# Error when using class variables with operators **URL:** \ **Category:** Ruby\ **Created:** [October 6, 2010, 8:22pm UTC](https://www.ruby-forum.com/t/error-when-using-class-variables-with-operators/196100 "2010-10-06T20:22:08Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![prichard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/90db22/32.png) [@prichard](https://www.ruby-forum.com/u/prichard)\ **Post date:** [October 6, 2010, 8:22pm UTC](https://www.ruby-forum.com/t/error-when-using-class-variables-with-operators/196100/1 "2010-10-06T20:22:08Z")
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 ---
**Author:** ![prichard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/90db22/32.png) [@prichard](https://www.ruby-forum.com/u/prichard)\ **Post date:** [October 6, 2010, 8:25pm UTC](https://www.ruby-forum.com/t/error-when-using-class-variables-with-operators/196100/2 "2010-10-06T20:25:51Z")
On Wednesday 06 October 2010, Paul R. 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 ---
**Author:** ![prichard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/90db22/32.png) [@prichard](https://www.ruby-forum.com/u/prichard)\ **Post date:** [October 6, 2010, 8:57pm UTC](https://www.ruby-forum.com/t/error-when-using-class-variables-with-operators/196100/3 "2010-10-06T20:57:45Z")
Stefano C. wrote: > On Wednesday 06 October 2010, Paul R. 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 @@ ---
**Author:** ![prichard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/90db22/32.png) [@prichard](https://www.ruby-forum.com/u/prichard)\ **Post date:** [October 6, 2010, 10:01pm UTC](https://www.ruby-forum.com/t/error-when-using-class-variables-with-operators/196100/4 "2010-10-06T22:01:17Z")
On Wednesday 06 October 2010, Paul R. wrote: > |\>\> | > |The problem is I intialise the attributes first… > |mltn.each do |trg| > |def off\_target > |@@target1 = @@target1 - @@target2 > |end > | > |So I want to make off\_target accessable which is why I use @@ I truly can’t understand how your code should work. However, here are some remarks: 1. have you tried following my suggestion and displaying the content of the class variables before trying the comparison? If so, are you they’re not nil? 2. In the code you posted in your second message, there’s no comparison between class variables. 3. It generally makes little sense to initialize class variables from the initialize method. This way, every time you create an instance of your class, the class variables are set to new values. There may be situations where this is the correct behaviour, but I think they’re quite rare. Usually, in this case, instance variables are the correct choice 4. You call the off\_target instance method from the on\_target class method. This can’t work. Either you made a mistake while writing your mail (meaning that both method are class methods or both are instance methods) or your code will fail as soon as has to execute the else clause, because off\_target can only be called on an instance of your class, while in the body of the on\_target? method, the implicit receiver, that is, self, is the class itself. Stefano ---
**Author:** ![prichard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/90db22/32.png) [@prichard](https://www.ruby-forum.com/u/prichard)\ **Post date:** [October 6, 2010, 9:59pm UTC](https://www.ruby-forum.com/t/error-when-using-class-variables-with-operators/196100/5 "2010-10-06T21:59:31Z")
On 10/6/2010 2:01 PM, Paul R. wrote: > > > |if @@target1 \< @@target2 > > > The problem is I intialise the attributes first… > > > mltn.each do |trg| > > > 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 ---
**Author:** ![prichard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/90db22/32.png) [@prichard](https://www.ruby-forum.com/u/prichard)\ **Post date:** [October 6, 2010, 10:20pm UTC](https://www.ruby-forum.com/t/error-when-using-class-variables-with-operators/196100/6 "2010-10-06T22:20:36Z")
On 10/6/2010 2:58 PM, Jeremy B. wrote: > > > > | > > > > def self.on\_target?(mltn) > > class variables shared among all instances of your class, which is > end > @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. 🙂 Hmm. Looking closer at what you’re trying to do, I see that you do need the class variables in your current implementation. The problem is that I can’t figure out why you are trying to use a class method (on\_target?) for this rather than an instance method. It looks like Stefano is also confused about what you’re trying to do here. Please understand that you are mixing together class and instance methods. For now you should probably stick to instance methods since they are more common: class MyClass def my\_instance\_method … end end Rather than… class MyClass def self.my\_class\_method … end end Once you have a handle on instance methods, understanding class methods and how they differ from instance methods will be easier. -Jeremy ---
**Author:** ![prichard](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/90db22/32.png) [@prichard](https://www.ruby-forum.com/u/prichard)\ **Post date:** [October 6, 2010, 11:59pm UTC](https://www.ruby-forum.com/t/error-when-using-class-variables-with-operators/196100/7 "2010-10-06T23:59:43Z")
Jeremy B. wrote: > On 10/6/2010 2:58 PM, Jeremy B. wrote: > > > > > > | > > > > > > def self.on\_target?(mltn) > > > > class variables shared among all instances of your class, which is > > end > > @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. 🙂 > > Hmm. Looking closer at what you’re trying to do, I see that you do need > the class variables in your current implementation. The problem is that > I can’t figure out why you are trying to use a class method (on\_target?) > for this rather than an instance method. It looks like Stefano is also > confused about what you’re trying to do here. > > Please understand that you are mixing together class and instance > methods. For now you should probably stick to instance methods since > they are more common: > > class MyClass > def my\_instance\_method > … > end > end > > Rather than… > > class MyClass > def self.my\_class\_method > … > end > end > > Once you have a handle on instance methods, understanding class methods > and how they differ from instance methods will be easier. > > -Jeremy Thanks guys for the help. Stefano, I stuck a ‘p’ in and I got a nil. Jeremy I understand where you are coming from when you say stick to instance methods before using class methods. However I need to make one method a class method (it’s more complex than the example I gave above). I will then be calling a method from here (presumingly this has to be a class method too?) Thanks for the patience, I’m slow on the uptake