C Program to check Armstrong number

In this example, we will write a C Program to check Armstrong number. An Armstrong number (also known as narcissistic number) is a number that is equal to the sum of its own digits each raised to the power of the number of digits.

For example, 370 is an Armstrong number

//We are adding cubes of individual digits
//because the number of digits in number is 3
370 = 3*3*3 + 7*7*7 + 0*0*0
    = 27 + 343 + 0
    = 370

C Program to check if a number is Armstrong number or not

The explanation of the program is available at the end of this code.

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

int main() {
    int number, originalNumber, remainder, numDigits = 0;
    double result = 0.0;

    // Prompt the user to enter a number
    printf("Enter an integer: ");
    scanf("%d", &number);

    //Copying the number to a different variable
    //so that the number doesn't get changed in calculations
    originalNumber = number;

    // Counting number of digits in the number
    while (originalNumber != 0) {
        originalNumber /= 10;
        ++numDigits;
    }

    originalNumber = number;

    // Calculate the sum of the digits raised to the power of numDigits
    while (originalNumber != 0) {
        //get the last digit
        remainder = originalNumber % 10;

        //add the last digit raised to the power of number of digits
        result += pow(remainder, numDigits);
   
        //remove last digit from number
        originalNumber /= 10;
    }

    // Check if the result is equal to the number
    if ((int)result == number)
        printf("%d is an Armstrong number.\n", number);
    else
        printf("%d is not an Armstrong number.\n", number);

    return 0;
}

Output:

Enter an integer: 153
153 is an Armstrong number.

Explanation of the program:

1. Include header files:
   • stdio.h is included for scanf and printf.
   • math.h is included for the pow function which calculates the power of a number.
2. Declare variables:
   • number to store the number entered by user.
   • originalNumber to hold the copy of number for calculation purposes.
   • remainder to get the last digit of number on every iteration.
   • numDigits to store the number of digits in the number.
   • result to store the sum of each digit raised to the power of the number of digits.
3. Input:
   • Prompt the user to enter a number using printf and then use scanf to read & store the entered value.
4. Calculate the number of digits:
   • Dividing number by 10 (which removes last digit from number) and increasing the value of numDigits by 1 on every iteration.
5. Calculate the Armstrong number:
   • Get last digit of number using modulo % operator on every iteration of while loop.
   • Raise each digit to the power of numDigits using pow function and add to result.
   • Divide the number by 10 to remove last digit so that the second last digit gets processed on next iteration.
6. Check and output the result:
   • Compare the original number with the calculated result, if equal, print that the input number is an Armstrong number.
   • Otherwise, print that the input number is not an Armstrong number.