In this program, two user-defined functions checkPrimeNumber() and checkArmstrongNumber() are created.

The checkPrimeNumber() function returns 1 if the number entered by the user is a prime number. Similarly, checkArmstrongNumber() function also returns 1 if the number entered by the user is an Armstrong number.

Example: Check Prime and Armstrong

#include <math.h>
#include <stdio.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;
}

// function to check prime number
int checkPrimeNumber(int n){
   int i, flag = 1, squareRoot;

   // computing the square root
   squareRoot = sqrt(n);
   for (i = 2; i <= squareRoot; ++i) {
      // condition for non-prime number
      if (n % i == 0) {
         flag = 0;
         break;
      }
   }
   return flag;
}

// function to check Armstrong number
int checkArmstrongNumber(int num){
   int originalNum, remainder, n = 0, flag;
   double result = 0.0;

   // store the number of digits of num in n
   for (originalNum = num; originalNum != 0; ++n) {
      originalNum /= 10;
   }

   for (originalNum = num; originalNum != 0; originalNum /= 10) {
      remainder = originalNum % 10;

      // store the sum of the power of individual digits in result
      result += pow(remainder, n);
   }

   // condition for Armstrong number
   if (round(result) == num)
      flag = 1;
   else
      flag = 0;
   return flag;
}

Output

Enter a positive integer: 407
407 is not a prime number.
407 is an Armstrong number.