Thread: Prime Number Program for class

You're increasing the prime count whenever a number in question isn't divisible by another number, but you're suppose to only increase the count when the number in question isn't divisible by all other numbers greater than 1 and less than itself (subtle difference here).

Adding/removing code willy nilly without understanding what's going on is not a good debugging technique. Examine what your algorithm for checking if a number is prime is actually doing (i.e. work it out as if you were the computer). Try it out with the number 5 (which is prime), as well as with the number 6 (which is not prime). Pay attention to when the prime count increases, then see if you can't figure out a better way to check for what I suggested earlier (hint: you'll probably end up needing another if statement and a boolean flag).

You haven't learned about booleans yet? Hmm, that seems kind of odd. Booleans are values which can either be true or false.

boolean isPrime = false; // a boolean variable is set to false
if(isPrime) // equivalent to if(isPrime == true)
{
    System.out.println("this will never execute");
}

Without booleans, you'd need to use breaks and instead of keeping track of how many numbers are prime, you'd need to keep track of how many numbers are composite. Once you've figured out how many composite numbers there are, you know that the rest of the numbers are prime.