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

Posted by Diane Blamaud
Diane Blamaud
4,166 Points

Task 4/4, function works when tested on python shell but isn't accepted as a correct answer to pass the challenge

''' def reverse_evens(iterable): return iterable[::-2] ''' This functions returns [5, 3, 1] when passed the example [1, 2, 3, 4, 5] in python shell but isn't accepted on the challenge page. Therefore I cannot determine where the mistake is. Can someone see where the mistake might be?

slices.py 
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):
    return iterable[::-2]

1 Answer

Kip Yin
Kip Yin
4,847 Points

The problem is that your code (iterable[::-2]) only works when the length of the iterable is odd.

The first thing comes to me is using a conditional:

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

or if you prefer:

def reverse_evens(iterable):
    if len(iterable) % 2 == 0:
        return iterable[-2::-2]
    else:
        return iterable[::-2]
Kip Yin
Kip Yin
4,847 Points

Just for fun, the solution can be even shorter:

def reverse_evens(iterable):
    return iterable[::-2] if len(iterable) % 2 else iterable[-2::-2]
Diane Blamaud
Diane Blamaud
4,166 Points

Ohh I hadn't thought about this! I wasn't testing it enough. Thanks!

Kip Yin
Kip Yin
4,847 Points

No problem! If the answer is helpful, a "Best Answer" would be appreciated. Good luck on learning!

Idris Abdulwahab
Idris Abdulwahab
Courses Plus Student 2,961 Points

Although this was not posted by me but I really benefited from the insight and hints. Thank you Kip.

Kip Yin
Kip Yin
4,847 Points

I'm glad it helped!