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

Posted by Mikaela Land
.a{fill-rule:evenodd;}techdegree
Mikaela Land
Python Web Development Techdegree Student 2,324 Points

Couldn't find reverse evens

So on the reverse_ evens I'm not sure why treehouse says it can't find it. Any ideas. Oh and I'm not even sure what I did with the for i part but I was recommended it. I asked but got no response. Any explanations?

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

def first_and_last_4 (first_last):
    first = first_last [:4]
    last = first_last [-4:]
    return first + last

def odds(iter):
    result_iter = []
    for i in range(len(iter)):
        if i % 2 != 0:
            result_iter.append(iter[i])
    return result_iter


def reverse_evens (it):
    it_reverse: []
    for i in range(len(it)):
        if (i) % 2 == 0:
            it_reverse.append [::-2]
    return it_reverse

I also tried: def reverse_evens (it): for i in range(len(it)): if (i) % 2 == 0: it_reverse = it [::-2] return it_reverse

1 Answer

Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

I don't think you can use the slice operator on a method name. And you can do this without a loop.

To always return reverse evens using only slice(s), there's two basic strategies:

  • compute the starting position based on the list size
  • extract the even index values first, then reverse them

Either one will pass the challenge when implemented correctly.

Mikaela Land
.a{fill-rule:evenodd;}techdegree
Mikaela Land
Python Web Development Techdegree Student 2,324 Points

Still not understanding I'm sorry. I've tried getting the starting postion but it seems I've forgotten how. and I did a for loop for the index values and tried to reverse them. No luck. Keep getting a TypeError

Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

The second method is the easiest. Use one slice to get the even-indexed items, and then another slice to put them in reverse order. No loop is needed and it can be done on one line.

Mikaela Land
.a{fill-rule:evenodd;}techdegree
Mikaela Land
Python Web Development Techdegree Student 2,324 Points

OMG! I got it. It's been two days of messing with it! Thanks so much Steven!