**Example to check if an integer (entered by the user) can be expressed as the sum of two prime numbers of all possible combinations with the use of functions.**

**Example to check if an integer (entered by the user) can be expressed as the sum of two prime numbers of all possible combinations with the use of functions.**

To understand this example, you should have the knowledge of following C++ programming topics:

- C++ for Loop
- C++ if, if…else and Nested if…else
- C++ Functions
- Types of User-defined Functions in C++

This program takes a positive integer from user and checks whether that number can be expressed as the sum of two prime numbers.

If the number can be expressed as a sum of two prime numbers, the output shows the combination of the prime numbers.

To perform this task, a user-defined function is created to check prime number.

## Example: Check Whether a Number can be Expressed as a Sum of Two Prime Numbers

#include <iostream> using namespace std; bool checkPrime(int n); int main() { int n, i; bool flag = false; cout << "Enter a positive integer: "; cin >> n; for(i = 2; i <= n/2; ++i) { if (checkPrime(i)) { if (checkPrime(n - i)) { cout << n << " = " << i << " + " << n-i << endl; flag = true; } } } if (!flag) cout << n << " can't be expressed as sum of two prime numbers."; return 0; } // Check prime number bool checkPrime(int n) { int i; bool isPrime = true; for(i = 2; i <= n/2; ++i) { if(n % i == 0) { isPrime = false; break; } } return isPrime; }

**Output**

Enter a positive integer: 34 34 = 3 + 31 34 = 5 + 29 34 = 11 + 23 34 = 17 + 17