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

Write a C program to enter any number from user and check whether given number is Armstrong number or not using for loop. C program for Armstrong number. Logic to check Armstrong numbers in C program.



Input number: 371


371 is armstrong number

Required knowledge

Basic C programming, For loop, If else

What is Armstrong number?

An Armstrong number is an n-digit number that is equal to the sum of the nth powers of its digits. Below are few examples of Armstrong numbers:
6 = 61 = 6
371 = 33 + 73 + 13 = 371

Logic to check Armstrong number

There are three important concepts required for checking Armstrong number. The entire logic is based upon these three concepts.

Must know -

Below is the step by step descriptive logic to check Armstrong number.

  1. Input a number from user to be checked for Armstrong. 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 it in some 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 extract the last digit of num. Store it in some variable say lastDigit = num % 10.
  6. Now comes the real calculation to find sum of power of digits. Perform sum = sum + pow(lastDigit, digits). Here pow(x, y) returns xy.
  7. Since the last digit of num is processed. Hence, remove last digit by performing num = num / 10. Repeat step 5-7 till num > 0.
  8. After loop according to definition 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;


Enter any number to check Armstrong number: 371

Happy coding 😉

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