POOD - chapter2.rb

class Gear
  attr_reader :chainring, :cog, :wheel

  def initialize(chainring, cog, wheel = nil)
    @chainring = chainring
    @cog       = cog 
    @wheel     = wheel
  end 

  def ratio
    chainring / cog.to_f
  end 

  def gear_inches
    ratio * wheel.diameter
  end 
end

class Wheel
  attr_reader :rim, :tire

  def initialize(rim, tire)
    @rim  = rim 
    @tire = tire
  end 

  def diameter
    rim + (tire * 2)
  end 

  def circumference
    diameter * Math::PI
  end 
end

wheel = Wheel.new(12, 3)
p wheel.circumference

p Gear.new(52, 11, wheel).gear_inches
p Gear.new(52, 11).gear_inches

# 56.548667764616276
# 85.0909090909091
# /Users/banyan/lang/ruby/POOD/chapter2.rb:15:in `gear_inches': undefined method `diameter' for nil:NilClass (NoMethodError)
#    from /Users/banyan/lang/ruby/POOD/chapter2.rb:40:in `<main>'