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

reverse_evens function not working

My code is:

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

I tried this on an online compiler and on my own laptop. Both work. I dont know why it won't work here.

I dont know why it won't work even if it works elsewhere.

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

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

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

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

1 Answer

Steven Parker
Steven Parker
230,230 Points

This strategy will work with some test data, but not every time. You managed to test with "lucky data".

To create a function that will work reliably with all data, there are two basic approaches:

  • compute which item to start with based on the size (or actually the "even or odd-ness") of the list, or
  • use one slice to extract all the even indexed values, and then another slice to reverse them

Either method will work when implemented correctly. Bonus hint: the second method might be easier to implement.

Thank you so much! I didnt think of that.