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

Posted by Tiago Ramos
Tiago Ramos
2,380 Points

why can't I jump one step in the reverse slice?

Hi guys I've done some research and I think my reverse_evens() function is ok, why doesn't it work?

thanks

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

def first_and_last_4(ite):

    head = ite[:4]
    tail = ite[-4:]

    concat = head + tail
    return concat

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

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

1 Answer

Steven Parker
Steven Parker
231,269 Points
Steven Parker
Steven Parker
231,269 Points

You're not accounting for the possibility of different list lengths.

This approach will only work on lists that have an odd number of items. If the list passed to the function has an even number of items, this will return reverse odds instead of reverse evens.

You'll either need to calculate the starting position based on the list size, or you'll need to extract the evens first and then reverse them in a separate step.

Tiago Ramos
Tiago Ramos
2,380 Points

hey steven thank you for the tips, but shouldn't there be a way to present a reverse list, and jump one step, without removing the evens?

Tiago Ramos
Tiago Ramos
2,380 Points

And I have no ideia how to calculate the staring position, and how can I use that to return the reverse evens

Steven Parker
Steven Parker
231,269 Points
Steven Parker
Steven Parker
231,269 Points

If the list had an even number of items, you'd need to start at the second-to-last item (instead of the very last item) to get the even-numbered indexes ("iterable[-2::-2]").