Question

Consider the following code

class CheckOut 
    @rules
    @total = 0
    @basket = Hash.new 

    def initialize(rules, discounts)
        @total=0
                #if i use the line below everything is ok.
        #@basket = Hash.new
        @rules = rules
    end

     def scan sku
          price = @rules[sku]
          if @basket.has_key?(sku) #I get NoMethodError: undefined method `has_key?' for nil:NilClass
             @basket[sku] += 1
          else 
              @basket[sku] = 1
          end
          @total += price
     end    

     def total
        @total
     end
end

If I run the code as is I get a noMethodError on has_key? but if I create the Hash in initialize everything works. Why can’t I create the Hash at the declaration?

Answer 1

When you define an instance variable in the class body, it’s a class instance variable defined on CheckOut, which is an instance of Class, and doesn’t exist in an instance of CheckOut. Instead you need to define the instance variables in your initialize as you’ve found (since initialize runs in the context of the new CheckOut instance):

class CheckOut
  def initialize(rules, discounts)
    @total = 0
    @basket = Hash.new
    @rules = rules
  end
  ...
end

Here’s a quick example illustrating this further:

class Foo
  @bar = "class bar!"
  @baz = "class baz!"
  def initialize
    @bar = "instance bar!"
  end
end

Foo.instance_variable_get(:@bar)  #=> "class bar!"
Foo.new.instance_variable_get(:@bar)  #=> "instance bar!"

Foo.instance_variable_get(:@baz)  #=> "class baz!"
Foo.new.instance_variable_get(:@baz)  #=> nil

This also shows that all instance variables default to nil, even if they’ve never been referenced before. This is why your error is a NoMethodError for nil:NilClass and not a NameError.