Example: C program to check whether a number can be expressed as the sum of two prime numbers

// C program to check whether a number can be expressed as the sum of two prime numbers
#include <stdio.h>

// @custom function to the value is prime or not
int IsPrimeNumber(int random_number) {
    int flag = 1;
    for (int i = 2; i <= random_number / 2; i++) {
        if (random_number % i == 0) {
            flag = 0;
            break;
        }
    }
    return flag;
}

// @driver function to call & check the expressed
// @As sum of two prime numbers
int main() {
    int random_number, flag = 0;

    printf("Enter a positive integer number: ");
    scanf("%d", &random_number);

    printf("\n");
    for (int i = 2; i <= random_number / 2; i++) {

        // It will check (i) to be a prime number
        if (IsPrimeNumber(i) == 1) {

            // It will check (random_number - 1) to be a prime number
            if (IsPrimeNumber(random_number - i) == 1) {
                printf("%d = %d + %d\n", random_number, i, random_number - i);
                flag = 1;
            }
        }
    }
    if (flag == 0)
        printf("%d can not be expressed as the sum of two prime numbers!\n", random_number);
    return 0;
}