What is :: (double colon) in numpy like in myarray[0::3]?

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What is :: (double colon) in Python?

I read the question What is :: (double colon) in Python when subscripting sequences?, but this not answer what myarray[x::y] mean.

It appears to me that other question does answer yours as well. `myarray[0::3]` is extended slice syntax that means start at element 0, step by 3, and stop at the end of `myarray`. — Dave Costa, Aug 19 '11 at 15:31
However, it really looks like the answer you linked is also valid for your question: get every `y`th element of a list, starting at the `x`th element — mdeous, Aug 19 '11 at 15:32

score 83 · Accepted Answer · answered Aug 19 '11 at 15:30

83

It prints every y^th element from the list / array

>>> a = [1,2,3,4,5,6,7,8,9]
>>> a[::3]
[1, 4, 7]

The additional syntax of a[x::y] means get every y^th element starting at position x

ie.

>>> a[2::3]
[3, 6, 9]

answered Aug 19 '11 at 15:30

GWW

4

Of note that `a[::3]` includes the item at the 0th index too, which is the `1` in the array. So you could also write it as `a[0::3]` – Monica Heddneck May 10 '18 at 19:29
2

and what about the [x:i:y]? what i signifies here? – KCK Jul 25 '19 at 14:04
3

For 1D arrays x is the start, i is the end of the range, and y is the step. – GWW Jul 25 '19 at 15:18
5

a[::-1] could be used to reverse an array. Found from original question :-) – Pradeep Singh Dec 14 '19 at 04:16

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