Armstrong Number Program in Java

What is an Armstrong number

An Armstrong number (for 3 digits) equals the sum of the cubes of its own digits. The classic example is 153: 1³ + 5³ + 3³ = 1 + 125 + 27 = 153. Checking one is a great exercise in extracting digits and using a loop.

Extracting digits

int num = 153;
while (num > 0) {
    int digit = num % 10;   // last digit
    System.out.println(digit);
    num = num / 10;         // remove last digit
}

The trick % 10 gets the last digit and / 10 removes it. Repeating peels digits off from right to left until the number becomes 0.

The Java program

public class Armstrong {
    public static void main(String[] args) {
        int num = 153;
        int original = num;    // keep a copy
        int sum = 0;

        while (num > 0) {
            int d = num % 10;
            sum += d * d * d;  // cube each digit
            num = num / 10;
        }
        if (sum == original)
            System.out.println(original + " is Armstrong");
        else
            System.out.println(original + " is not Armstrong");
    }
}

We save original, then loop: take each digit, add its cube to sum, and remove the digit. At the end we compare sum with the original number.

Dry run

For any number of digits

A true Armstrong number raises each digit to the power of the number of digits, not always 3. For a 4-digit number, you cube... no, you use the 4th power. So count the digits first, then raise each digit to that power. The 3-digit version above uses cubes because it has 3 digits.

Common mistakes

• Not saving the original. The loop reduces the number to 0 — keep a copy to compare.
• Wrong power. For n digits, raise each digit to the n-th power, not always 3.
• Swapping % and /. % 10 gets the digit; / 10 removes it.
• Wrong loop condition. Loop while num > 0 to process all digits.

Summary

• An Armstrong number equals the sum of its digits raised to the digit-count power.
• For 3 digits, that means the sum of cubes (153 = 1³+5³+3³).
• Get digits with % 10 and remove them with / 10.
• Save the original number to compare at the end.
• Compare the sum to the original to decide.

Next, practise digit work in the palindrome number program, or loops in Java loops.