c++ floating point exception

Question

I did this for the Euler project problem 5, but for some reason I get a floating point exception:

#include <iostream>
using namespace std;

int main ()
{
    long num;
    bool isnum = false;
    long i = 20;

    while (isnum == false)
    {
        for (int j = 0; j <= 20; j++)
        {
            if (i % j != 0)
            {
                break;
            }
            else
            {
                num = i;
                isnum = true;
            }
        }
        i+=20;
    }
    cout << num << endl;
    return 0;
}

What I don't understand is how there can be a floating point exception when I do nothing with my code that would output a non-integer number.

Answer 1

i % j

There's a division by zero here because j is initialized to 0.

Answer 2

Since you use i % j with j being initialized to zero, you get undefined behavior, according to 5.6 [expr.mul] paragraph 4:

... If the second operand of / or % is zero the behavior is undefined. ...

This can yield, for example, a floating point exception. I can also cause nastier things to happen.

Answer 3

You are doing i % j with j == 0 at the first iteration of the for loop.

Thus the floating point exception.