Separate prime numbers from array

Question

public class HelloWorld {

    public static void main(String []args) {

        int [] arr = {2, 5, 9, 6, 7, 13, 24, 42, 8};
        int [] arr1 = new int[4];
        int [] arr2 = new int[arr.length - arr1.length];

        for(int i = 0; i < arr.length; i++) {
            for(int j = 2; j <= arr[i]/2; j++) {
                if(arr[i] % j == 0) {
                    System.out.println("Number is not prime " + arr[i]);
                    break;
                }
                else {
                    System.out.println("Number is prime " + arr[i]);
                    break;
                }
            }
        }
    }
}

The program should check an array of numbers and print if the given number is prime. There is something wrong, since the first 2 are not marked as prime. Then I don't know why 9 was taken as a prime number.

Answer 1

There are multiple problems in your code.

1. Why 2 is not taken as prime. As explained by others, j <= arr[i]/2 is the culprit. According to this condition, j <= 1 and j == 2. So, the loop does not get executed.

2. Why 9 is taken as prime. For the the first time when j == 2 and arr[i] == 9. As, 9%2 != 0, the number is printed as prime.

   for(int j = 2; j <= arr[i]/2; j++){
       if(arr[i] % j == 0){
           System.out.println("Number is not prime " + arr[i]);
           break;
       }
       else {
           System.out.println("Number is prime " + arr[i]);
           break;
       }
   }
   

Advice: Instead of checking for arr[i]/2, you can use square root of number to check instead.

You can refer the following program if needed:

public class PrimeNumber {
    public static void main(String []args){

        int [] arr = {2,3,4,5,9,6,7,13,24,42,8,400,101};
        int [] arr1 = new int[4];
        int [] arr2 = new int[arr.length - arr1.length];
        boolean flag = true;
        for(int i = 0; i < arr.length; i++){
            if(arr[i] == 2 || arr[i] == 3 )
            {
                System.out.println("Number is prime " + arr[i]);
                continue;
            }
            flag = true;
            for(int j = 2; j <= Math.sqrt( arr[i] ); j++){
                if(arr[i] % j == 0){
                    System.out.println("Number is not prime " + arr[i]);
                    flag = false;
                    break;
                }
            }
            if ( flag )
            {
                System.out.println("Number is prime " + arr[i]);
            }
        }
    }
}

Answer 2

for (int j = 2; j <= arr[i] / 2; j++) {
    if (arr[i] % j == 0) {
        System.out.println("Number is not prime " + arr[i]);
        break;
    } else {
        System.out.println("Number is prime " + arr[i]);
        break;
    }
}

Here you iterate only once and check if given number is dividable by 2 or not (i.e. this is equal to arr[i] % 2 == 0). To check if given number is prime or not, you have to check all numbers from 2 to sqrt(val).

I recommend you to select this check into separate method.

import java.util.function.IntPredicate;

final IntPredicate isPrime = val -> {
    if (val < 2)
        return false;

    for (int i = 2, sqrt = (int)Math.sqrt(val); i <= sqrt; i++)
        if (val % i == 0)
            return false;

    return true;
};

And your code looks much simpler:

int[] arr = { 2, 5, 9, 6, 7, 13, 2, 4, 42, 8 };

for (int val : arr) {
    if (isPrime.test(val))
        System.out.println("Number is prime " + val);
    else
        System.out.println("Number is not prime " + val);
}