Question

I know this is not the best nor the most efficient way to find prime numbers; However, I can’t seem to find the reason why 169 counts as a prime number (for smaller numbers it works OK as far as I’m concerned).

public static int checkPrime(int num, int i)
{
    if (i == num)
        return 1;

    else
    {
        if (num % i == 0)
            return 0;
        else
            checkPrime(num, i+1);

    }

    return 1;
}

Main Class:

        System.out.println("Type a number");
        number = reader.nextInt();

        if ((number % 10) % 2 == 0)
            result = 0;

        else
            result = checkPrime(number, 2);

        if (result == 1 || number == 2)
            System.out.println(number + " is a prime number");
        else
            System.out.println(number + " is NOT a prime number");

Answer 1

Replacing int by boolean for better expressiveness, and returning the value of the recursive call, your method looks like this:

public static boolean checkPrime(int num, int i)
{
    if (i == num)
        return true;    
    else
    {
        if (num % i == 0)
            return false;
        else
            return checkPrime(num, i+1);
    }
}

It still doesn’t work for num < 2, it’ll run until i overflows. So as your main prime checking function you can write something like:

public static boolean checkPrime(int num)
{
   if(num<2)
     return false;
   else
     return checkPrime(num, 2);
}

btw (number % 10) % 2 is equivalent to number % 2.