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

jacobtydings
jacobtydings
4,262 Points
Posted by jacobtydings
jacobtydings
4,262 Points

Having trouble with negative_evens

Hey, I seem to be having some trouble getting it to accept the code I wrote for the last function. I don't see why returning the list at [::-2] wouldn't work, as it seems to anywhere else I go. The slicing of the iterable is just there so that I don't throw things off in the workspace, and I think for the sake of the challenge, it's optional. What am I doing wrong?

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

def first_and_last_4(iterable):
    iterable = iterable[:4]+iterable[-4:]
    return iterable

def odds(iterable):
    iterable = iterable[1::2]
    return iterable

def reverse_evens(list1):
    list1 = list1[:]
    list2 = list1[::-2]
    return list2

1 Answer

Steven Parker
Steven Parker
204,727 Points
Steven Parker
Steven Parker
204,727 Points

A slice with a step of -2 will actually work half the time, based on the length of the list. The other half of the time it will return the odd indexed items reversed instead of the evens.

To work properly every time, there's two basic strategies:

  • use the length (actually even/odd-ness) of the list to determine the starting position
  • extract the even indexed items first and then reverse them in a separate operation

Hint: Either strategy is effective, but the second one might be easier to implement.