Java Program to Find Sum of Digits Using While Loop

You can find the sum of digits of a number in Java using a while loop by repeatedly extracting the last digit with num % 10, adding it to a sum, and then removing the last digit with num = num / 10 until the number becomes zero.

Examples

Input123

OutputSum of digits: 6

Input0

OutputSum of digits: 0

Input99999

OutputSum of digits: 45

How to Think About It

To find the sum of digits, think of peeling off one digit at a time from the right side of the number. Use the remainder operator % to get the last digit, add it to a total sum, then remove that digit by dividing the number by 10. Repeat this until no digits remain.

Algorithm

1

Get the input number.

2

Initialize sum to 0.

3

While the number is greater than 0:

4

Extract the last digit using remainder operator.

5

Add the digit to sum.

6

Remove the last digit by dividing the number by 10.

7

After the loop ends, return or print the sum.

Code

java

import java.util.Scanner;

public class SumOfDigits {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = scanner.nextInt();
        int sum = 0;
        while (num > 0) {
            sum += num % 10;
            num /= 10;
        }
        System.out.println("Sum of digits: " + sum);
        scanner.close();
    }
}

Output

Enter a number: 123
Sum of digits: 6

Dry Run

Let's trace the input 123 through the code to see how the sum is calculated.

1

Initial values

num = 123, sum = 0

2

First iteration

sum = 0 + 123 % 10 = 0 + 3 = 3; num = 123 / 10 = 12

3

Second iteration

sum = 3 + 12 % 10 = 3 + 2 = 5; num = 12 / 10 = 1

4

Third iteration

sum = 5 + 1 % 10 = 5 + 1 = 6; num = 1 / 10 = 0

5

Loop ends

num is now 0, exit loop; sum = 6

num	num % 10	sum	num after division
123	3	3	12
12	2	5	1
1	1	6	0

Why This Works

Step 1: Extract last digit

Using num % 10 gets the last digit of the number because it gives the remainder when divided by 10.

Step 2: Add digit to sum

Add the extracted digit to the running total stored in sum.

Step 3: Remove last digit

Divide the number by 10 using integer division (num /= 10) to drop the last digit and prepare for the next iteration.

Alternative Approaches

Using recursion

java

public class SumOfDigits {
    public static int sumDigits(int num) {
        if (num == 0) return 0;
        return num % 10 + sumDigits(num / 10);
    }
    public static void main(String[] args) {
        int num = 123;
        System.out.println("Sum of digits: " + sumDigits(num));
    }
}

Recursion is elegant but uses more memory due to function calls.

Using for loop

java

public class SumOfDigits {
    public static void main(String[] args) {
        int num = 123;
        int sum = 0;
        for (; num > 0; num /= 10) {
            sum += num % 10;
        }
        System.out.println("Sum of digits: " + sum);
    }
}

For loop is a compact alternative but functionally similar to while loop.

Complexity: O(d) time, O(1) space

Time Complexity

The loop runs once for each digit in the number, so the time depends on the number of digits, denoted as d.

Space Complexity

Only a few variables are used, so space is constant, O(1).

Which Approach is Fastest?

The while loop and for loop approaches have similar speed and memory use; recursion uses more memory due to call stack overhead.

Approach	Time	Space	Best For
While loop	O(d)	O(1)	Simple and clear iteration
For loop	O(d)	O(1)	Compact iteration syntax
Recursion	O(d)	O(d)	Elegant code but uses more memory