Java - Prime Number Algorithm Implementation doesn't work

Question

I wrote this code which is supposed to search for prime numbers and put them into an array. Here it is:

int[] prime_array = new int[(int)s.upper_bound];
int index_in_array = 0;
boolean are_we_done = false;
int index = 1;
boolean is_prime = true;

while (!are_we_done) {
    try {
        for (int i = 1; i < index; i++) {
            if ((index%i)==0) {
                is_prime = false;
            }
        }

        if (is_prime) {
            prime_array[index_in_array] = index;
            index_in_array++;
        }

        index++;
        is_prime = true;
    } 
    catch (IndexOutOfBoundsException e) {
        are_we_done = true;
        break;
    }
}

Unfortunately, the only prime it catches is 1. Any idea why it doesn't work?

Answer 1

Your for loop is looking for factors starts with i=1 , and if index%i==0 you decide it is not a prime. But n%1==0 for all integers n .

The lowest factor that indicates that something is not prime is 2 .

Edit:

Here are some other suggestions:

• Use a properly bounded loop to cycle through your candidate primes.
• Only check factors up to the square root of the candidate prime.
• Only check 2 and odd numbers as primes (because no even number above 2 is prime, and 1 is a unit, not a prime)
• Only check 2 and odd numbers as factors (because no even number above 2 can be a prime factor)
• Use a sieve instead.