# C Program to Check Prime or Armstrong Number Using User-defined Function

In this example, you will learn to check whether an integer is a prime number or an Armstrong or both by creating two separate functions.

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

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.

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## Check Prime and Armstrong Number

``````#include <math.h>
#include <stdio.h>
int checkArmstrongNumber(int n);
int main() {
int n, flag;
printf("Enter a positive integer: ");
scanf("%d", &n);

// check prime number
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 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 num) {
int original, rem, result = 0, n = 0, flag;
original = num;
while (original != 0) {
original /= 10;
++n;
}
original = num;
while (original != 0) {
rem = original % 10;
result += pow(rem, n);
original /= 10;
}
// condition for Armstrong number
if (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.
```