Question

… or alternatively an Array which prevents duplicate entries.

Is there some kind of object in Ruby which:

• responds to [], []= and <<
• silently drops duplicate entries
• is Enumerable (or at least supports find_all)
• preserves the order in which entries were inserted

?

As far as I can tell, an Array supports points 1, 3 and 4; while a Set supports 1, 2 and 3 (but not 4). And a SortedSet won’t do, because my entries don’t implement <=>.

Answer 1

There isn’t one as far as I know, and Set by its mathematical nature is meant to be unordered (or at least, implementationally, meant not to guarantee order – in fact its usually implemented as a hash table so it does mess up order).

However, it’s not hard to either extend array directly or subclass it to do this. I just tried it out and this works:

class UniqueArray < Array
  def initialize(*args)
    if args.size == 1 and args[0].is_a? Array then
      super(args[0].uniq)
    else
      super(*args)
    end
  end

  def insert(i, v)
    super(i, v) unless include?(v)
  end

  def <<(v)
    super(v) unless include?(v)
  end

  def []=(*args)
    # note: could just call super(*args) then uniq!, but this is faster

    # there are three different versions of this call:
    # 1. start, length, value
    # 2. index, value
    # 3. range, value
    # We just need to get the value
    v = case args.size
      when 3 then args[2]
      when 2 then args[1]
      else nil
    end

    super(*args) if v.nil? or not include?(v)
  end
end

Seems to cover all the bases. I used OReilly’s handy Ruby Cookbook as a reference – they have a recipe for “Making sure a sorted array stays sorted” which is similar.