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

This question already has answers here: Closed 11 years ago.

Possible Duplicate:

What is :: (double colon) in Python?

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

---

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]