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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?

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    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] Commented Feb 3, 2016 at 19:42
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    th = l[2::3] following your logic Commented Feb 3, 2016 at 19:43
  • 0 % 3 = 0, 3 % 3 = 0...etc the math works out. Commented Feb 3, 2016 at 19:44
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    @kuhe you can check the stackoverflow post Explain Python's slice notation, there is some useful answers about python index/slice notation. Commented Feb 3, 2016 at 19:47

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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).

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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.

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