Casting out nines clear solution for Sum of Digits by amandel - python coding challenges

Enable Javascript in your browser and then refresh this page, for a much enhanced experience.

Casting out nines solution in Clear category for Sum of Digits by amandel

def sum_digits(num: int) -> int:
    d = num % 9
    return 0 if not num else d if d else 9


print("Example:")
print(sum_digits(38))

assert sum_digits(38) == 2
assert sum_digits(0) == 0
assert sum_digits(10) == 1
assert sum_digits(132) == 6
assert sum_digits(232) == 7
assert sum_digits(811) == 1
assert sum_digits(702) == 9

print("The mission is done! Click 'Check Solution' to earn rewards!")

Sept. 12, 2022

Comments:

mithilkotak on Oct. 2, 2022, 6:04 a.m.
Your solution can be turned into a compact one-liner: <pre class='brush: python'>def sum_digits(num:int)->int:return 0 if not num else d if(d:=num%9) else 9 </pre> Very fast solution and clever use of divisibility rule for 9. It is well known that a number is divisible by 9 if and only if the continued sum of its digits till only one digit remains is 9. Note that continued sum of digits of a number till only one digit remains can be 0 if and only if the number is 0, otherwise it is 1 or 2 or 3 or ... or 9. Therefore, if num%9==0 then continued sum of digits till only one digit remains is 9 and if num%9!=0 then it is num%9 because num=(greatest multiple of 9 less than num)+num%9.

amandel on Oct. 2, 2022, 10:57 a.m.
I have improved it even more. I posted a new solution, named Logical It is a ridiculously simple one-liner: return num and ( num % 9 or 9 )

Strings Theory

0 %

You ClassRoom Demo is Ready With the new tool CheckiO ClassRoom, you will be able to analyze your students' progress, have your own leaderboard, change missions order on the map, and so on.