CODEWITHSUNDEEP
pythonrecursiontree
dannyyy
dannyyy

Reputation: 149

Why are my variables outside of recursive function not keeping their values?

So I'm just working on some simple tree functions. I'm stumped on the way recursion is working in my code. I know it's probably simple in what I am missing, but hoping someone on here can explain to me in simple terms why my code isn't working the way I want.

So, I'm trying to just trying to do a preorder traversal through a tree and append the node content to a string i declare outside of the preorder function.

t:

   1
  / \
 2   3

def preorderPrint(t):
    tree = "TREE: "

    def preorder(tree, string):
        if tree is None:
            string += f"None "
            return

        string += f"#{tree.val} "
        print(string)

        if tree.left:
            preorder(tree.left, string)
        if tree.right:
            preorder(tree.right, string)

    preorder(t, tree)

    return tree

The output from print statements looks like this:

TREE: #1 
TREE: #1 #2 
TREE: #1 #3 
TREE:

The last print is me printing what is returned from the function call.

So what Im trying to understand is, why do the contents of the string change? I can somewhat see that as it moves up the recursive stack, it doesn't retain its original value, but I'm just confused on what exactly is happening. If anyone can explain, I would greatly appreciate it!

Upvotes: 0

Views: 592

Answers (2)

Florian
Florian

Reputation: 56

If you pass immutable arguments like integers, strings or tuples to a function, the passing acts like call-by-value. You need to return the string at the end of your function and concatenate the returned value again.

Upvotes: 3

prhmma
prhmma

Reputation: 953

you function preorder does not return any values and does numbers printed inside your recursive function. consider you first #1 which is true and expected, the next node will be #2 as it is the left node. your code prints it out when the inner function is executing with this arguments preorder(2,'#1'). after this, there are no left nodes and it will get back to your first node #1. then it will consider your right node that will be your third answer. if you want something like #1 #2 #3, you should return values from your preorder function and append it to a string variable which comulativly gather your data and print it out when your search is done.

Upvotes: 0

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