Python slice return a wrong values?

Question

I am learning python, and I have a problem with list slice. when I try to get all element at the third position, I got the wrong one:

l = [9, 0, 7, 1, 7, 5, 5, 9, 8, 0]
th = l[::3]
>> [9, 1, 5, 0]

but in my logic it should be:

>> [7, 5, 8]

Why it returns a wrong result?

You are stepping 3 starting from the 0 index so you get what you ask for, to get what you want would be `l[2::3]`, if it makes it simpler you are basically doing `l[0:len(l):3]` — Padraic Cunningham, Feb 03 '16 at 19:42
@kuhe you can check the stackoverflow post [Explain Python's slice notation](http://stackoverflow.com/questions/509211/explain-pythons-slice-notation), there is some useful answers about python index/slice notation. — mgc, Feb 03 '16 at 19:47

score 2 · Answer 1 · answered Feb 03 '16 at 19:45

2

The problem is that the Python slice operator starts at the first index (index 0), while you want it to start at the third (index 2). [2::3] should get what you want, as this will tell it to start at index 2 and take it and every third element after.

answered Feb 03 '16 at 19:45

StephenTG

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score 2 · Accepted Answer · edited Apr 05 '19 at 12:28

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l[::3] means start at 0 and go till the end of list and step 3 each time so at each step, it will output item at indices 0, 3, 6, 9. Which corresponds to the result that Python returned back. try l[2::3] if you want your desired output (every third element starting from the third).

edited Apr 05 '19 at 12:28

banan3'14

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answered Feb 03 '16 at 19:47

Harkamal Jot Kumar

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Python slice return a wrong values?

2 Answers2