Function not retaining a variable value during recursion.

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I am trying to write a simple code to find the sum of all values within a given range in BST using recursion but for some reason, the 'summ' variable is not functioning the way I think it should work. Here is the code

def rangeSumBST(self, root: TreeNode, low: int, high: int , summ = 0) -> int:

    if root:
        if root.val >= low and root.val <= high:
            print(root.val)
            summ += root.val
            print('summ: ' + str(summ))
        self.rangeSumBST(root.left , low, high , summ)
        self.rangeSumBST(root.right, low, high,summ)
    return summ

Input tree: [10,5,15,3,7,null,18]

My output:-

10
summ: 10
7
summ: 17 # 10 + 7
15
summ: 25 # 10 + 15

Desired Output:-
10
summ: 10
7
summ: 17 # 10 + 7
15
summ: 32 # 17 + 15
I am confused can anyone please help and elaborate the reason.

What I have tried:

I am new to recursion and I am finding it very hard so I didn't tried anything significant

Posted 26-Jan-21 10:36am

Dr Doggo

Updated 26-Jan-21 20:00pm

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Richard MacCutchan 27-Jan-21 4:16am

You need to use the debugger to step through the code to identify exactly which values are being passed to rangeSumBST at each step. It is not clear from your question what these values are, or the structure of your Treenodes.

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OriginalGriff · Answer 1 · 2021-01-26T20:00:00

Writing your code does not mean it is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ? See here if you don;t know how to use it: pdb — The Python Debugger — Python 3.9.1 documentation[^]
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!