From: Stefan Salewski <mail@...>
Date: 2011-04-12T21:32:06+09:00
Subject: puts box.dup.enlarge(4, 5)  -- unexpected result

Hello,

I wonder why the statement "puts box.dup.enlarge(4, 5)" below gives
output 8 instead of (0, 1, 6, 8).

Thanks, Stefan Salewski

stefan@AMD64X2 ~/pet $ cat btest.rb 
#!/usr/bin/ruby -w
module Bounding
  class Box
    attr_accessor :x1, :y1, :x2, :y2
    def initialize(x1, y1, x2, y2)
      @x1, @x2 = [x1, x2].minmax
      @y1, @y2 = [y1, y2].minmax
    end
    def to_s
      "(#{@x1}, #{@y1}, #{@x2}, #{@y2})"
    end
    def enlarge(x, y)
      if x < 0 then @x1 += x else @x2 += x end
      if y < 0 then @y1 += y else @y2 += y end
    end
  end
end # Bounding

box = Bounding::Box.new(0, 1, 2, 3)
puts box

#this works as exspected
nb = box.dup
nb.enlarge(4, 5)
puts nb

#this works NOT as expected
puts box.dup.enlarge(4, 5)

stefan@AMD64X2 ~/pet $ ruby btest.rb
(0, 1, 2, 3)
(0, 1, 6, 8)
8