From: Robert Klemme <bob.news@...>
Date: 2004-11-11T21:53:28+09:00
Subject: Re: array.each restart when array is changed


"Kevin B�rgens" <kevin@boergens.de> schrieb im Newsbeitrag
news:2vfjurF2ljhdfU1@uni-berlin.de...
> Hi!
>
>
> How to restart a "each" iteration when the iterated array is changed? I
do
> it like this
>
> changed=false
> while (changed)
> changed=false
> array.each {|e|
>   if (today_is_christmas)
>     array=array+=["Hello santa claus"]

This is sufficient:
array += +=["Hello santa claus"]

>     changed=true
>   end
> }
>
> Is there a more elegant way to do this?

I would not change the array in place while iterating.  Without more
knowledge about your scenario I'd do this:

added = []

begin
  added.clear

  array.each do |e|
    if (today_is_christmas)
      added << "Hello santa claus"
    end
  end

  array.concat added
end until added.empty?


If you want to ensure, that each element is traversed only once, a queue
approach is better:

queue = array.dup

until queue.empty?
  e = queue.shift

  if today_is_christmas(e)
    x = "Hello santa claus"
    queue << x
    array << x
  end
end

This is much more performant because with the other solution you end up
running through the same array members over and over again.

Are you doing some kind of BFS?

Kind regards

    robert