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Re: What meaning is 'a[0:10:2]'?

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From Tim Chase <python.list@tim.thechases.com>
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Subject Re: What meaning is 'a[0:10:2]'?
Date Sun, 15 Nov 2015 18:48:04 -0600
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On 2015-11-15 16:27, fl wrote:
> When I learn slice, I have a new question on the help file. If I
> set:
> 
> pp=a[0:10:2]
> 
> pp is array([1, 3])
> 
> I don't know how a[0:10:2] gives array([1, 3]).
> 
> I know matlab a lot, but here it seems quite different. Could you
> tell me what meaning a[0:10:2] is?

Well, if it a matlab.array was a well-behaved object it would just
give you "0".  As your copy/paste on the help for a slice stated, the
first number is where it starts (0), the second number is where it
ends (10, exclusive), and the 3rd number (2) is the stride.  To
demonstrate:

  >>> a = list(range(20))
  >>> a
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
  >>> a[0:10:2]
  [0, 2, 4, 6, 8]
  >>> a[:10:2]
  [0, 2, 4, 6, 8]
  >>> a[0:10]
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

So note that the stride of 2 provides every other one while a stride
of three provides every 3rd value

  >>> a[0:10:3]
  [0, 3, 6, 9]

-tkc

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Thread

What meaning is 'a[0:10:2]'? fl <rxjwg98@gmail.com> - 2015-11-15 16:27 -0800
  Re: What meaning is 'a[0:10:2]'? Ben Finney <ben+python@benfinney.id.au> - 2015-11-16 11:46 +1100
    Names [was Re: What meaning is 'a[0:10:2]'?] Steven D'Aprano <steve@pearwood.info> - 2015-11-16 23:57 +1100
      Re: Names [was Re: What meaning is 'a[0:10:2]'?] Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-11-16 20:06 -0500
  Re: What meaning is 'a[0:10:2]'? Tim Chase <python.list@tim.thechases.com> - 2015-11-15 18:48 -0600
  Re: What meaning is 'a[0:10:2]'? Steven D'Aprano <steve@pearwood.info> - 2015-11-16 23:29 +1100

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