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Python while loops and digit processing

Repeat until a changing condition becomes false.

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Before you start

Integer division, remainder, and conditions.

Repeat until a changing condition becomes false.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A while loop checks its condition before every iteration, so its body may run zero times. A termination argument explains why the loop eventually stops.

For a nonnegative integer, n % 10 extracts the last decimal digit and n // 10 removes it. The function uses abs so negative inputs have the same digit sum as their magnitudes.

Code example

Python

def digit_sum(number):
    number = abs(number)
    total = 0
    while number > 0:
        total += number % 10
        number //= 10
    return total

print(digit_sum(204))
assert digit_sum(0) == 0
assert digit_sum(-204) == 6

Output

Example details

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1def digit_sum(number):

2 number = abs(number)

3 total = 0

4 while number > 0:

5 total += number % 10

6 number //= 10

7 return total

9print(digit_sum(204))

10assert digit_sum(0) == 0

11assert digit_sum(-204) == 6

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Start with 204.	`number = 204`	—
Initialize the sum.	`total = 0`	—
Extract the last digit 4.	`number = 204, total = 4`	—
Remove that digit.	`number = 20`	—
Add the next digit, 0.	`total = 4`	—
Remove the zero.	`number = 2`	—
Add the final digit 2.	`total = 6`	—
Reach zero, so the next condition is false.	`number = 0`	—
Return and print 6.	`number = 0`	6
Check zero and negative inputs without extra output.	`number = 0`	6

Behind the scenes

How Python processes this code

Python runs this source through its interpreter. It creates objects for values and resolves names as each statement runs.

Cost and common mistakes

O(d) iterations and O(1) stored variables for d decimal digits; arbitrary-precision arithmetic costs grow with integer size.

Omitting number //= 10 would prevent termination for positive inputs.
This example accepts integers, not decimal strings or floating-point values.

Try it yourself

Trace digit_sum(1002).

Reveal answer and reasoning

The extracted digits are 2, 0, 0, 1 and the result is 3.