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In this example, you will learn to check if an integer entered by the user can be expressed as the sum of two prime numbers of all possible combinations.

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

• C if…else Statement
• C for Loop
• C Functions
• C User-defined functions

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To accomplish this task, we will create a function named checkPrime().

The checkPrime() returns 1 if the number passed to the function is a prime number.

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Integer as a Sum of Two Prime Numbers

#include 
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) {
                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