From: Gregory Brown <gregory.t.brown@...>
Date: 2009-01-29T14:51:28+09:00
Subject: Re: Real life use of each_cons?

On Thu, Jan 29, 2009 at 12:19 AM, Mischa Fierer <f.mischa@gmail.com> wrote:
> Hello,
>
> I've been going through the enumerable/enumerator methods that I'm
> unfamiliar with and came across each_cons. Apart from a post by a guy
> who wasn't sure where it was even defined, I have only come across one
> use of it in the wild:
>
> (From Prawn)
>
>  def polygon(*points)
>      move_to points[0]
>      (points << points[0]).each_cons(2) do |p1,p2|
>        line_to(*p2)
>      end
>    end
>
>
> Does anyone have any ideas on other cases where each_cons would be
> useful? Or perhaps insight into why it's in Ruby?

I think you may have been confused by my ugly code there.  I have
replaced it with:

    def polygon(*points)
      move_to points[0]
      (points[1..-1] << points[0]).each do |point|
        line_to(*point)
      end
    end

The reason why it's not needed is because we draw the lines from point to point.

But if we were drawing them segment by segment, it'd make sense.

>> [[1,2],[3,4],[5,6],[1,2]].each_cons(2) { |a| p a }
[[1, 2], [3, 4]]
[[3, 4], [5, 6]]
[[5, 6], [1, 2]]

I imagine I had refactored a line p1, p2 call down to just line_to(p2)
without fixing the each_cons()... sorry about that.

In general each_cons is useful when you need a sliding window of size
n across a dataset.

-greg

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