Variable Scope Issue in Python

Question

I am new to Python and I have been working with it for a bit, but I am stuck on a problem. Here is my code:

def collatz(num,ctr):
    if(num != 1):
        ctr+=1
        if(num%2==0):
            collatz(num/2,ctr)
        else:
            collatz(num*3+1,ctr)
    return ctr
test=collatz(9,0)

For any number I put in for num, let's say 9 for instance, and 0 for ctr, ctr always comes out as 1. Am I using the ctr variable wrong?

EDIT: I am trying to print out how many times the function is recursed. So ctr would be a counter for each recursion.

Answer 1

I changed your recursive calls to set the value received back from the recursive calls into ctr. The way you wrote it, you were discarding the values you got back from recursing.

def collatz(num,ctr):
    if(num != 1):
            ctr+=1
            if(num%2==0):
                    ctr=collatz(num/2,ctr)
            else:
                    ctr=collatz(num*3+1,ctr)
    return ctr

test=collatz(9,0)