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

Nathan Tallack
Nathan Tallack
22,160 Points
Posted by Nathan Tallack
Nathan Tallack
22,160 Points

Should work returning reverse stepped list.

The last task asks us to make a function named reverse_evens that accepts a single iterable as an argument. Return every item in the iterable with an even index...in reverse.

For example, with [1, 2, 3, 4, 5] as the input, the function would return [5, 3, 1]. Here is my code.

def reverse_evens(i):
    return i[-1::-2]

But it keeps telling me it didn't get the right values from reverse_evens even though I am pretty sure my code should give the expected result.

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

def first_and_last_4(i):
    return i[:4] + i[-4:]

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

def reverse_evens(i):
    return i[-1::-2]

3 Answers

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

You're halfway there. Your function will return the reverse evens if the list has an odd number of elements. But otherwise it will return "reverse odds" instead.

To always return reverse evens, you'll need to compute the starting position based on the list size, or extract the even indexes first and then reverse them in a separate step.

Nathan Tallack
Nathan Tallack
22,160 Points
Nathan Tallack
Nathan Tallack
22,160 Points

Ah, see, I was working to only satisfy the example, not all possible cases. My bad. Thanks for that. :)

Thomas Bråten
Thomas Bråten
1,408 Points
Thomas Bråten
Thomas Bråten
1,408 Points

I've been going over this over and over again and I just can't get it in my brain. Looking at different examples i see almost the same explanations but I just dont get it. So, if I understand it correctly have an if to check if length of string that is passed in is odd or even. If odd make, this list else make other. Even writing this down just spins my head. Could someone post a working example of the last challenge? And if it can be explained as well. Its something I just don't get and I don't even get what I don't get. I'm sure I'll have a facepalm at the end of this.

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

The simplest approach is to extract the even indexes first and then reverse them with a separate slice:

def reverse_evens(i):
    return i[::2][::-1]
Thomas Bråten
Thomas Bråten
1,408 Points
Thomas Bråten
Thomas Bråten
1,408 Points

Thank you Steven, I'm gonna go through it a couple times. Sometimes I need to see it different ways :)