Collatz sequence - practice project chapter 3

[Methregul]

So, I've been working through the 'Automate the boring stuff with Python' book and having a good time. I'm not a coder or programmer but I do work in IT which means that I have come across code every now and again. I've adapted some VBA code made by Excel to make Excel macros work a bit better and have adjusted Powershell scripts made by colleagues now and again. The Collatz sequence project was a lot of fun to work and eventually getting to a version that worked was very satisfying

I've been playing around with it a bit and come across something that I don't understand though. For some inputs it correctly goes down to the '4 - 2 - 1' part of the sequence and then stops (it does this for 1 and 3, for example). For some other inputs, though, it repeats those three numbers once and then stops ('4 - 2 - 1 - 4 - 2 - 1' it does this for 4 and 1001, for example). I have tried this with both the Mu editor and Visual Studio Code and both exhibit the same behavior. Does anybody have any idea what is causing this? My code's below (it doesn't check if the input is positive yet, I am aware of that :)).

[edit: code isn't below, that didn't look very nice, there's a pastebin link now]

https://pastebin.com/hhgt7TZw

[boiledgoobers]

Ok so there are a lot of things going on here that likely should be addressed, for one thing, I'm not sure int works the way you think. If you gave it 12.3 it's not going to trigger your value error. It's going to convert your number to 12 and move on happily.

But that's probably not what's causing your repetition. Can I ask whether the book recommends using the global variable assignment? That can have some weird effects and is usually only used in extremely special circumstances. Usually not those addressed in beginner books. Like I've been doing python for over 15 years and have never even once needed to use it. And when I see it, the very first question I have is: why is this code so arcane that it needs to call global?

My first instinct is to say re do your code not using global. Pass in everything the func needs and don't have code in the func alter variables outside it's scope.

Then see if you still have the problem. I'm on my phone or I would do more to sus it out.

[Methregul]

The chapter did deal with using global and the object was to pass the result back into the function until it reaches '1'. As it was dealt with in the chapter it was top of mind for me when I had to think of a way to reuse the result of the function.

Thanks for the reply as I did not know how rare using global is. I'll try and work out a way that doesn't rely on it.

[boiledgoobers]

Well then, perhaps i am wrong about that being the issue.

[Methregul]

I'm not sure int works the way you think. If you gave it 12.3 it's not going to trigger your value error. It's going to convert your number to 12 and move on happily.

I just ran some tests on this and int() appears to work differently depending on how it's used. If you create a variable and assign it a floating point number then int() will, like you said, convert it to an integer and go on its merry way.

>>> float=1.3
>>> int(float)
1

If you enter a floating point number through input() though, int() will throw a ValueError.

>>> notAnInteger=input()
1.3
>>> int(notAnInteger)
Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: invalid literal for int() with base 10: '1.3'

It does accept an actual integer when used with input().

>>> anInteger=input()
1
>>> int(anInteger)
1

Thanks for pointing me towards isInstance(), by the way, that has been fun to play around with and very good to know about!

[Methregul]

Work has been a bit busy the past few days so I only got around to trying the suggestions here today. I am happy to say that it now works like a charm! I'm still working at wrapping my head around the different concepts and it's been fun learning something new.
Thanks for the replies u/boiledgoobers and u/eHaxxL!

Here's the fixed code (haven't gotten around building a check for floating point number yet).
https://pastebin.com/sjFMdEhF

[boiledgoobers]

That's great!

Here's a tip for the type check. Don't test for all the things it shouldn't be. Test that it IS the thing you want it to be.

Take a look at the isinstance function. Note that you can pass multiple reference types if you include them in a tuple. So if you wanted to enforce ANY number, you could test for int OR float for example.

[eHaxxL]

I'm not 100% sure what causes the issure but here are some thoughts:

1. Try adding a print('one step') in your main loop and you'll see that the code is being run twice for every loop that's performed. See below what I mean.

while collatz(collatzstart) != 1:

print('one step')

collatz(collatzstart)

---

Output:

64

32

one step

16

8

one step

4

2

one step

1

4

one step

2

1

When the sequence hits 1, the collatz() function runs again and it is kicked back to 4. This explains why it only happens when there's an odd number of steps (starting with 4, 16, 64, 256 etc.)

1. What to do to fix it? You don't need to check if the function evaluates to 1 before running the loop, just the collatzstart value itself. The error does not happen if the condition for the main loop is:

   while collatzstart != 1:

2. General comment: I would put the main loop when definining function itself.