A trivial Ruby circle class

circle.rb

class Circle
  attr_reader :r
  
  def initialize(x = 0, y = 0, r = 1)
    @x = x
    @y = y
    @r = r
  end
  
  def center
    [@x, @y]
  end
  
  def area
    Math::PI * @r * @r
  end
  
  def perimeter
    Math::PI * 2.0 * @r
  end
  
  def expand!(factor)
    @r *= factor
    self
  end
  
  def move!(dx, dy)
    @x += dx
    @y += dy
    self
  end
  
  def to_s
    "<Circle at (#{@x},#{@y}) with radius #{@r}>"
  end
end