When we build objects through our own classes we know we can add behavior to the objects through instance methods. But how do we give our objects data? For example, how would we teach a dog what its name is? Or what breed it is? How do we put data inside of an instance of an object so that each dog can have its own name and own breed? How do we give objects attributes or properties?
Run your tests with
learn to get an overview of what we are trying to do. Then follow along below for how to get your tests to pass.
We've been working with variables for a while now. For example:
bro_greeting = "Sup, bro?"
The code above sets a variable,
bro_greeting, equal to the string
"Sup, bro?". Now we can use that variable to read and operate on that string.
bro_greeting #=> "Sup, bro?" bro_greeting.upcase #=> "SUP, BRO?"
bro_greeting variable is what's known as a local variable, so named because it can only be accessed in a specific, local environment.
A local variable that is defined inside one method, for example, cannot be accessed by another method. In order to get around this limitation, we can use instance variables inside our Ruby classes.
An instance variable is a variable that is accessible in any instance method in a particular instance of a class.
Let's say we have a class called
Dog that is responsible for producing individual dog objects. We want each dog to be able to have a name and show its name. So we need to write some methods:
dog.rb inside the
lib folder. This is where we'll create our Dog class. Type in the following code.
class Dog def name=(dog_name) this_dogs_name = dog_name end def name this_dogs_name end end
Here we've defined two instance methods, the
name=, or "name equals" method, and the
name method. The first method takes in an argument of a dog's name and sets that argument equal to a variable,
this_dogs_name. The second method is responsible for reporting, or reading the name. The methods act as mechanisms to expose data from inside of the object to the outside world.
Our two methods therefore are responsible for "setting" and "getting" an individual dog's name.
Here's the functionality we want to achieve:
lassie = Dog.new lassie.name = "Lassie" lassie.name #=> "Lassie"
Our new dog instance should be assigned a name and know its name.
dog.rb file add the above code. This should create a new dog instance, give it a name (Lassie) and try to access- or read- its name.
learn again. You should see an error message, with the following snippet inside it:
`name': undefined local variable or method `this_dogs_name''
Uh-oh. Looks like the
#name method doesn't know about the
this_dogs_name variable from the
#name= method. That is because
this_dogs_name is a local variable. A local variable has a local scope. That means that it cannot be accessed outside of the method in which it is defined.
We define an instance variable by prefacing the variable name with an
Instance variables are bound to an instance of a class. That means that the value held by an instance variable is specific to whatever instance of the class it happens to belong to. Instance variables hold information about an instance, usually an attribute of that instance, and can be called on throughout the class, without needing to be passed into other methods as arguments (as would be the case with local variables).
Let's refactor our
Dog class to use an instance variable instead of a local variable to set and get an individual dog's name.
dog.rb and change the
Dog class in the following way:
class Dog def name=(dogs_name) @this_dogs_name = dogs_name end def name @this_dogs_name end end lassie = Dog.new lassie.name = "Lassie" puts lassie.name
learn again and your tests should now be passing.
Why did it work? Inside the
#name= method, we set the value of
@this_dogs_name equal to whatever string is passed in as an argument. Then, we are able to call on that same instance variable in a totally separate method, the
As we dive deeper into object-oriented Ruby, we'll be using instance variables frequently to pass information around the instance methods of a class. Think of instance variables as the containers for instance-specific information. The ability of instance variables to store information and be accessible within different instance methods is one of the things that makes it possible for us to create similar but unique objects in object-oriented Ruby.
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