Passing value to a function in recursion in Python

Question

I have been trying to pass value from my recursive helper function to the parent, but it is passing None. I am learning Python and recursion and trying to understand how the values are passed between functions.

def superDigitSum(n,k):
    num2str = [int(i) for i in list(str(n))*k] #creates the list[1,4,8,1,4,8,1,4,8]
    result=0
    return superDigitSumHelper(num2str, result)

def superDigitSumHelper(num2str, helper_result):
    if not num2str :
        return helper_result
    else:
        helper_result+=num2str[-1]
        superDigitSumHelper(num2str[:-1],helper_result)

print(superDigitSum(148,3))

The code creates the list [1,4,8,1,4,8,1,4,8] and sums the values. I expect the result to be 39 (3 * (1+4+8)).

If you have an answer which suits your needs, please dont forget to mark it as such for people to later use as a point of reference. :) — Fallenreaper
– Fallenreaper, Commented Jun 7, 2018 at 17:14
Python programmers use snake_case instead of lowerCamelCase. You should name your function super_digit_sum instead of superDigitSum. — Mulan
– Mulan, Commented Jun 7, 2018 at 19:01
When you get to a resolution, please remember to up-vote useful things and accept your favourite answer (even if you have to write it yourself), so Stack Overflow can properly archive the question. — Prune
– Prune, Commented Jun 7, 2018 at 21:07

Sam · Accepted Answer · 2018-06-07 15:14:43Z

1

try

else:
    helper_result+=num2str[-1]
    return superDigitSumHelper(num2str[:-1],helper_result)

Without returning the results of the recursive call you are just returning None

answered Jun 7, 2018 at 15:14

Sam

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Comments

Fallenreaper · Accepted Answer · 2018-06-07 15:15:00Z

1

Your function is slightly wrong.

You need to add a return the following to your superDigitSumHelper function:

return superDigitSumHelper(num2str[:-1], helper_result)

in your else conditional.

Essentially, it is returning none because after the function ends, it isnt returning anything so the default value is none.

answered Jun 7, 2018 at 15:15

Fallenreaper

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Comments

Prune · Accepted Answer · 2018-06-07 15:15:28Z

1

This clause returns None to the next call up the stack. You need to return the last value:

else:
    helper_result+=num2str[-1]
    superDigitSumHelper(num2str[:-1],helper_result)

Instead,

else:
    helper_result+=num2str[-1]
    return superDigitSumHelper(num2str[:-1],helper_result)

answered Jun 7, 2018 at 15:15

Prune

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Comments

Mulan · Accepted Answer · 2018-06-07 18:57:29Z

If you're allowed to do things like str(n) or list*k, or even * at all, it kind of defeats the purpose of the exercise

def super_digit_sum (n, k):
  return sum (map (int, list (str (n)))) * k

print (super_digit_sum (148, 3)) # 39

But I'm sure the point of this is to use primitive features and operations as a means of learning how to write simple recursive programs...

We start with a simple function that sums the digits of a single input number

def sum_digits (n):
  if n < 10:
    return n
  else:
    return n % 10 + sum_digits (n // 10)

print (sum_digits (148))
# 13

Then we make a simple function that can multiply two numbers

def mult (n, m):
  if n is 0:
    return 0
  else:
    return m + mult (n - 1, m)

print (mult (3, 7))
# 21

Then combining the two, we write your main function

def super_digit_sum (n, k):
  return mult (sum_digits (n), k)

print (super_digit_sum (148, 3))
# 39

If you're allowed to use *, you can skip mult altogether and simply write

def super_digit_sum (n, k):
  return sum_digits (n) * k

print (super_digit_sum (148, 3))
# 39

This is a better program because you arrive at the answer in a straightforward manner. Compare that to the other approach of taking a number, converting it into a string, chopping it up into single characters, converting those characters back to digits, then summing the digits, and finally multiplying the result by a constant.

Both approaches arrive at the same result, but only one results in a program worth reading, imo. I talk about this roundabout-style of programming in another python Q&A: How to count neighbors in a list? I think you will find it helpful.

Sure thing. I will do my best to use primitive features to solve the exercises. :)

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