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

Chastin Davis
PLUS
Chastin Davis
Courses Plus Student 2,299 Points
Posted by Chastin Davis
Chastin Davis
Courses Plus Student 2,299 Points

I believe I need assistance on returning a reversed sliced list

I have return item[::-2] , in my method at the bottom of my code. I am still told this is wrong. The -2 moves me backward in the list and :: means I'm starting from one end and moving to the opposite end. I even try in workspace and I see that my method skips all odd indexes and returns the even indexes. What am I missing?

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

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

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

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

2 Answers

Unsubscribed User
Unsubscribed User
9,496 Points

This one is tricky, because you need to actually do 2 things... get the even index, and then reverse them. I got hung up on this for a good minute, but I think it's because I didn't actually understand the full question. So, there are 2 ways to do this:

you can get the indexes first, store in a variable, and then reverse them: 

item = item[::2] # get the items at even indexes
return item[::-1] # reverse them

or you can do a double slice

return item[::2][::-1]
Chastin Davis
Chastin Davis
Courses Plus Student 2,299 Points
Chastin Davis
Chastin Davis
Courses Plus Student 2,299 Points

Thanks. It is still unacceptable if return item[::-2] shows me as correct in workspace.. I'll apply your answer however.

Unsubscribed User
Unsubscribed User
9,496 Points

[::-2] reverses everything stepping over every 2nd index from the last. It won't always get "even" indexes. If there are 6 things in a list for example, then it gets item[5], item[3], item[1].
you need to use [::2] instead, to get every even index starting at the beginning: item[0], item[2], item[4], and so on... then you just reverse it with [::-1]

hope that helps

Patrick Tse
Patrick Tse
10,682 Points
Patrick Tse
Patrick Tse
10,682 Points

I agree the example is a bit confusing: "For example, with [1, 2, 3, 4, 5] as the input, the function would return [5, 3, 1]." It should include " with [1, 2, 3, 4, 5, 6] as the input, the function would also return [5, 3, 1]."