Python Program to Find Sum of Digits Using While Loop

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

Examples

Input123

Output6

Input0

Output0

Input9999

Output36

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 num % 10 to get the last digit and add it to a total sum. Then remove that digit by dividing the number by 10 using num //= 10. Repeat this until the number is zero.

Algorithm

1

Get the input number.

2

Initialize sum to zero.

3

While the number is greater than zero:

4

Extract the last digit using modulo 10.

5

Add the digit to sum.

6

Remove the last digit by integer division by 10.

7

After the loop ends, return or print the sum.

Code

python

num = int(input("Enter a number: "))
sum_digits = 0
while num > 0:
    digit = num % 10
    sum_digits += digit
    num //= 10
print("Sum of digits:", sum_digits)

Output

Enter a number: 123
Sum of digits: 6

Dry Run

Let's trace the input 123 through the code

1

Initial values

num = 123, sum_digits = 0

2

First iteration

digit = 123 % 10 = 3; sum_digits = 0 + 3 = 3; num = 123 // 10 = 12

3

Second iteration

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

4

Third iteration

digit = 1 % 10 = 1; sum_digits = 5 + 1 = 6; num = 1 // 10 = 0

5

Loop ends

num is now 0, exit loop

num	digit	sum_digits
123	3	3
12	2	5
1	1	6

Why This Works

Step 1: Extract last digit

Using num % 10 gets the last digit of the number because modulo gives the remainder after division by 10.

Step 2: Add digit to sum

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

Step 3: Remove last digit

Use integer division num //= 10 to drop the last digit, moving to the next digit in the next loop.

Alternative Approaches

Using recursion

python

def sum_digits(n):
    if n == 0:
        return 0
    return n % 10 + sum_digits(n // 10)

num = int(input("Enter a number: "))
print("Sum of digits:", sum_digits(num))

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

Using string conversion

python

num = input("Enter a number: ")
sum_digits = sum(int(d) for d in num)
print("Sum of digits:", sum_digits)

Converting to string is simple and readable but less efficient for very large numbers.

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

Time Complexity

The loop runs once for each digit in the number, so time grows linearly with the number of digits, O(d).

Space Complexity

Only a few variables are used regardless of input size, so space complexity is O(1).

Which Approach is Fastest?

The while loop approach is efficient and uses constant space, while recursion uses extra stack space and string conversion may be slower for large inputs.

Approach	Time	Space	Best For
While loop	O(d)	O(1)	Efficient and memory-friendly
Recursion	O(d)	O(d)	Elegant but uses more memory
String conversion	O(d)	O(d)	Simple code, good for small inputs