From: Jim Freeze <jim@...>
Date: 2003-04-08T12:58:53+09:00
Subject: Array Sutraction

Ok, this has been discussed at length previously,
and it appears that the implementation has
changed.

Consider:

  # in 1.6.7
  [1,1,2,2,3,3] - [3] => [1,2]

  # in 1.8.0
  [1,1,2,2,3,3] - [3] => [1,1,2,2]

I think this last algorithm was chosen for
performance reasons. The problem is, the #- operation
is neither explainable by the Array being considered a bag
or a set. Plus, since this operation seems to change
with revisions, it is hard to rely upon.

The one explanation that is consistent between the two
is if #uniq is applied to both sides of the equation.
Tbat is,

  ([1,1,2,2,3,3].uniq - [3].uniq).uniq  #=> [1,2].uniq     => [1,2]
  ([1,1,2,2,3,3].uniq - [3].uniq).uniq  #=> [1,1,2,2].uniq => [1,2]

So it would seem that Array#- is incomplete and the user
is required to apply #uniq to the result.

All I can say is, Why?


-- 
Jim Freeze
----------
Mandrell: "You know what I think?"
Doctor:   "Ah, ah that's a catch question. With a brain your size you
	  don't think, right?"
		-- Dr. Who