Is there an easier way?

class Store
  def check_inventory
    @inventory ||= []
    @inventory.each { ... }
  end
end

Can the lines with the instance variables开发者_开发知识库 on them be turned into a one-liner?

Yes!! There is always a cool hack in Ruby!

class Store
  def check_inventory
    @inventory.to_a.each { ... }
  end
end

The reason this works is Cool Ruby Feature number 9,123: as it happens, NilClass implements a #to_a method that returns []! How awesome is that?!

edtq ross$ irb --prompt-mode simple
>> nil.to_a
=> []
>> @this_does_not_exist.to_a
=> []

DigitalRoss's answer is almost the same, but it will never change @inventory; your code will ensure that @inventory is always an array. If you need that behavior, you can just straightforwardly combine the two lines:

class Store
  def check_inventory
    (@inventory ||= []).each { ... }
  end
end