bool isPrime( int );// C++

using namespace std;

bool isPrime( int );// Prototype for this function.

bool isPrime( int num ) {
    for ( int divisor = 2; divisor <= num / 2; divisor++ )// For all divisors from 2 to 1/2 of the number...
        if ( num % divisor == 0 )// If it divides evenly with no remainder...
            return false;// then it is not a prime since it's dividable by something.

    return true;// It wasn't dividable by anything from 2 to 1/2 of the number, so it must be prime.
}