Digit Sum

Write a function that sums the digits of a non-negative integer.

digit_sum(n)

Do it with a while loop and arithmetic (% and //) — don't convert n to a string.

Examples

digit_sum(1234)   // → 10   (1+2+3+4)
digit_sum(9)       // → 9
digit_sum(100)     // → 1
digit_sum(55555)   // → 25

Walkthrough

Thinking it through

This is exactly the kind of problem where you don't know the number of iterations ahead of time — a number could have 1 digit or 10 — which is the signature of a while loop from the lesson.

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

Two arithmetic tricks do all the work:

• n % 10 is the remainder when dividing by 10 — for any integer, that's exactly its last digit. 1234 % 10 is 4.
• n // 10 is floor division by 10 — it removes the last digit. 1234 // 10 is 123.

Each loop iteration peels off one digit (adding it to total) and shortens n by one digit, until n reaches 0 and the loop condition n > 0 becomes false — that's the "something moves toward making the condition false" requirement from the for-vs-while lesson, and it's what guarantees this loop actually terminates.

Trace 1234: total=0, n=1234 → add 4, n=123 → add 3, n=12 → add 2, n=1 → add 1, n=0, loop stops. total = 4+3+2+1 = 10. ✓

Sign in to join the discussion.