Teaching Kids Programming – Compute the Sum of the Digits using Three Methods

Teaching Kids Programming: Videos on Data Structures and Algorithms

Given a positive integer num, return the sum of its digits. Bonus: Can you do it without using strings?

Constraints
0 ≤ num < 2 ** 31

Example 1
Input
num = 123
Output
6
Explanation
Since 6 = 1 + 2 + 3

Example 2
Input
num = 50
Output
5
Explanation
Since 5 = 5 + 0

Sum of the Digits by Converting to String

By converting the number to string, we can actually iterate the digits easily:

def sumOfDigits(n):
  s = str(n)
  ans = 0
  for i in s:
    ans += i
  return ans

A more fancy Python code using list comprehension via sum method:

def sumOfDigits(n):
  return sum([int(x) for x in str(n)])

Recursive Summing up the Digits

We know the sum of the least significant digit (rightmost), and we can add it together with the others – which can be recursively solved. The terminal case is that when given number is zero, the sum is zero.

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

Suming up digits by Extracting Digits one by one

Similar to the above recursive approach, but implemented using a loop iterativedly.

def sumOfDigits(n):
  ans = 0
  while n > 0:
    ans += n % 10
    n //= 10
  return ans

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