From: Jeremy Bopp <jeremy@...>
Date: 2010-10-07T04:58:49+09:00
Subject: Re: Error when using class variables with operators

On 10/6/2010 2:01 PM, Paul Roche wrote:
> Stefano Crocco wrote:
>> On Wednesday 06 October 2010, Paul Roche wrote:
>>> |Hi. When I use class variables with operators I get the following error
>>> |message........
>>> |
>>> | undefined method `>' for nil:NilClass
>>> |
>>> |
>>> |here's a piece of code that gives this error.....
>>> |
>>> |if @@target1 <  @@target2
>>> |   @@status = true
>>
>> It means that @@target1 is nil. Try
>>
>>   p @@target1
>>
>> just before the if and see what it shows.
>>
>> Stefano
> 
> The problem is I intialise the attributes first.......
> 
> def initialize(tar1, tar2)
> @@target1 = tar1
> @@target2 = tar2
> end
> 
> 
> Then I use this method....
> 
> def self.on_target?(mltn)
> mltn.each do |trg|
> if trg.target1 > trg.target2
> then p "on target"
> else
> off_target
> end
> end
> end
> 
> and call this method in the method above...
> 
> def off_target
> @@target1 = @@target1 - @@target2
> end
> 
> So I want to make off_target accessable which is why I use @@

You don't need @@ in order to make off_target accessible.  That method
is already accessible from any instance of your class.  What you
probably mean is that you need @@ to make target1 and target2
accessible.  However, the @@target1 and @@target2 variables are actually
class variables shared among all instances of your class, which is
probably not what you want.

class MyClass
  def initialize(value)
    @@value = value
  end

  def get_value
    @@value
  end
end

my_class1 = MyClass.new(1)
my_class1.get_value                   # => 1

my_class2 = MyClass.new("unexpected")

my_class1.get_value                   # => "unexpected"


What you want instead are instance variables such as @target1 and
@target2.  I found these notations a little hard to remember at first,
but you'll find that defining class variables is pretty rare and that
you can usually just use @variable. :-)

-Jeremy