= b; i++)
{
 for (j = 2; j <= i; j++) // Changed the < to <=, and got rid of semicolon
 {
    if (!(i%j)&&(i!=j)) break;
    if (j==i) 
    {
              cout << i << endl;
              sum += i;
              cout << sum ;

    }
 }
}

The variable sum gives me rubbish results.

Answer 1

It is impossible to know without the exact details but two most likely possibilities are:

1. sum was not initialized to 0 prior to usage
2. You are encountering an overflow, since the sum of numbers is too large to fit in it. It obviously depends on the type of sum, and the range.

EDIT:

The editted code works for me, for small ranges (note that for larger ranges, one should also consider issue #2).

You might be misreading the results, try adding endl to cout << sum ;