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Python

Posted by thieny
thieny
5,403 Points

Why does this code does not work?

Hi, I eventually completed the slice functions challenge, but I'm curious as to why my first attempt was unfruitful.

Here it is:

 def reverse_evens(item):
    if len(item) % 2 == 0:
        reversed = item[-1::-2]
    else:
        reversed = item[-2::-2]
    return reversed

My idea was to check if the length of the iterable was odd or even to calibrate where I should start the slice. I played around in the IDLE and I guess it has something to do with the fact that when presented with a slice, my code count quotation marks as an element instead of ignoring it?

Thank you,

2 Answers

Logan R
Logan R
22,989 Points
Logan R
Logan R
22,989 Points

Hello!

So the meat of challenge 4 is: Return every item in the iterable with an even index...in reverse.

Given the list: [1, 2, 3, 4, 5], we want the following indexes: 0, 2, and 4. Our list would become [1, 3, 5]. Reversed, this is [5, 3, 1]. Your code works great for this example!

But what if our list is not an odd length. What if we had [1, 2, 3, 4, 5, 6] for example? We want the following indexes: 0, 2, and 4. Our list would become [1, 3, 5] then reversed to [5, 3, 1].

Here is where your code presents a problem.

>>> x = [1, 2, 3, 4, 5]
>>> y = [1, 2, 3, 4, 5, 6]

>>> x[-1::-2]
[5, 3, 1]
>>> y[-1::-2]
[6, 4, 2]

Your code starts at the last index, not at the last even index.

I hope this helps! If you still don't understand anything, feel free to reply :)

( Taken from my reply here: https://teamtreehouse.com/community/reverseeven )

thieny
thieny
5,403 Points

Hi Logan, My code actually takes this into account and check if the iterable length is odd or even. This is why I'm wondering why it doesn't work anyway

Logan R
Logan R
22,989 Points

Hey Thieny,

The issue is that when you do [-2::-2], you start 2 indexes back. This cuts off the first entry in the array.

For example:

>>> x = [1, 2, 3, 4, 5]
>>> y = [1, 2, 3, 4, 5, 6]
>>> x[-2::-2]
[4, 2]
>>> y[-1::-2]
[6, 4, 2]
thieny
thieny
5,403 Points
thieny
thieny
5,403 Points

Sure, but Isn't the cut-off entry odd anyway, in this case?