From: Michael Halcrow <mah69@...>
Date: 2002-09-26T06:46:09+09:00
Subject: Set Union on Array of Objects

Why do array subtraction and array union not behave in the same manner
when operating on an array of user-defined objects? Specifically, how
can I get the Array union operator to use the user-defined "==" method
of the objects, as the Array subtraction operator does? What is the best
way to get what I expect?

Thanks,
Mike

#!/usr/bin/ruby

class Foo

  attr_accessor :a, :b
  
  def initialize( a, b )
    @a = a
    @b = b
  end

  def ==( foo )
    puts "Foo.== Called!"
    if( $comparemode == 0 )
      @a == foo.a
    else
      @b == foo.b
    end
  end

  def to_s
    "#{@a}:#{@b}"
  end
    
end

x = [ Foo.new( 'a', 1 ), Foo.new( 'a', 2 ) ]
y = [ Foo.new( 'a', 2 ), Foo.new( 'b', 3 ) ]

puts "Set Subtraction on Attribute:"
puts "EXPECT (empty)"
$comparemode = 0
puts x - y
puts "Set Subtraction on Value:"
puts "EXPECT a:1"
$comparemode = 1
puts x - y
puts "Set Union on Attribute:"
puts "EXPECT a:1, b:3"
$comparemode = 0
puts x | y
puts "Set Union on Value:"
puts "EXPECT a:1, a:2, b:3"
$comparemode = 1
puts x | y

-----------------------

Set Subtraction on Attribute:
EXPECT (empty)
Foo.== Called!
Foo.== Called!
Set Subtraction on Value:
EXPECT a:1
Foo.== Called!
Foo.== Called!
Foo.== Called!
a:1
Set Union on Attribute:
EXPECT a:1, b:3
a:1
a:2
a:2
b:3
Set Union on Value:
EXPECT a:1, a:2, b:3
a:1
a:2
a:2
b:3


-- 
---------------------------------------- | ------------------------
Michael Halcrow                          | mhalcrow@byu.edu    
Research Assistant, Network Security Lab | Dept. of Comp. Science  
                                         | Brigham Young University
What has it got on its portses?          |
---------------------------------------- | ------------------------