ISC COMPUTER SCIENCE PRACTICAL 2018 QUESTION 1

QUESTION -1

A Goldbach number is a positive even integer that can be expressed as the sum of two odd primes.

Note: All even integer numbers greater than 4 are Goldbach numbers. Example: 6 = 3 + 3
10 = 3 + 7
10 = 5 + 5

Hence, 6 has one odd prime pair 3 and 3. Similarly, 10 has two odd prime pairs, i.e. 3 and 7, 5 and 5.

Write a program to accept an even integer ‘N’ where N > 9 and N < 50. Find all the odd prime pairs whose sum is equal to the number ‘N’.
Test your program with the following data and some random data: Example 1:

INPUT: N = 14
OUTPUT: PRIME PAIRS ARE:
3, 11
7, 7

Example 2: INPUT: N = 30
OUTPUT: PRIME PAIRS ARE
7, 23
11, 19
13, 17

Example 3: INPUT: N = 17
OUTPUT: INVALID INPUT. NUMBER IS ODD.

Example 4: INPUT: N = 126
OUTPUT: INVALID INPUT. NUMBER OUT OF RANGE.

JAVA SOURCE CODE

import java.util.*;

public class ISC2018Q1 {

       public static void main(String[] args) {

             int N;

             Scanner in = new Scanner(System.in);

             System.out.println("Enter the value of N:");

             N = in.nextInt();

             while( N < 9 || N >= 50 || N%2 !=0) {

                    if(N%2 !=0)

                          System.out.println("INVALID INPUT. NUMBER IS ODD.");

                    else

                          System.out.println("INVALID INPUT. NUMBER OUT OF RANGE.");

                    System.out.println("Enter the value of N:");

                    N = in.nextInt();  

             }

             Goldbach obj = new Goldbach(N);

             System.out.println("OUTPUT: PRIME PAIRS ARE");

             obj.PrimePairGenerator();

             in.close();

             }

}

class Goldbach{

public void PrimePairGenerator() {

       for (int i=3; i<Numb;i++) {

             for(int j=3; j<Numb/2+1;j++) {

                    if(i+j == Numb) // if i+j equals N then we check whether both i and j are prime or not

                          if(IsPrime(i))

                                 if(IsPrime(j))

                                       System.out.println(i+","+j);

                    } // end of inner-for

             } // end of outer-for

       }

private boolean IsPrime(int N) {

       boolean flag=false;

       for(int x=0; x<Prime_Numbers.length;x++) {

             if(N==Prime_Numbers[x])

                    flag=true;

             }

       return(flag);

       }

Goldbach(int N){

       Numb=N;

       }

       private int Numb; // since the range of N is too small no need to write down the prime function

       private int[] Prime_Numbers = {3,5,7,11,13,17,19,23,29,31,37,41,43,47};

}           // this is will save time! 2 is not included as only odd pair primes are needed