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.
Example
Input
Input number: 371
Output
371 is armstrong number
Required knowledge
Basic C programming, If else, While loop
What is Armstrong number?
An Armstrong number is a ndigit number that is equal to the sum of the n^{th} power of its digits. For example –
6 = 6^{1} = 6
371 = 3^{3} + 7^{3} + 1^{3} = 371
Logic to check Armstrong number
Step by step descriptive logic to check Armstrong number.
 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
.  Count total digits in the given number, store result in a variable say digits.
 Initialize another variable to store the sum of power of its digits ,say
sum = 0
.  Run a loop till
num > 0
. The loop structure should look likewhile(num > 0)
.  Inside the loop, find last digit of num. Store it in a variable say
lastDigit = num % 10
.  Now comes the real calculation to find sum of power of digits. Perform
sum = sum + pow(lastDigit, digits)
.  Since the last digit of num is processed. Hence, remove last digit by performing
num = num / 10
.  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);
}
else
{
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.

round()
function rounds a fractional number to nearest integer. 
pow()
function is used to find power of any number.
Output
Enter any number to check Armstrong number: 371 371 is ARMSTRONG NUMBER
Happy coding 😉