superstewie superstewie - 3 months ago 8
Python Question

Collatz function not exiting correctly

Here's a program that is intended to count the length of a Collatz sequence recursively:

def odd_collatz ( n ):
return (3 * n) + 1

def even_collatz ( n ):
return int(n / 2)

def collatz_counter ( initialNumber, initialLength ):
length = initialLength

while True:
if initialNumber == 1:
return length

elif initialNumber != 1:
length += 1

if initialNumber % 2 == 0:
collatz_counter(even_collatz(initialNumber), length)

else:
collatz_counter(odd_collatz(initialNumber), length)

print(collatz_counter(13, 1)


The expected answer should be 10. However, the program gets stuck in an infinite loop. At the second to last step of the sequence
initalNumber
equals 2. The program functions as expected:
collatz_counter
is called using
even_collatz
and the number 10.

The expected action of the next step would be to run
collatz_counter
with an
initialNumber
of 1 and an
initialLength
of 10. What I would expect would happen is that the first if statement would evaluate to true,
collatz_counter
should return
length
and then exit. This is not, however, what happens:

What actually happens is that the function evaluates the first if statement, runs the
return length
line, and then jumps to the line of code under
if initialNumber % 2...
and the whole process repeats itself over and over and over in an infinite loop.

Any ideas as to why this might be happening?

Answer

You are mixing recursion and looping in a bit of an odd way. The problem is while True:. Because you never return anything from within the loop there is nothing stopping it from going on forever. Your code reaches 1 then just keeps adding to the length. Here is a fixed version.

def odd_collatz ( n ):
    return (3 * n) + 1

def even_collatz ( n ):
    return int(n / 2)

def collatz_counter ( initialNumber, initialLength ):
    length = initialLength


    if initialNumber == 1:
        return length

    elif initialNumber != 1:
        length += 1

        if initialNumber % 2 == 0:
            return collatz_counter(even_collatz(initialNumber), length)

        else:
            return collatz_counter(odd_collatz(initialNumber), length)

print(collatz_counter(13, 1))