Determining if a number is prime

Question

I have perused a lot of code on this topic, but most of them produce the numbers that are prime all the way up to the input number. However, I need code which only checks w

Answer 1

If you know the range of the inputs (which you do since your function takes an int), you can precompute a table of primes less than or equal to the square root of the max input (2^31-1 in this case), and then test for divisibility by each prime in the table less than or equal to the square root of the number given.

Answer 2

This is a quick efficient one:

bool isPrimeNumber(int n) {
    int divider = 2;
    while (n % divider != 0) {
        divider++;
    }
    if (n == divider) {
        return true;
    }
    else {
        return false;
    }
}

It will start finding a divisible number of n, starting by 2. As soon as it finds one, if that number is equal to n then it's prime, otherwise it's not.