circle.rb

class Circle
  def initialize(radius)
    @radius = radius
  end

  def diameter
    2 * @radius
  end

  def circumference
    2 * Math::PI * @radius
  end

  def area
    Math::PI * @radius ** 2
  end

  def to_s
    "#{self.class}\n  radius: #{@radius}\n  diameter: #{diameter}\n  circumference: #{circumference}\n  area: #{area}\n"
  end
end

c = Circle.new(3)
puts c