Question

I’m trying to find prime numbers with a specific condition in Java.

The challenge is to show all the prime numbers (under 100.000) which contain a ‘3’ four times.
I already have a code which shows all the prime numbers under 100.000, but I can’t seem to figure out how to count the ones that contain the number ‘3’ four times.
I can however count all the prime numbers.

Can someone help me with this?

Here’s the code I have, where am I going to put the numbers into strings?

    package Proeftentamen;

import java.util.regex.*;

/**
 *
 * @author Stefan
 */
public class Vraag_6 {

    /// priemgetallen waar 4x een 3 in voor komt???? wtf...
    public static void main(String[] args) {
        boolean[] lijst = new boolean[1000000]; // hoeveelheid getallen
        vularray(lijst);
        lijst = zeef(lijst);
        drukaf(lijst);
    }

    public static void vularray(boolean[] lijst) {
        for (int i = 2; i < lijst.length; i++) {
            lijst[i] = true;
        }
    }

    public static boolean[] zeef(boolean[] lijst) {
        for (int i = 2; i < lijst.length / 2; i++) {
            if (lijst[i]) {
                for (int j = 2 * i; j < lijst.length; j += i) {
                    lijst[j] = false;
                }
            }
        }
        return lijst;
    }

    public static void drukaf(boolean[] lijst) {
        int count = 0;
        for (int i = 2; i < lijst.length; i++) {
            if (lijst[i] == true) {
                System.out.println(i + " " + lijst[i]);
                count++;
            }
        }
        System.out.println("Aantal priemgetallen: " + count);
    }
}

Answer 1

This question really sounds like a homework, so you should write down what you have come up with and what you tried so far.

There are a lot of ways to count numbers. Just to give you a clue, you can use the reminder operation (in Java – %):

56 % 10 = 6
25 % 5  = 0

So, when you divide by 10 and use a reminder operation you can get the last digit of your number. Now use a loop and counter and you’ll be fine.

Another option (very ugly, so don’t really use it 🙂 ) – to turn your number into a String and iterate (loop) over its characters.

Hope this helps and good luck!