Python Program to Find Sum of Digits of a Number

You can find the sum of digits of a number in Python by converting it to a string and summing the integer value of each character using sum(int(digit) for digit in str(number)).

Examples

Input123

Output6

Input405

Output9

Input0

Output0

How to Think About It

To find the sum of digits, think of the number as a sequence of single digits. Extract each digit one by one, convert it to a number if needed, and add it to a total sum. Repeat until all digits are added.

Algorithm

1

Get the input number.

2

Convert the number to a string to access each digit.

3

For each character in the string, convert it back to an integer.

4

Add each integer digit to a running total.

5

Return or print the total sum.

Code

python

number = int(input("Enter a number: "))
sum_of_digits = sum(int(digit) for digit in str(abs(number)))
print("Sum of digits:", sum_of_digits)

Output

Enter a number: 123
Sum of digits: 6

Dry Run

Let's trace the input 123 through the code

1

Input number

number = 123

2

Convert to string

str(number) = '123'

3

Convert each digit to int and sum

int('1') + int('2') + int('3') = 1 + 2 + 3 = 6

4

Print result

Sum of digits: 6

Digit	Integer Value	Running Sum
'1'	1	1
'2'	2	3
'3'	3	6

Why This Works

Step 1: Convert number to string

Converting the number to a string lets us access each digit individually as a character.

Step 2: Convert each character to integer

Each character digit is converted back to an integer so we can perform arithmetic.

Step 3: Sum all digits

Adding all integer digits together gives the total sum of the digits.

Alternative Approaches

Using a while loop and modulo

python

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

This method uses math operations without converting to string, which can be more efficient for very large numbers.

Using recursion

python

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

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

This recursive method breaks down the problem into smaller parts but may be less efficient for very large numbers.

Complexity: O(n) time, O(n) space

Time Complexity

The program processes each digit once, so time grows linearly with the number of digits (n).

Space Complexity

Converting the number to a string uses extra space proportional to the number of digits (n).

Which Approach is Fastest?

The modulo and division method uses constant space and is generally faster for very large numbers, while string conversion is simpler and more readable.

Approach	Time	Space	Best For
String conversion and sum	O(n)	O(n)	Simplicity and readability
Modulo and division loop	O(n)	O(1)	Efficiency with large numbers
Recursion	O(n)	O(n)	Learning recursion, less efficient