Question

How to check whether a number can be expressed as sum of two prime numbers in Java

Answer 1

public class MyClass {
    static boolean isPrime(int n) {
        if (n < 2 || (n % 2 == 0 && n != 2)) {
            return false;
        }
        
        int count = (int)Math.floor(Math.sqrt(n));
        for (int i = 3; i <= count; i += 2) {
            if (n % i == 0) {
                return false;
            }
        }
        return true;
    }
    public static void main(String args[]) {
        int n = 38;
        boolean has_primes = false;
        
        for (int i = 2; i <= n / 2; i++) {
            if (isPrime(i)) {
                if (isPrime(n - i)) {
                    System.out.printf("%d = %d + %d\n", n, i, n - i);
                    has_primes = true;
                }
            }
        }

        if (!has_primes)
            System.out.println(n + " cannot be expressed as a sum of two prime numbers");
  }
}




/*
run:
 
38 = 7 + 31
38 = 19 + 19
 
*/

 

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