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Python Python Collections (2016, retired 2019) Slices Slice Functions

Posted by Mikaela Land
.a{fill-rule:evenodd;}techdegree
Mikaela Land
Python Web Development Techdegree Student 2,324 Points

Slice Functions: Local variable error

In the odds function I keep getting:

UnboundLocalError: local variable 'odds_only' referenced before assignment

Not sure how to fix it. I keep trying to change things around to test it but I've got nothing. I got it to work once but it stopped working the second time.

Any ideas?

slices.py 
def first_4(lst):
    return lst[:4]

def first_and_last_4 (lst2):
    first = lst2 [:4]
    last = lst2 [-4:]
    both = first + last
    return both

def odds (num):
    if len(num) % 2 == 0:
    odds_only = num [1::2]
    return odds_only

1 Answer

Alexander Davison
Alexander Davison
65,469 Points

I think you are a bit lost. The odds function takes in an iterable still, not a number. (It seems that you believe that it takes in an iterable.)

The problem is asking to return an iterable which has all of the odd indexed values from the passed in iterable.

For instance, if I pretended I was within the Python shell:

>>> odds([1, 2, 3, 4, 5])
[2, 4]
>>> odds('abcdefg')
['b', 'd', 'f']
>>> odds((1, 2, 3))
[2]

Here's my solution to this problem:

(Let's call the input iterable iter and the output iterable result_iter)

  • Go through every number from zero to N-1, where N is len(iter). Let's call the "current number" i.
  • If i is odd, add to result_iter iter[i]. (Because i is an index, not the item itself)
  • Return result_iter.

Implemented in code, it may look somewhat like this:

def odds(iter):
    result_iter = []
    for i in range(len(iter)):
        if i % 2 != 0:
            result_iter.append(iter[i])
    return result_iter

Simpler way for the Pythonista:

def odds(iter):
    return [iter[i] for i in range(len(iter)) if i % 2 != 0]

I hope this helps! :grin:

Happy coding! :zap: ~Alex

Mikaela Land
.a{fill-rule:evenodd;}techdegree
Mikaela Land
Python Web Development Techdegree Student 2,324 Points

Thanks. I think I get it. I just have a question: why did you add range? Wouldn't that imply the list is just numbers? What's its purpose in the code?