C Programming Code Examples C > Mathematics Code Examples C Program to Check Prime or Armstrong Number Using User-defined Function C Program to Check Prime or Armstrong Number Using User-defined Function In this program, two user-defined functions checkPrimeNumber() and checkArmstrongNumber() are created. The checkPrimeNumber() returns 1 if the number entered by the user is a prime number. Similarly, checkArmstrongNumber() returns 1 if the number entered by the user is an Armstrong number. #include <stdio.h> #include <math.h> int checkPrimeNumber(int n); int checkArmstrongNumber(int n); int main() { int n, flag; printf("Enter a positive integer: "); scanf("%d", &n); // Check prime number flag = checkPrimeNumber(n); if (flag == 1) printf("%d is a prime number.\n", n); else printf("%d is not a prime number.\n", n); // Check Armstrong number flag = checkArmstrongNumber(n); if (flag == 1) printf("%d is an Armstrong number.", n); else printf("%d is not an Armstrong number.",n); return 0; } int checkPrimeNumber(int n) { int i, flag = 1; for(i=2; i<=n/2; ++i) { // condition for non-prime number if(n%i == 0) { flag = 0; break; } } return flag; } int checkArmstrongNumber(int number) { int originalNumber, remainder, result = 0, n = 0, flag; originalNumber = number; while (originalNumber != 0) { originalNumber /= 10; ++n; } originalNumber = number; while (originalNumber != 0) { remainder = originalNumber%10; result += pow(remainder, n); originalNumber /= 10; } // condition for Armstrong number if(result == number) flag = 1; else flag = 0; return flag; }