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

Dmitry Bruhanov
Dmitry Bruhanov
8,513 Points
Posted by Dmitry Bruhanov
Dmitry Bruhanov
8,513 Points

reverse_events

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

print reverse_evens([1,2,3,4,5]) 
# this line was not typed in the challenge. 
#Added this to check the result in an online python compiler

This code prints out the required list: [5, 3, 1]. Why does the challenge say that it is a mistake? Maybe I misunderstood the task? Thanks in advance for you kind and swift reply.

slices.py 
def first_4(iterable):
    return iterable[0:4]

def first_and_last_4(iterable):
    return first_4(iterable) + iterable[-4:]

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

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

2 Answers

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.

Dmitry Bruhanov
Dmitry Bruhanov
8,513 Points
Dmitry Bruhanov
Dmitry Bruhanov
8,513 Points

Thanks for your swift reply. I checked whether the length of the iterable is even or odd and looped through the list accordingly and it worked!!!

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

You don't actually need a loop, it would be better to use slices since this is a slice challenge. :wink:

Dmitry Bruhanov
Dmitry Bruhanov
8,513 Points
Dmitry Bruhanov
Dmitry Bruhanov
8,513 Points

You got it right. By a 'loop' I meant taking every second item from the end like this:

iterable[::-2]

This is called a slice in Python, isn't it? However, since Python is built on "C" and it is only a built in function, behind the scenes this must work like a backward loop)) don't you think?

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

You're probably right about what goes on in the engine, but from a programming perspective, that's simply called a "slice". So I'm guessing when you say you "looped through the list accordingly" you mean "sliced the list with different arguments". :wink: