Java 2d Array Matrix Program（素数之和）

Question

for(i=2;i<=5;i++) {
        for(j=2;j<=(i/2);j++) {
            if(i%j== 0) {
                j=i;
                break;
            }
        }
        if(i!=j) {
            sum +=i;            
        }
    }
    System.out.println("The sum of all prime numbers is : "+sum);

What I'm trying to achieve is the sum of all prime numbers when I run my program, i am using this code but then the sum is not what I think it is. The sum is greater than what I expected it. I tried rewriting and changing some but the result is as is. Other informations: The matrix is randomized from 0-9, it is a (5x5)2D array.

Answer 1

You should take into account the specific requirement that array elements are randomized from 0 to 9, thus you have to check for the following primes below 10: [2, 3, 5, 7].

Then iterate the matrix and sum up all primes:

private static boolean isPrimeBelow10(int x) {
    return x == 2 || x == 3 || x == 5 || x == 7;
}

public static int sumPrimes(int[][] arr) {
    int sum = 0;
    for (int i = 0; i < arr.length; i++) {
        for (int j = 0; j < arr[i].length; j++) {
            if (isPrimeBelow10(arr[i][j])) {
                sum += arr[i][j];
            }
        }
    }
}

If you needed to handle larger elements, then you would have to maintain an extendable list of primes and add new elements to it as necessary.