Vfrdtyky

I am new to sets and I found this problem in a book:

Implement the sieve of Eratosthenes: a method for computing prime numbers, known to the ancient Greeks. Choose an n. This method will compute all prime numbers up to n. First insert all numbers from 2 to n into a set. Then erase all multiples of 2 (except 2); that is, 4, 6, 8, 10, 12, …. Erase all multiples of 3; that is, 6, 9, 12, 15, …. Go up to . Then print the set.

I have written this code:

import java.util.Iterator;

import java.util.Set;

import java.util.TreeSet;



public class SieveOfEratosthenes {

    public static void main (String args){

        System.out.print(generatePrime(20));

    }



    public static Set generatePrime(int n){

        Set<Integer> primes = new TreeSet<>();

        Iterator<Integer> iter = primes.iterator();



        //generate all numbers up to n and add them to the set

        for (int i = 2; i < n; i++){

            primes.add(i);

        }



        //for numbers up to root n

        for (int f = 2; f <= Math.sqrt(n); f++){

            while (iter.hasNext()){

                int current = iter.next();

                if (current % f == 0 && current != 2){

                    primes.remove(current);



                }

            }



        }



        return primes;

    }

}

The problem is that the code in the while loop doesn't implement. When I debugged the program I found that hasNext() is returning null. I couldn't figure out the reason of doing so although the list contains the numbers.

This is the output that I get from the code:

[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

Process finished with exit code 0

Thank you in advance!

You're creating the Iterator before you add elements to the Set when you should be creating it after (inside the for-loop):

Set<Integer> primes = new TreeSet<>();



//generate all numbers up to n and add them to the set

for (int i = 2; i < n; i++) {

    primes.add(i);

}



//for numbers up to root n

for (int f = 2; f <= Math.sqrt(n); f++){

    Iterator<Integer> iter = primes.iterator();

Also, I recommend changing the following:

primes.remove(current);

To:

iter.remove();

To avoid any ConcurrentModificationExceptions.

Lastly, it seems you still have an issue where 3 is not in the resulting Set, which you'll have to debug.