Question

I have the following code which spits out prime numbers between 1 and N. A friend came up with this solution but I believe there is a more efficient way to write this code. Such as making it so that if (i%j!=0) {System.out.print (i + " ");}. However I found this spat out numbers randomly all over the place…

import java.util.Scanner;

public class AllPrime {


public static void main(String[] args) {

    System.out.println("Enter a number:\n");
    Scanner input = new Scanner(System.in);
    int a = input.nextInt();

    for (int i = 2; i < a; i++) {
        boolean primeNum = true;
        for(int j=2; j<i; j++) {
            if (i%j==0) {
                primeNum =false;
            }
        }
        if (primeNum) {
            System.out.print(i + " ");
            }
        }
    }
}

Answer 1

Look at proper sieves, like the Sieve of Eratosthenes. You don’t need to be checking for % each time.