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

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

Visit these pages to learn to check whether a number is

## Example: Check Prime and Armstrong

``````#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;
}

// function to check prime number
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.
```

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.
• `0` if the number entered by the user is not a prime number.

Similarly, `checkArmstrongNumber()` function returns:

• `1` if the number entered by the user is an Armstrong number.
• `0` if the number entered by the user is not an Armstrong number.

Note: In `checkPrimeNumber()` and `checkArmstrongNumber()` functions, the flag variables are the return value of the functions.

In `main()`, the flag variable stores the values returned by `checkPrimeNumber()` and `checkArmstrongNumber()`.