nil：NilClass的未定义方法“ +”（NoMethodError）

Question

I'm writing a simple class that is initialized with a variable called "cash" which is an integer.

Below is the code. When I run this, I get the NoMethodError. I know I can easily fix this by referencing the local class variable with @cash, but a book I read on OOP recommend to almost never use the @, and instead set the attr and use simply 'cash'. I have set attr_accessor, but it doesn't work, and I'd like to understand why. Thanks

class Person
  attr_accessor :cash

  def initialize(cash)
    @cash = cash
  end

  def add_cash(amount)
    cash = cash + amount
  end
end

Answer 1

Local variable reference has precedence over method call with the same name.

Similarly, local variable assignment has precedence over method call with the same name. A writer method of the form foo= needs an explicit receiver. When the receiver is omitted, it is not recognized as a method, but as a local variable assignment.

def add_cash(amount)
  self.cash = cash + amount
end

Answer 2

Change add_cash method to:

  def add_cash(amount)
    @cash = cash + amount  # using cash= will define a local variable in the method,
                           # then cash + amount will become nil + amount
                           # or you could use self.cash instead of @cash
  end

UPDATE

You can do these compares:

  def add_cash_local(amount)
    puts "Instance cash: #{cash}"    # we can use cash for the same effect of self.cash or @cash                  
                                     # If there is no local variable assigned as cash
    puts "Instance cash + amount: #{cash + amount}"

    cash = 100
    puts "Local cash: #{cash}"       # after cash= assignment, cash is differ from self.cash or @cash
    puts "Local cash + amount: #{cash + amount}"

    puts "Instance cash: #{@cash}"
    puts "Instance cash + amount: #{@cash + amount}"

    puts "Instance cash: #{self.cash}"
    puts "Instance cash + amount: #{self.cash + amount}"

  end

Answer 3

The error should be (as for why you got `-', I have no idea unless you didn't show all your code and your method calls):

undefined method `+' for nil:NilClass (NoMethodError)

The error in the code stems from the fact that the cash variable is only a local variable. It is undefined before the call to add_amount, so you see the error to nil:NilClass.

In the docs for attr_accessor , that is creating set and get methods for an instance (@cash) variable. So, you need to reference the arguments as instance variables when you declare a class.