C++ program to check the Armstrong number

In this article, you will learn and get code to check whether an input number is an Armstrong number or not in C++. The program is created in these ways:

But before starting these programs, let's first understand what Armstrong's number means.

What is an Armstrong number?

a number that equals the sum of its own digits, where each digit is raised to the power of the number of digits. For example, 1634 is an Armstrong number because:

1634 = 14 + 64 + 34 + 44
     = 1 + 1296 + 81 + 256
     = 1297 + 337
     = 1634

The result is equal to the number itself. So it is an Armstrong number.

Note: Because the total number of digits in 1634 is 4, each of its digits is raised to the power of 4.

In C++, using the while loop, check the Armstrong number

This is the first part of the article. It asks the user to enter a number and checks whether it is an Armstrong number or not.

The question is: write a program in C++ that checks whether a given number, given by the user at run-time, is an Armstrong number or not. Here is its answer:

#include<iostream>
using namespace std;
int main()
{
    intnum, temp, noOfDigit=0, res=0, rem, pow, i;
    cout<<"Enter the Number: ";
    cin>>num;
    temp = num;
    while(num>0)
    {
        num = num/10;
        noOfDigit++;
    }
    num = temp;
    while(num>0)
    {
        rem = num%10;
        pow = 1;
        i = 0;
        while(i<noOfDigit)
        {
            pow = pow*rem;
            i++;
        }
        res = res + pow;
        num = num/10;
    }
    if(res==temp)
        cout<<"\nIt is an Armstrong Number";
    else
        cout<<"\nIt is not an Armstrong Number";
    cout<<endl;
    return 0;
}

This program was built and runs under the Code::Blocks IDE. Here is its sample run:

C++ program find Armstrong number

Now supply any number, say 1634, and press the ENTER key to check whether it is an Armstrong number or not, as shown in the snapshot given below:

C++ program check Armstrong number

And here is another sample run, with user input, 153:

check armstrong number c++

Because 153 is a three-digit number, each digit is raised to the power of three.

The following block of code:

while(num>0)
{
    num = num/10;
    noOfDigit++;
}

is used to count the total number of digits available in the given number.

The following is the dry run of the above program with user input, 153:

In C++, using the for loop, check the Armstrong number

Now let's create the same program as the previous one, using the for loop. That is, this program does the same job as the previous program. The only difference is that, in place of a "while" loop, we've used a "for" loop here:

#include<iostream>
using namespace std;
int main()
{
    intnum, temp, noOfDigit=0, res=0, rem, pow, i;
    cout<<"Enter the Number: ";
    cin>>num;
    for(temp=num; temp>0; temp=temp/10)
        noOfDigit++;
    for(temp=num; temp>0; temp=temp/10)
    {
        rem = temp%10;
        pow = 1;
        for(i=0; i<noOfDigit; i++)
            pow = pow*rem;
        res = res + pow;
    }
    if(res==num)
        cout<<"\nIt is an Armstrong Number";
    else
        cout<<"\nIt is not an Armstrong Number";
    cout<<endl;
    return 0;
}

This program produces exactly the same output as the previous program. Here is its sample run with user input, 371:

check armstrong number using for loop c++

And here is the last sample run, with user input: 567.

c++ check armstrong number

The same program in different languages

C++ Quiz


« Previous Program Next Program »


Liked this post? Share it!