# Trying to understand something about instance variables **URL:** \ **Category:** Rails\ **Created:** [March 2, 2007, 10:29pm UTC](https://www.ruby-forum.com/t/trying-to-understand-something-about-instance-variables/87483 "2007-03-02T22:29:35Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Kristen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/c4cdca/32.png) [@Kristen](https://www.ruby-forum.com/u/Kristen)\ **Post date:** [March 2, 2007, 10:29pm UTC](https://www.ruby-forum.com/t/trying-to-understand-something-about-instance-variables/87483/1 "2007-03-02T22:29:35Z")
My question is based on the following code from the agile book: File: depot\_f/app/controllers/store\_controller.rb private def find\_cart session[:cart] ||= Cart.new File: depot\_f/app/models/cart.rb class Cart Ö’attr\_reader :items def initialize @items = [] end def add\_product(product) @items \<\< product end end File: depot\_f/app/controllers/store\_controller.rb def add\_to\_cart @cart = find\_cart product = Product.find(params[:id]) @cart.add\_product(product) end Ok, so correct me if Im wrong, but dont instance variables only last till the end of the method? So how is it possible to keep appending items to the instance array named @items? Sorry for the newbie question, but I need to get this cleared up so I can move on. Thanks in advance Kristen ---
**Author:** ![Kristen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/c4cdca/32.png) [@Kristen](https://www.ruby-forum.com/u/Kristen)\ **Post date:** [March 2, 2007, 10:56pm UTC](https://www.ruby-forum.com/t/trying-to-understand-something-about-instance-variables/87483/2 "2007-03-02T22:56:40Z")
> class Cart > > till the end of the method? > > So how is it possible to keep appending items to the instance array > named @items? > > Sorry for the newbie question, but I need to get this cleared up so I > can move on. You’re right about instance variables… but way up at the top is the line: session[:cart] ||= Cart.new This line checks to see if sesion[:cart] exists and if it doesn’t, it sets it to a new Cart. Then it returns that cart object (either a new one or from the session). This happens a bit farthe down in the line: @cart = find\_cart At which point you have a valid @cart which has as instance variables @items as they were saved in the session from the last request. -philip ---
**Author:** ![Kristen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/c4cdca/32.png) [@Kristen](https://www.ruby-forum.com/u/Kristen)\ **Post date:** [March 3, 2007, 3:14am UTC](https://www.ruby-forum.com/t/trying-to-understand-something-about-instance-variables/87483/3 "2007-03-03T03:14:08Z")
> 1. Next, a Product object is created as a result of a find from the > database using an id parameter from the request. It’s only a local > variable and it’s called product. > 2. The product is added to the @cart. Hmm, Im kinda getting the idea now, but could you clarify the difference between an instance variable and a local variable. How do I know when I should use an instance or a local variable. Is there a rule of thumb? Thanks you for your replies so far. ---
**Author:** ![Kristen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/c4cdca/32.png) [@Kristen](https://www.ruby-forum.com/u/Kristen)\ **Post date:** [March 2, 2007, 11:16pm UTC](https://www.ruby-forum.com/t/trying-to-understand-something-about-instance-variables/87483/4 "2007-03-02T23:16:42Z")
Kristen, I think that the statement, “Instance variables only last till the end of the method” is incorrect. Instance variables live as long as their containing object lives. So, the Cart object has an instance variable (an array) called @items. If your statement were true, there would be no point in adding anything to it because @items would disappear before anything useful can be done. Here’s what’s going on starting with store\_controller#add\_to\_cart: 1. You’re creating a Cart object as a result of a find from the database and calling it @cart. 2. Next, a Product object is created as a result of a find from the database using an id parameter from the request. It’s only a local variable and it’s called product. 3. The product is added to the @cart. 4. The controller method ends. By default, Rails looks for a .rhtml file called /app/views//. By convention, store is the controller name, and add\_to\_cart is the action name. 5. Presumably, you want to show what’s in the cart in the view. That’s one reason why you’re using an instance variable-- to make it visible to the view. The local variable, product, is not available to the view. Hope it helps, -Anthony ---
**Author:** ![Kristen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/c4cdca/32.png) [@Kristen](https://www.ruby-forum.com/u/Kristen)\ **Post date:** [March 3, 2007, 3:59am UTC](https://www.ruby-forum.com/t/trying-to-understand-something-about-instance-variables/87483/5 "2007-03-03T03:59:06Z")
An instance variable will be available to embedded ruby calls in your rhtml templates; a local variable will only be available within the method it’s defined in. In the example from AWDWR you quoted: > def add\_to\_cart > @cart = find\_cart > product = Product.find(params[:id]) > @cart.add\_product(product) > end @cart is an instance variable (denoted with an @ symbol), because it’s an instance of the Cart object. Product is a local variable, its scope is local to the action (method) add\_to\_cart. @cart can be exposed in web pages, but product cannot. Further down AWDWR shows the instance variable @cart exposed in the add\_to\_cart.rhtml template: # Your Pragmatic Cart \<% for item in @cart.items %\>- \<%= item.title %\> \<% end %\> The rule of thumb is: to make data visible to your user, use an instance variable. The AWDWR book does a much better job of illustrating these details. Hope this helps! Regards, Dave * * * Information and Educational Technology Kwantlen University College - 604-599-2120 “So powerful is the light of unity that it can illuminate the whole earth.” --Bahá'u’lláh Kristen [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Sent by: removed\_email\_address@domain.invalid 02/03/2007 06:14 PM Please respond to removed\_email\_address@domain.invalid To removed\_email\_address@domain.invalid cc Subject [Rails] Re: Trying to understand something about instance variables. > 1. Next, a Product object is created as a result of a find from the > database using an id parameter from the request. It’s only a local > variable and it’s called product. > 2. The product is added to the @cart. Hmm, Im kinda getting the idea now, but could you clarify the difference between an instance variable and a local variable. How do I know when I should use an instance or a local variable. Is there a rule of thumb? Thanks you for your replies so far. – Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). ---
**Author:** ![Kristen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/c4cdca/32.png) [@Kristen](https://www.ruby-forum.com/u/Kristen)\ **Post date:** [March 4, 2007, 12:43am UTC](https://www.ruby-forum.com/t/trying-to-understand-something-about-instance-variables/87483/6 "2007-03-04T00:43:21Z")
> @cart is an instance variable (denoted with an @ symbol), because it’s > an > instance of the Cart object. Product is a local variable, its scope is > local to the action (method) add\_to\_cart. @cart can be exposed in web > pages, but product cannot. I realy appreciate the tips guys. There is one mroe thing that Im not quite clear about. The method def add\_to\_cart @cart = find\_cart product = Product.find(params[:id]) @cart.add\_product(product) end Is the @cart in @cart = find\_cart the same @cart on the line @cart.add\_product(product)? I understand that the first line is either retrieving or creating a new object in the session. So, is the third line just referencing that same @cart object or is it going into the model cart and invoking the add\_product method? Thank you! ---
**Author:** ![Kristen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/c4cdca/32.png) [@Kristen](https://www.ruby-forum.com/u/Kristen)\ **Post date:** [March 3, 2007, 8:07am UTC](https://www.ruby-forum.com/t/trying-to-understand-something-about-instance-variables/87483/7 "2007-03-03T08:07:19Z")
The decision about the visibility depends on the encapsulation principle in object oriented programming. The variables that you declare must have as minimal visibility as possible to get the job done. The reason is that this principle results in better quality software due to lower coupling. You should read about basic OOP book if you are not familiar with basic concepts. Here is a some good books: Object Oriented Software Construction by Bertrand Meyer, Object Oriented Design Heuristics by Arthur J. Riel and if you want a freebie, my book available for free at [http://guide.zepho.com/version1/](http://guide.zepho.com/version1/)[http://www.amazon.com/Object-Oriented-Software-Construction-Prentice-Hall-International/dp/0136291554/ref=pd\_bbs\_sr\_1/104-8023659-2852731?ie=UTF8&s=books&qid=1172905283&sr=1-1](http://www.amazon.com/Object-Oriented-Software-Construction-Prentice-Hall-International/dp/0136291554/ref=pd_bbs_sr_1/104-8023659-2852731?ie=UTF8&s=books&qid=1172905283&sr=1-1)