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

I am Stuck on this exercise please help me out

I have done whatever in the question is asked but i am still not able to pass the question.

def first_4(iterable):
    return iterable[0:4]

def first_and_last_4(list):
    return list[:4] + list[-4:len(list):1]

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

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

2 Answers

Steven Parker
Steven Parker
216,057 Points

You're half right. Your method will work when the list has an odd number of items. But when the list has an even number of items, it will return reverse odds instead.

To always return reverse evens, 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.

I would recommend first grabbing the even items in the list then reversing. Currently you're grabbing every other element starting with the last position. This will only grab even elements if the list length is even, otherwise you will grab all odd elements.

I'll post my solution below, also note the slightly easier code for first_and_last. You don't need to use the len function to return the last 4 of an iterable. Final note, just a reminder single character objects have the potential to give you huge headaches when debugging code, avoid them like the plague.

def first_4(iterable):
    return iterable[:4]

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

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

def reverse_evens(iterable):
    # note that I am first grabbing the even elements, then reversing them.
    return iterable[::2][::-1]