From: ahoward <ahoward@...>
Date: 2003-05-30T06:03:47+09:00
Subject: Re: Iteration - last detection

On Thu, 29 May 2003, Martin DeMello wrote:

> Orion Hunter <orion2480@hotmail.com> wrote:
> > Is there any built in functionality for iteration that will allow me to
> > detect when I am on the last element?  This would not be hard for an Array,
> > using array.length, but what about a Hash?  How would I know when I am at
> > the last element?
>
> Not built in, but this should work:
>
> module Enumerable
>   def each_with_lastp # no shortage of nicer names, I'm sure :)
>     flag = nil
>     i = nil
>     each {|j|
>       yield [i, false] if flag
>       i = j
>       flag = true
>     }
>     yield [i,true]
>   end
> end
>
> a = [1,2,3,4,5,6,7,8,9]
> a.each_with_lastp {|i, last|
>   print i
>   print last ? "" : ","
> }
>
> It's rather crude, in that there's the overhead of a call to 'if flag'
> with every iteration, but I couldn't think of a way around that.

you could use each_with_index to avoid the flag - but not th 'if':

module Enumerable
  def each_with_last(&block)
    l = nil
    each_with_index do |e, i|
      block.call l, false if i > 0
      l = e
    end
    block.call l, true
  end
end

btw.  if you don't mind me asking, why 'yield [i,true]'?  i remember some
posts awhile back about how params get passed to blocks and it was very
non-intuitive regarding list assignment and all...


-a
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