C Program to Check Whether a Number can be Expressed as Sum of Two Prime Numbers

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. Read - Programming language

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

• C if, if...else and Nested 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. 

Example: Integer as a Sum of Two Prime Numbers

#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