Practice Page

February 10, 2022

Program to check a number is Armstrong or not

In this program, we will find the number is Armstrong or not where the number should be entered by the user, we will look at different ways to write Armstrong number in C.

Most websites have given incorrect definitions/codes for Armstrong of a number.

Number-is-Armstrong-or-not

Very Important

On most websites, Armstrong number definition is given wrong. They all assume any number where the sum of the cubes of its digits is equal to the number itself is Armstrong Number.

However, Armstrong number is any number following the given rule –

  • abcd… = an + bn + cn + dn + …
  • Where n is the order(length/digits in number)
Armstrong Number in C

Positive or Negative number

Even or Odd number 

Sum of First N Natural numbers

Sum of N natural numbers

Sum of numbers in a given range

Prime number within a given range

C Code:-

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

// Armstrong number is any number following the given rule
// abcd... = a^n + b^n + c^n + d^n + ...
// Where n is the order(length/digits in number)

// Example = 153 (order/length = 3)
// 153 = 1^3 + 5^3 + 3^3 = 1 + 125 + 27 = 153

// Example = 1634 (order/length = 4)
// 1634 = 1^4 + 6^4 + 3^4 + 4^4 = 1 + 1296 + 81 + 256 = 1634

// number of digits in a number is order
int order(int x)
{
    int len = 0;
    while (x)
    {
        len++;
        x = x/10;
    }
    return len;
}

int armstrong(int num, int len){

    int sum = 0, temp, digit;
    temp = num;
    
    // loop to extract digit, find power & add to sum
    while(temp != 0)
    {
        // extract digit
        digit = temp % 10;

        // add power to sum
        sum = sum + pow(digit,len);
        temp /= 10;
    };

    return num == sum;
}

// Driver Code
int main ()
{
    int num, len;
 
    printf("Enter a number: ");
    scanf("%d",&num);
 
    // function to get order(length)
    len = order(num);
    
    // check if Armstrong
    if (armstrong(num, len))
        printf("%d is Armstrong", num);
    else
        printf("%d is Not Armstrong", num);

}

Output:-

Enter a number: 1634
1634 is Armstrong

Enter a number: 209
209 is Not Armstrong

Enter a number: 153
153 is Armstrong

Method 2

This method uses recursion in C

Code

#include<stdio.h> 
#include <math.h>
int order(int x)
{
    int len = 0;
    while (x)
    {
        len++;
        x = x/10;
    }
    return len;
}
int getArmstrongSum(int num, int order){
    
    if(num == 0)
        return 0;
    
    int digit = num % 10;
    
    return pow(digit, order) + getArmstrongSum(num/10, order);
}


// Driver Code
int main ()
{
    int num, len;
 
    printf("Enter a number: ");
    scanf("%d",&num);
 
    // function to get order(length)
    len = order(num);
    
    // check if Armstrong
    if (num == getArmstrongSum(num, len))
        printf("%d is Armstrong", num);
    else
        printf("%d is Not Armstrong", num);

}

Output

Enter a number: 1634
1634 is Armstrong

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While loop in C