C Program to Generate Armstrong Numbers up to Limit n

Here's a C Program to generate Armstrong numbers up to a limit n using C concepts like For Loop, While Loop, IF statement and Modulus. This post also contains output of this program and an explanation of how the program generates Armstrong Numbers up to a limit n.

What is an Armstrong Number? A number is an Armstrong number if the sum of cubes of each digit of that number is equal to the number itself e.g. 153 is an Armstrong number i.e. 1³ + 5³ + 3³ = 153.

# include <stdio.h>
# include <conio.h>
void main()
{
int i, a, r, s, n ;
clrscr() ;
printf("Enter the limit : ") ;
scanf("%d", &n) ;
printf("\nThe armstrong numbers are :\n\n") ;
for(i = 0 ; i <= n ; i++)
{
a = i ;
s = 0 ;
while(a > 0)
{
r = a % 10 ;
s = s + (r * r * r) ;
a = a / 10 ;
}
if(i == s)
printf("%d\t", i) ;
}
getch() ;
}

Output of above program - 

Enter the limit : 1000
The armstrong numbers are :
0       1       153       370        371        407

Explanation of above program -

The above program first asks the user to enter a limit n and then using a for loop, it calculates all the Armstrong numbers that lie between 0 and n. To understand the working of this program let’s take an example –

Suppose the user enters 1000 as limit n. In this case, the for loop will run for the value of loop variable i = 0 to i = 1000 (or 1001 times).  Let’s understand the process inside the for loop.

Working of for loop –

• The first line inside the loop is creating a copy of the current number (loop variable i) that we’re checking to be an Armstrong number and storing its value in the variable “a”. Further operations are done on this copy. In this way the original value is not modified and we can easily do the comparison needed to check whether that number is an Armstrong number or not.
• In the second line of the loop, we’re initializing the sum s to zero at the start of new iteration, every time for all values of i.
• Next, there is a while loop, the process inside this while loop is same as the process used to generate the reverse of a number. The only difference is in the second line of while loop where we are adding the cubes of every digit of that number. (Take a look at the Program to Reverse an Integer if you need/)
• After this while loop, the variable s holds the sum of cubes of all digits of a number.
• Now to check whether that number is Armstrong or not all we have to do is to check whether the value of i and s are equal. Here i holds the original number we’re checking to be Armstrong and s holds the sum of cubes of all digits of that number. If these two value i.e. i and s are equal then that number “i” is Armstrong number and we print its value otherwise we continue with the next value of i. Also refer to this - Program to Check Whether the Given Number is Armstrong or Not . (This post contains further explanation about checking whether a number is Armstrong or not).