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Java Program to Check Whether a Number can be Expressed as Sum of Two Prime Numbers
In this article, we will understand how to check whether a number can be expressed as sum of two prime numbers. Prime numbers are special numbers who have only two factors 1 and itself and cannot be divided by any other number.
A number is a prime number if its only factors are 1 and itself. 11 is a prime number. Its factors are 1 and 11 itself. Some examples of prime numbers are 2, 3, 5, 7, 11, 13 and so on. 2 is the only even prime number. All other prime numbers are odd numbers.
Below is a demonstration of the same −
Input
Suppose our input is −
Input number : 43
Output
The desired output would be −
The number can be expressed as sum of two prime numbers. The possible solutions are : 43 = 2 + 41
Algorithm
Step 1 - START Step 2 - Declare two integer values namely my_input and i Step 3 - Read the required values from the user/ define the values Step 4 - Define a function IsPrime which takes an integer value and checks if the value is a prime number or not. Step 5 - Using a for loop, iterate from 2 to half of ‘my_input’ value, check if the ‘i’ value and ‘my_input’ – ‘i’ values are both prime numbers. If yes, store both the values. Step 6 - Display the result Step 7 - Stop
Example 1
Here, the input is being entered by the user based on a prompt. You can try this example live in ourcoding ground tool 
.
import java.util.Scanner;
public class SumOfPrimes {
public static void main(String[] args) {
int my_input, i;
boolean my_temp = false;
my_input = 43;
System.out.println("Required packages have been imported");
Scanner my_scanner = new Scanner(System.in);
System.out.println("A reader object has been defined ");
System.out.print("Enter the number : ");
my_input = my_scanner.nextInt();
for (i = 2; i <= my_input / 2; ++i) {
if (IsPrime(i)) {
if (IsPrime(my_input - i)) {
System.out.println("The number can be expressed as sum of two prime numbers.");
System.out.println("The possible solutions are :");
System.out.printf("%d = %d + %d\n", my_input, i, my_input - i);
my_temp = true;
}
}
}
if (!my_temp)
System.out.println(my_input + " cannot be expressed as the sum of two prime numbers.");
}
static boolean IsPrime(int num) {
boolean my_prime = true;
for (int i = 2; i <= num / 2; ++i) {
if (num % i == 0) {
my_prime = false;
break;
}
}
return my_prime;
}
}Output
Required packages have been imported A reader object has been defined Enter the number : 43 The number can be expressed as sum of two prime numbers. All the possible solutions are : 43 = 2 + 41
Example 2
Here, the integer has been previously defined, and its value is accessed and displayed on the console.
public class SumOfPrimes {
public static void main(String[] args) {
int my_input, i;
boolean my_temp = false;
my_input = 43;
System.out.println("The number is defined as " +my_input);
for (i = 2; i <= my_input / 2; ++i) {
if (IsPrime(i)) {
if (IsPrime(my_input - i)) {
System.out.println("The number can be expressed as sum of two prime numbers.");
System.out.println("The possible solutions are :");
System.out.printf("%d = %d + %d\n", my_input, i, my_input - i);
my_temp = true;
}
}
}
if (!my_temp)
System.out.println(my_input + " cannot be expressed as the sum of two prime numbers.");
}
static boolean IsPrime(int num) {
boolean my_prime = true;
for (int i = 2; i <= num / 2; ++i) {
if (num % i == 0) {
my_prime = false;
break;
}
}
return my_prime;
}
}Output
The number is defined as 43 The number can be expressed as sum of two prime numbers. All the possible solutions are : 43 2 + 41
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