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 if...else Statement
- C Programming for Loop
- C Programming Functions
- C Programming User-defined functions

To accomplish this task, `checkPrime()`

function is created.

The `checkPrime()`

returns 1 if the number passed to the function is a prime number.

```
#include <stdio.h>
int checkPrime(int n);
int main()
{
int n, i, flag = 0;
printf("Enter a positive integer: ");
scanf("%d", &n);
for(i = 2; i <= n/2; ++i)
{
// condition for i to be a prime number
if (checkPrime(i) == 1)
{
// condition for n-i to be a prime number
if (checkPrime(n-i) == 1)
{
// n = primeNumber1 + primeNumber2
printf("%d = %d + %d\n", n, i, n - i);
flag = 1;
}
}
}
if (flag == 0)
printf("%d cannot be expressed as the sum of two prime numbers.", n);
return 0;
}
// Function to check prime number
int checkPrime(int n)
{
int i, isPrime = 1;
for(i = 2; i <= n/2; ++i)
{
if(n % i == 0)
{
isPrime = 0;
break;
}
}
return isPrime;
}
```

**Output**

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