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

Posted by Samuel Zhen
Samuel Zhen
33,571 Points

Challenge task 4 bummer! My function return the expected result but the challenge refuse the answer.

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

Any idea?

slices.py 
def first_4(iterable):
    return iterable[0:4]

def first_and_last_4(iterable):
    return iterable[0:4] + iterable[len(iterable) - 4:]

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

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

2 Answers

Dave StSomeWhere
Dave StSomeWhere
19,870 Points
Dave StSomeWhere
Dave StSomeWhere
19,870 Points

Well, you think you returned the expected result but didn't do complete testing.

If you run:

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

print(reverse_evens([1,2,3,4,5]))
print(reverse_evens([1,2,3,4,5,6]))

You'll see the output for case 1 is correct while case 2 returns the odd/incorrect values:

[5, 3, 1]
[6, 4, 2]
Schaffer Robichaux
Schaffer Robichaux
21,729 Points
Schaffer Robichaux
Schaffer Robichaux
21,729 Points

Hey Samuel, The keyword that Kenneth uses in the video is 'even' indexes. This provides the clue that you need to determine the inclusiveness of your slice based off of whether or not the starting index is even. To further Dave's point above, I often build very simple "test harnesses" to evaluate my scripts that produce seemingly correct answers- they've always helped me with debugging and are well worth the time. A sample for your problem is below:

test1 = list(range(10))
test2 = 'potato'
test3 = list('potato')
test4 = [1, 2, 3, 4, 5]

def test_harness_1(func, test1, test2, test3):
    func(test1)
    func(test2)
    func(test3)

test_harness_1(reverse_evens, test1, test2, test3)

You can add print statements in your code as needed to evaluate the results.