Sum of Digits

Two operations peel digits off a number:

• n % 10 gives the last digit
• Math.floor(n / 10) (or n // 10 in Python) removes the last digit

Repeat both until n becomes 0:

total = 0
while n > 0:
    total += n % 10
    n //= 10

This digit-peeling loop is the backbone of the next several lessons: reversing, palindromes, Armstrong numbers and digital roots.