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

Ben Hedgepeth
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Ben Hedgepeth
Python Web Development Techdegree Student 10,287 Points
Posted by Ben Hedgepeth
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Ben Hedgepeth
Python Web Development Techdegree Student 10,287 Points

Slicing even indexes in a list

Please refer to the reverse_evens function. I wrote an if statement to evaluate whether the length of the iterable is even or odd. If it length of the iterable happens to be even the last index will be odd; and I want to start removing indexes from position -2 and iterate -2 steps.

For iterables that have an odd length, I want to remove the even indexes starting at position -1 (because the last index is even) and iterate -2 steps.

slices.py 
def reverse_evens(iterable):
    if len(iterable) % 2:
        # the last index of an iterable that happens to have an even length is odd
        return iterable[-2::-2]
    else:
        # the last index of an iterable that happens to have an odd length is even
        return iterable[-1::-2]

1 Answer

Steven Parker
Steven Parker
231,007 Points
Steven Parker
Steven Parker
231,007 Points

Your concept is sound, but the implementation has the cases reversed. When the length is odd, the starting offset should be the last item (-1). And when the length is even, the offset should be the second-to-last item (-2).