Question

Create a class named HW that has the main method. Leave the main method empty for now.
• Create a method named allPairs that takes an integer array parameter named a that contains only positive integers and does not return anything. The method should print out all pairs of numbers that have a sum that is prime. If there are no pairs that have a sum that is prime, print out "No pairs". Note that in the first example, (2, 1) is not the same as (1, 2) because there are two 2’s in the array.
Test your allPairs method in the main method using the sample usages.
Sample Method Usage Sample Output

int[] a1 = { 2, 1, 2, 9, 10, 4 };

allPairs(a1); returns (2, 1) (2, 9) (1, 2) (1, 10) (1, 4) (2, 9) (9, 10) (9, 4)

int[] a2 = { 20, 1, 19, 5 };

allPairs(a2);    returns    No pairs

int[] a3 = { 17, 12, 3 };

allPairs(a3);     returns (17, 12)

Answer 1

Here is the completed code for this problem. Comments are included, go through it, learn how things work and let me know if you have any doubts or if you need anything to change. If you are satisfied with the solution, please rate the answer. Thanks

// HW.java

public class HW {

      // required method

      public static void allPairs(int array[]) {

            // boolean flag denoting if at least one target pair is found

            boolean found = false;

            // looping through each possible unique pair

            for (int i = 0; i < array.length; i++) {

                  for (int j = i + 1; j < array.length; j++) {

                        // finding sum of elements at i and j

                        int sum = array[i] + array[j];

                        // checking if sum is prime

                        if (isPrime(sum)) {

                              // found

                              found = true;

                              // displaying pair

                              System.out.printf("(%d, %d) ", array[i], array[j]);

                        }

                  }

            }

            // at the end, if not found, displaying No pairs

            if (!found) {

                  System.out.println("No pairs");

            } else

                  System.out.println(); // printing line break

      }

      // private helper method implemented to check if a number is prime or not

      // designed to improve neatness of the code

      private static boolean isPrime(int n) {

            if (n <= 1) {

                  // any number less than 2 = not prime

                  return false;

            }

            // looping from 2 to n/2

            for (int i = 2; i <= n / 2; i++) {

                  // if any number divides n evenly, then n is not prime

                  if (n % i == 0) {

                        return false;

                  }

            }

            return true; // prime

      }

      public static void main(String[] args) {

            //testing

            int[] a1 = { 2, 1, 2, 9, 10, 4 };

            allPairs(a1); // displays (2, 1) (2, 9) (1, 2) (1, 10) (1, 4) (2, 9) (9,

                                    // 10) (9, 4)

            int[] a2 = { 20, 1, 19, 5 };

            allPairs(a2); // displays No pairs

            int[] a3 = { 17, 12, 3 };

            allPairs(a3); // displays (17, 12)

      }

}

/*OUTPUT*/

(2, 1) (2, 9) (1, 2) (1, 10) (1, 4) (2, 9) (9, 10) (9, 4)

No pairs

(17, 12)