From: Florian Gross <flgr@...>
Date: 2004-03-15T02:54:39+09:00
Subject: Re: iterating with multiple values

gabriele renzi schrieb:

> Hi gurus and nubys,

Moin!

> Suppose you are dealing with an array like [1,2,3,4], and suppose ypou
> want to deal, at any iteration, with 3 values.I mean something like:
> 
> [1,2,3,4].each_3 do |pre,cur,next|
>  print pre,cur,next,"\n"
> end
 >
> nil,nil,1
> nil,1,2
> 1,2,3
> 2,3,4
> 3,4,nil
> 4,nil,nil

I've implemented this like this:

module Enumerable
   def each_window(window)
     window = 0 ... window unless window.is_a? Range
     window_end = window.end + (window.exclude_end? ? 0 : 1)
     elements, pushed = [], 0
     each_with_index { |item, index|
       elements.push(item)
       pushed += 1
       if pushed % window_end == 0
         start = [0, window.begin - window_end + pushed].max
         finish = [0, window_end + pushed].max
         yield(elements[start ... finish])
       end
     }
   end
end

You can use it like this for your example:

[1, 2, 3, 4].each_window(-2..0) { |items| p items }
[1]
[1, 2]
[1, 2, 3]
[2, 3, 4]

It's a bit different from your example however. (It won't yield trailing 
nils.)

It can also be used to iterate groups of three elements like this:

[1, 2, 3, 4, 5, 6].each_window(3) { |items| p items }

Or you can skip elements like this:

["foo", 1, 2, "bar", 3, 4].each_window(1..2) { |items| p items }

Maybe it is at least of some use to you.

Regards,
Florian Gross