FAQ: Decision Trees - Recursive Tree Building

Frequently Asked Questions Python FAQ
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This community-built FAQ covers the “Recursive Tree Building” exercise from the lesson “Decision Trees”.

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This exercise can be found in the following Codecademy content:

Data Science

Machine Learning

FAQs on the exercise Recursive Tree Building

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2

In part 7 of this exercise, we’re calling build_tree inside its own definition - what is that doing exactly?

Is it the case that because there already is a return statement in the definition, the function calls that up again? (seems to be the case from the output of the print call on it)

3 Likes
3

I don’t get it either! @toastedpitabread I know you are quite good with Python, could you help us? I get it that the objective is to return the branches with a Counter(labels) in the end, but I’m not getting how that translates into the tree we saw at the beginning of the exercise, I can’t visualize the following as a tree:

Splitting
--> Branch 0:
  Counter({'unacc': 4})
--> Branch 1:
  Splitting
  --> Branch 0:
    Counter({'good': 1})
  --> Branch 1:
    Counter({'acc': 1})
  --> Branch 2:
    Counter({'unacc': 1})
--> Branch 2:
  Splitting
  --> Branch 0:
    Counter({'vgood': 1})
  --> Branch 1:
    Counter({'acc': 1})
  --> Branch 2:
    Counter({'acc': 1})
4

I can’t access the code because I don’t have pro but it sounds like it’s using a recursive function.

I know the comp sci path in CC has a chapter on recursion, and binary trees, so that might be helpful to look up too.

If one hasn’t dealt with recursion before I personally find it easiest to write out by hand what happens (maybe even making a simpler model at first to get it). Like, if this is the first time encountering recursion, it might be better to experiment with a small function that emulates the factorial notation in math recursively.

There’s a lot of videos online explaining recursion trees, here’s one from MIT:

2 Likes
5

Yea I don’t get it either, how can you have multiple returns in a function?

6

It’s not about having multiple returns, it’s about having recursive returns.

def hello(repeat):
     if(repeat==0):
             return
     print("hello")
     return(hello(repeat-1))

hello(5)
# output 
# hello
# hello
# hello
# hello
# hello
1 Like