C program to check whether a number is armstrong number or not

Write a C program to input a number from user and check whether given number is Armstrong number or not. How to check Armstrong numbers in C program. Logic to check Armstrong numbers in C programming.



Input number: 371


371 is armstrong number

Required knowledge

Basic C programming, If else, While loop

What is Armstrong number?

An Armstrong number is a n-digit number that is equal to the sum of the nth power of its digits. For example –
6 = 61 = 6
371 = 33 + 73 + 13 = 371

Logic to check Armstrong number

Step by step descriptive logic to check Armstrong number.

  1. Input a number from user. Store it in some variable say num. Make a temporary copy of the value to some another variable for calculation purposes, say originalNum = num.
  2. Count total digits in the given number, store result in a variable say digits.
  3. Initialize another variable to store the sum of power of its digits ,say sum = 0.
  4. Run a loop till num > 0. The loop structure should look like while(num > 0).
  5. Inside the loop, find last digit of num. Store it in a variable say lastDigit = num % 10.
  6. Now comes the real calculation to find sum of power of digits. Perform sum = sum + pow(lastDigit, digits).
  7. Since the last digit of num is processed. Hence, remove last digit by performing num = num / 10.
  8. After loop check if(originalNum == sum), then it is Armstrong number otherwise not.

Program to check Armstrong number

 * C program to check Armstrong number
#include <stdio.h>
#include <math.h>

int main()
    int originalNum, num, lastDigit, digits, sum;

    /* Input number from user */
    printf("Enter any number to check Armstrong number: ");
    scanf("%d", &num);

    sum = 0;

    /* Copy the value of num for processing */
    originalNum = num;

    /* Find total digits in num */
    digits = (int) log10(num) + 1;

    /* Calculate sum of power of digits */
    while(num > 0)
        /* Extract the last digit */
        lastDigit = num % 10;

        /* Compute sum of power of last digit */
        sum = sum + round(pow(lastDigit, digits));

        /* Remove the last digit */
        num = num / 10;

    /* Check for Armstrong number */
    if(originalNum == sum)
        printf("%d is ARMSTRONG NUMBER", originalNum);
        printf("%d is NOT ARMSTRONG NUMBER", originalNum);

    return 0;

In the above program round() and pow() are mathematical functions present in math.h header file.


Enter any number to check Armstrong number: 371

Happy coding 😉