From: Robert Klemme <shortcutter@...>
Date: 2007-02-24T18:55:05+09:00
Subject: Re: enum_cons with tail

On 23.02.2007 20:50, S. Robert James wrote:
> enum_cons chops off the tail of the collection, if it isn't evenly
> divisble:
> 
> e.g.:
>     (1..10).each_cons(3) {|a| p a}
>     # outputs below
>     [1, 2, 3]
>     [2, 3, 4]
>     [3, 4, 5]
>     [4, 5, 6]
>     [5, 6, 7]
>     [6, 7, 8]
>     [7, 8, 9]
>     [8, 9, 10]
> 
> I'd also like to get:
> [9, 10, nil]
> [10, nil, nil]

You want semantics that are different from #each_cons.  #each_cons will 
give you a sliding window of n elements from the collection (see you 
example output).  This can be used to e.g. smooth a measurement curve.

> Is there anyway to do this?  (The collection is an opened File, too
> large to load entirely in memory.)

Yes, see below (not the best implementation though).  But maybe you want 
to change the algorithm that needs this.  What are you trying to accomplish?

> I don't have a clear grasp as to how internally Ruby treats a File as
> Enumerable.

You can cook your own version:

require 'enumerator'
module Enumerable
   def sub_range(n)
     ar = inject([]) do |rg, x|
       rg.push x
       rg.shift if rg.length > n
       yield *rg if rg.length == n
       rg
     end
     (n-1).times do
       ar.push nil
       ar.shift if ar.length > n
       yield *ar if ar.length == n
     end
     self
   end

   def each_add(n,&b)
     each(&b)
     n.times { b[nil] }
     self
   end

   def sub_range2(n,&b)
     to_enum(:each_add, n-1).enum_cons(n).each {|a| b[*a]}
   end
end

irb(main):036:0> [1].sub_range(3){|*x| p x}
[1, nil, nil]
=> [1]
irb(main):037:0> [1,2,3].sub_range(3){|*x| p x}
[1, 2, 3]
[2, 3, nil]
[3, nil, nil]
=> [1, 2, 3]
irb(main):038:0> (1..10).sub_range(3){|*x| p x}
[1, 2, 3]
[2, 3, 4]
[3, 4, 5]
[4, 5, 6]
[5, 6, 7]
[6, 7, 8]
[7, 8, 9]
[8, 9, 10]
[9, 10, nil]
[10, nil, nil]
=> 1..10
irb(main):250:0> (1..1).sub_range2(3) {|*a| p a}
[1, nil, nil]
=> nil
irb(main):251:0> (1..10).sub_range2(3) {|*a| p a}
[1, 2, 3]
[2, 3, 4]
[3, 4, 5]
[4, 5, 6]
[5, 6, 7]
[6, 7, 8]
[7, 8, 9]
[8, 9, 10]
[9, 10, nil]
[10, nil, nil]
=> nil

Kind regards

	robert