CODEWITHSUNDEEP
pythonrecursion
Stanleyrr
Stanleyrr

Reputation: 875

Having trouble understanding this recursion function

I was reviewing the topic of recursion in a tutorial site and came across the problem and solution below:

Problem:

Given an integer, create a function which returns the sum of all the individual digits in that integer. For example: if n = 4321, return 10 since 4+3+2+1=10.

Solution:

def sum_func(n):
    if n<10:
        return n
    else:
        return n%10 + sum_func(int(n/10))
    pass

I understand the "if n<10" is the base case - if n is single digit, then just return n. What I couldn't wrap my head around is the "n%10 + sum_func(int(n/10))" line of code. Assume n is 235, n%10 will equal 5 and int(n/10) will equal 23. Then ultimately 5 + 23 will become 28 and not 10.

Could someone please help me understand what "n%10 + sum_func(int(n/10))" does? If you could walk through the logic one line at a time that would be greatly appreciated!

Upvotes: 0

Views: 151

Answers (2)

Toothpick Anemone
Toothpick Anemone

Reputation: 4646

if n = 235 then int(n/10) is 23. However, you do not add 5 and 23. You add sum_func(int(n/10)) and 5.

sum_func(int(n/10)) is not int(n/10)

sum_func(int(n/10)) adds the digits in the number "23" together.

As such, sum_func(int(n/10)) == 2 + 3 == 5

sum_func(235) == sum_func(235)
              == sum_func(23)    + 5
              == sum_func(2) + 3 + 5
              == 2           + 3 + 5

Upvotes: 4

David Waterworth
David Waterworth

Reputation: 2931

As you say if there's only 1 digit return it.

The % is the modulus operator - i.e. the remainder when dividing by 10, or simply the last digit (i.e. 235%10 = 5)

int(n/10) drops the last digit since the int() function rounds down - i.e. 235 -> 23

Now what you seem to be confused by is within sum_func it calls sum_func again with the remainder once the last digit has been dropped i.e. 23 (this is the recursion part) which will then return 5.

i.e. you have

sum_func(235)
=5 + sum_func(23)
=5 + (3 + sum_func(2))
=5 + (3 + (2))
=10

Upvotes: 2

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