From: Jacob Fugal <lukfugl@...>
Date: 2006-06-06T05:18:14+09:00
Subject: Re: Object Troubles - undefined method `+'

On 6/5/06, Paul D. Kraus <paul.kraus@gmail.com> wrote:
> ./parse.rb:14:in `incndainvoice': undefined method `+' for nil:NilClass
> (NoMethodError)
>         from ./parse.rb:31
>         from ./parse.rb:20

<snip>

> #!/usr/bin/env ruby
>
> class Customer
>     attr_reader :invoices, :ndainvoices
>     attr_writer :invoices, :ndainvoices
>     def initialize
>         @invoices = 1
>         @ndainvoices = 0
>     end
>     def increment_invoice
>         invoices = invoices + 1
>     end
>     def increment_ndainvoice
>         ndainvoice = ndainvoice + 1 # this is line 14
>     end
> end

You're problem here is that on line 14, you are actually using a local
variable. This is a little tricky, since it's actually your assignment
that is changing the context of the right hand side. What happens is
that the parser sees "ndainvoice =" and assumes you're creating a new
local variable called "ndainvoice"; this new local variable then masks
the "ndainvoice" method. Since it's value is nil (the default value of
an unassigned-to variable), the effect of the line is:

  ndainvoice = nil + 1 # ndainvoice is a local variable here

In order to get ruby to recognize the assignment as a method call,
rather than an assignment to a new local variable, you need to prefix
"self.":

  self.ndainvoice = ndainvoice + 1

The reference to ndainvoice on the right hand side is correctly
interpreted as a method call since there is no local variable of the
same name to mask it. Alternatively, you can use the instance variable
directly by prefixing an "@" (as you do in initialize):

  @ndainvoice = @ndainvoice + 1

As a final step, you can also use += to shorten the "a = a + b" pattern:

  self.ndainvoice += 1
  # or
  @ndainvoice += 1

Incidentally, you have the same problem on line 11.

Jacob Fugal