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

Joseph Jackiewicz
Joseph Jackiewicz
2,102 Points
Posted by Joseph Jackiewicz
Joseph Jackiewicz
2,102 Points

Struggling to concatenate two slices

I was able to answer the first question and I believe I can correctly return the two slices but I keep getting the message Bummer! Try again! and I'm not sure why

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

def first_and_last_4(item):
    return (item[0:4] + item[:-4:-1]

2 Answers

Dustin James
Dustin James
11,364 Points
Dustin James
Dustin James
11,364 Points

You are very close here. If the list was [1, 2, 3, 4, 5, 6, 7, 8] you would get back [1, 2, 3, 4, 8, 7, 6] because -1 in item[:-4:-1] is going from the end of the list back to, but not including the fourth from last item.

Try this:

def first_and_last_4(item):
    return first_4(item) + item[-4:]  # corrected my original answer.

EDIT: My original post I wrote on my iPhone and missed typed my answer. I had put:

def first_and_last_4(item):
    return first_4(item) + item[:-4]  # this will return the first 4 (by using the previous method) but concatenates a slice from index 0 - 4th from last.

This is incorrect as Mark Mneimneh points out below.

markmneimneh
markmneimneh
14,132 Points 
This won't work
try it with a [1,2,3,4,5,6,7,8,9,10,11,12]

you will get 
[1, 2, 3, 4, 1, 2, 3, 4, 5, 6, 7, 8]
which is incorrect

item[:-4] means, start from beginning and end @ end -4.  which is not same as first 4 and last four.
markmneimneh
markmneimneh
14,132 Points
markmneimneh
markmneimneh
14,132 Points

see if this will work

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

and keep in mind ... there are other ways to achieve same thing.

Dustin James
Dustin James
11,364 Points

You are correct. My code will not work. I meant to type item[-4:] but I was typing on my phone. I have corrected my answer.

Your answer will work, but it does seem to be redundant. When using a slice, it is not necessary to get the length of the object being sliced.

item[len(item)-4:]  # you are essentially writing item[(12-4):] where 12 is from your example the amount of items.
item[-4:]  # this is just saying start from the fourth from last item and continue to the end. There is no need to get the length.

but as you have stated, there are many ways to do the same thing.