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

Posted by ARCHIT RANA
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ARCHIT RANA
Full Stack JavaScript Techdegree Graduate 27,853 Points

The code seems correct but the challenge task prompts an error---"Bummer. Could'nt get the right values"

I am getting an error that the function reverse_evens() could'nt return the right value. Please help...

slices.py 
def first_4(items):
    return items[0:4]
def first_and_last_4(items):
    return items[0:4] + items[-4:]
def odds(items):
    return items[1::2]
def reverse_evens(items):
    return items[::-2]

1 Answer

Alexander Davison
Alexander Davison
65,469 Points

This is a common mistake made by beginners.

items[::-2] doesn't always return the "reverse evens!"

Watch:

>>> lst1 = [1, 2, 3, 4]
>>> lst2 = [1, 2, 3, 4, 5]
>>> lst1[::-2]
[4, 2]
>>> lst2[::-2]
[5, 3, 1]

Notice when you enter lst1[::-2], it indeed returns the "reverse evens." But, notice that lst2[::-2] returns the reverse odds!

The correct way to get the reverse evens is with a simple items[::2][::-1].

items[::2][::-1]

The [::2] bit gets the even-indexed items, and the [::-1] bit reverses the evens.

I hope this helps.

ARCHIT RANA
.a{fill-rule:evenodd;}techdegree seal-36
ARCHIT RANA
Full Stack JavaScript Techdegree Graduate 27,853 Points

It worked!!!

Thank you. That was very helpful.

Alexander Davison
Alexander Davison
65,469 Points

Your welcome :grin: