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

Posted by mateoniksic3
mateoniksic3
2,255 Points

Why is it not working?

It works when I type it in console...

slices.py 
def first_4(list1):
    return list1[:4]

def first_and_last_4(list2):
    return list2[:4] + list2[-4:]

def odds(list3):
    return list3(1::2)

3 Answers

Jeff Muday
MOD
Jeff Muday
Treehouse Moderator 28,722 Points
Jeff Muday
Jeff Muday
Treehouse Moderator 28,722 Points

you need square brackets on the odds return statement.

mateoniksic3
mateoniksic3
2,255 Points

Thanks, but now this one does not work and it works in console...

def first_4(list1):
    return list1[:4]

def first_and_last_4(list2):
    return list2[:4] + list2[-4:]

def odds(list3):
    return list3[1::2]

def reverse_evens(list4)
    return list4[-1::-2]
Jeff Muday
MOD
Jeff Muday
Treehouse Moderator 28,722 Points
Jeff Muday
Jeff Muday
Treehouse Moderator 28,722 Points

Cool... you got pretty far and you are close. The reverse evens is trickier than it seems. Here's the hint. Reverse the list, then return the even elements.

recall that reversing a list can be accomplished by the slice below. I think you know how to do evens!

Best of luck!!

mylist = [1,2,3,4]
mylist_reverse =mylist [::-1]
mateoniksic3
mateoniksic3
2,255 Points

It still does not work..

It's weird, why does it work in console and not here?

def first_4(list1):
    return list1[:4]

def first_and_last_4(list2):
    return list2[:4] + list2[-4:]

def odds(list3):
    return list3[1::2]

def reverse_evens(list4)
    rev_list = list4[::-1]
    return rev_list[::2]
Jeff Muday
MOD
Jeff Muday
Treehouse Moderator 28,722 Points
Jeff Muday
Jeff Muday
Treehouse Moderator 28,722 Points

You are so close!! But a couple of tiny issues here.

  1. You were missing a colon on reverse_evens
  2. My bad advice ;)

I'll explain my bad advice: I interpreted the question wrong. When Kenneth asks for "reverse evens" that can be interpreted as either approach "A" or approach "B". "A" is the correct interpretation.

A) get the evens then reverse the list

mylist[::2][::-1]

B) get the reverse of the list then take the even elements.

mylist[::-1][::2]

Here's why--

On list [1,2,3,4,5] either A or B passes: reverse_evens_A= [5, 3, 1] reverse_evens_B= [5, 3, 1]

On the list [1,2,3,4,5,6] reverse_evens_A= [6, 4, 2] reverse_evens_B= [5, 3, 1]

I swear this passes the challenge (below) ;-) 

def reverse_evens(list4):
    return list4[::2][::-1]