From: Pieter Hugo <pieter@...>
Date: 2009-11-26T16:39:44+09:00
Subject: Re: Difference between << and += for Strings and Arrays. Bug?

Hi Robert (and everyone else) - thanks for the well reasoned responses. 
I'll get the hang of it I'm sure

> Why do you want to have two copies of your Array?

I have a setup where I have Folder objects. A folder can have many other 
children (members) as sub folders, but it can also have many other 
folders as parents (groups it belongs to). I get this done via a 
crosslink table. When creating a new parent-child relationship I need to 
make sure that the child is not somehow an ancestor or the parent I am 
trying to subordinate it to (as I need to avoid circular reference)

So I wrote the folder function:

  def ancestors
    ancestors = self.groups  #all the immediate parents are obviously 
ancestors
  scanfolders = [] #set up a stack to iterate through,
                         #looking for grandparents etc
  scanfolders += ancestors #the stack starts with the current ancestors
  if !scanfolders.nil? then
    while scanfolders.length > 0 do # while there are items on the stack
      scanitem = scanfolders.pop # get the last one and reduce the stack
      if scanitem then
        if !scanitem.groups.nil? then #if this item has parents
                                            #add them to the stack
         scanfolders += scanitem.groups
         scanfolders.uniq!
         ancestors += scanitem.groups #and record this item as an
                                            #ancestor
         ancestors.uniq!
        end
      end
    end
  end
  return ancestors
  end

So - to answer the question - I need to arrays that are initially the 
same (direct parents), But the one will eventually contain all ancestors 
and the other will be empty after iterating through all ancestors and 
testing them for further ancestors.

I should just replace

scanfolders = []
scanfolders += ancestors

with

scanfolders = ancestors.dup

Regards

Pieter
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