Find Sum of Digits in Python

What You’ll Learn

Digit sum means adding every digit of a number: 12345 → 1+2+3+4+5 = 15. Extract digits with % 10 and // 10, use abs() for negatives, and stop when the value becomes 0. This tutorial covers a live preview, worked Python examples, edge cases, and complexity.

📝 Problem & Approach

Given an integer n, return the sum of its decimal digits (sign ignored).

# 12345 -> 1+2+3+4+5 = 15
# -802  -> 8+0+2     = 10
# 0     -> 0

Inputs & Outputs

Minimal workflow

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

Method comparison

🔮 Live Preview

Enter any integer (e.g. 12345 or -802) and see the digits plus their sum.

Press “Compute”.

Examples Gallery

Three complete Python programs — sum digits of 12345, read user input, and trace peeling for a negative value. Click View Output to reveal sample console results.

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

number = 12345
print(f"The sum of digits of {number} is: {digit_sum(number)}")

The sum of digits of 12345 is: 15

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

number = int(input("Enter a number: ").strip())
print(f"Sum of digits: {digit_sum(number)}")

Enter a number: 12345
Sum of digits: 15

n = -802
x = abs(n)
total = 0
print(f"Starting from abs({n}) = {x}")
while x != 0:
    digit = x % 10
    total += digit
    print(f"  take {digit}, remaining {x // 10}, total={total}")
    x //= 10
print(f"Digit sum: {total}")

Starting from abs(-802) = 802
  take 2, remaining 80, total=2
  take 0, remaining 8, total=2
  take 8, remaining 0, total=10
Digit sum: 10

🔎 Worked Walkthrough — 12345

Watch digits peel from the right while the accumulator climbs to 15.

Same steps for -802 after abs: 2, then 0, then 8 → total 10.

⚖️ Facts Worth Knowing

Handy follow-ups interviewers sometimes ask.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

d is the number of digits — about floor(log₁₀ |n|) + 1 for n ≠ 0.

💡 Best Practices