From: Edgardo Hames <ehames@...>
Date: 2004-08-30T22:59:19+09:00
Subject: Flexible operations for a collection class

Hi you all.

I'm writing a collection class, which looks like this (by heart, don't
have the code here)

class Collection
  def initialize(elements=nil)
    @elements=elements
  end
  def add(elements)
    elements.each{ |o| @elements << o unless @elements.include? o}
  end
  def sortable_by=(element_attribute)
    @sortable_by=element_attribute
  end
end

I need some help with the following issues:

0) I would like to be able to pass either an array or an single object
to the constructor, but (a) @elements should be a list of objects (not
an array containing arrays). I tried this

def initialize(o, *array)
  @elements << o
  array.each{ |o| @elements << o unless @elements.include? o}
end

but (a) is not satisfied when I do

c = Collection.new([1,2,3,4])

The same goes for the #add method.

1) I would like to use the #sortable_by method to indicate which
attribute of the collection elements should be compared when sorting
it. Right now, I'm sorting it like this

def sort(&block)
  if block_given?
    @elements.sort(&block)
  else
    @elements.sort{|x,y| x.send(@sortable_by) <=> y.send(@sortable_by)}
  end
end

but I would like to implement the #<=> method in my objects, so I can
delegate the #sort method to @elements. I came up with this,

def sortable_by=(element_attribute)
  @sortable_by=element_attribute
  @elements.each{|o| o.sortable_by=@sortable_by}
end

and I update every new object I add. If I had a Java Comparator like
class, that would be trivial.

Can anybody point out some suggestions on how to do this in a better way.
Thanks for reading such a long message.

Kind regards,
Ed

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan