Question

Rohan was asked by his teacher to develop a program which takes in a dynamic user input and prints all the prime numbers till that number and then prints all the composite numbers till that number. Help him out by developing the program to do so

Answer 1

import java.util.Scanner;

public class Main {
    private static final boolean[] numbers = new boolean[Integer.MAX_VALUE / 2];

    private static void markNumbers() {
        for (int i = 2; i * i < numbers.length; i++) {
            if (!numbers[i]) {
                for (int j = i * i; j < numbers.length; j += i) {
                    numbers[j] = true;
                }
            }
        }
    }

    private static void showNumbers(int n) {
        System.out.print("Primes:");
        for (int i = 2; i <= n; i++) {
            if (!numbers[i]) {
                System.out.print(" " + i);
            }
        }
        System.out.println();
        System.out.print("Composites:");
        for (int i = 2; i <= n; i++) {
            if (numbers[i]) {
                System.out.print(" " + i);
            }
        }
        System.out.println();
    }

    public static void main(String[] args) {
        markNumbers();
        Scanner in = new Scanner(System.in);
        int n;
        do {
            System.out.print("Enter the number(0 to stop): ");
            n = in.nextInt();
            if (n > 1 && n < numbers.length) {
                showNumbers(n);
            }
        }while (n != 0);
    }
}