Collatz Sequence - getting a None at the end

Question

Learning from "Automate The Boring Stuff" by Al Sweigart. At the end of Chapter 3, the Collatz Sequence is given as practice. The output seems correct, but the last row has a 'None' in it. In the code below, I am guessing that when p = 1, it gets out of the while loop, and then there is nothing to print, so it gives a None (?). Can someone point me in the right direction of why the None is added, and how to fix it?

See code right below and example results further down:

def collatz (p):
    while p != 1:
        if p % 2 ==0:
           p = (p//2)
           print(p)
        else:
           p = ((3*p) + 1)
           print(p) 

print ('Select a number between 1 and 10')
entry = float(input())
number = round(entry)
if number >10 or number <1:
   print('Your selection must between 1 and 10. Please try again')
else:       
   Program = collatz(number)
   print (Program)

** Example results: If I input number as 3, I get:

Your function does not return anything explicitly. Any such function implicitly returns `None`. — DYZ, Mar 13 '19 at 22:02
You're printing the return value of `collatz()`, which is `None.`. The function itself prints information, so there is no need to print its result. — kindall, Mar 13 '19 at 22:15

score 1 · Accepted Answer · answered Mar 13 '19 at 23:48

1

As was already pointed out in the comments, your function returns None. I thought I'd take your function and make it a generator which you can iterate over and print the values that way. This has several advantages like making your code much more flexible and reusable:

def collatz (p):
    while p != 1:
        if p % 2 == 0:
           p = p / 2 # You don't need the double slash here because of the if before it
        else:
           p = (3*p) + 1

        yield p 

print('Select a number between 1 and 10')
number = int(input()) # You really want int here I think and leave out the rounding

if 1 <= number <= 10: # Slightly more pythonic
   for sequence_item in collatz(number):
       print(sequence_item)
else:
   print('Your selection must between 1 and 10. Please try again')

Feel free to ask anything or correct me on possibly wrong assumptions! :)

answered Mar 13 '19 at 23:48

whiterock

190
2
11

Thanks. This is helpful. The reason I used float, is if someone types 5.5, then I get an error if 'int' is used. So, wanted to convert the input and then round it so there are no errors, no matter what number type the user types. – Philomath Mar 15 '19 at 00:39
@Philomath Would you mind giving me an upvote if you say you found it useful? :) – whiterock Aug 11 '19 at 14:26
1

Already did. It says 'Thanks for the feedback! Votes cast by those with less than 15 reputation are recorded, but do not change the publicly displayed post score' – Philomath Aug 13 '19 at 14:54

Collatz Sequence - getting a None at the end

1 Answers1