Question

I have this class:

class Account
    attr_accessor :balance
    def initialize(balance)
            @balance = balance
    end
    def credit(amount)
            @balance += amount
    end
    def debit(amount)
            @balance -= amount
    end
end

Then, for example, later in the program:

bank_account = Account.new(200)
bank_account.debit(100)

If I call the debit method with the “-=” operator in it (as shown in the class above) the program fails with the following message:

bank2.rb:14:in `debit': undefined method `-' for "200":String (NoMethodError)
from bank2.rb:52:in `<main>'

But if I remove the minus sign and just make it @balance = amount, then it works. Obviously I want it to subtract, but I can’t figure out why it doesn’t work. Can math not be done with instance variables?

Answer 1

Your value passed into initialize() is a string, rather than an integer. Cast it to an int via .to_i.

def initialize(balance)
   # Cast the parameter to an integer, no matter what it receives
   # and the other operators will be available to it later      
   @balance = balance.to_i
end

Likewise, if the parameter passed to debit() and credit() is a string, cast it to an int.

def credit(amount)
    @balance += amount.to_i
end
def debit(amount)
    @balance -= amount.to_i
end

Finally, I’ll add that if you plan to set @balance outside the initialize() method, it is recommended to define its setter to call .to_i implicitly.

def balance=(balance)
  @balance = balance.to_i
end

Note: This assumes you want and only intend to use integer values. Use .to_f if you need floating point values.