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

Vladimir Lapcevic
Vladimir Lapcevic
6,363 Points
Posted by Vladimir Lapcevic
Vladimir Lapcevic
6,363 Points

Why even function does not work?

Please review my code and explain why my fourth function does not work properly. Thanks

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

def first_and_last_4(somethinga):
    return somethinga[:4] + somethinga[-4:]

def odds(somethingb):
    return somethingb[1::2]

def reverse_evens(som):
    return som[::-2]

3 Answers

Ty Yamaguchi
Ty Yamaguchi
25,397 Points
Ty Yamaguchi
Ty Yamaguchi
25,397 Points

You could try something like this:

def reverse_evens(som):
    evens = som[::2]
    return evens[::-1]
Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

:warning: Posting explicit solutions without explanations is strongly discouraged by Treehouse, and may be subject to redaction by staff or moderators.

Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

Your function will work, half of the time. But depending on the length (actually the even/odd-ness) of the list, it might return reverse odds instead.

To guarantee reverse evens, there's two basic strategies:

  • use the length of the list to compute the "start" index
  • extract the evens first, then reverse them in a separate operation (slice)

Either method will work when properly implemented. (hint: the second one might be easier to do)