Instance Variables Code-Along

Learning Goals

Define instance variables
Distinguish instance variables from local variables
Describe how instance variables give objects attributes and properties

Introduction

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? In other words, how do we give objects attributes or properties?

Instructions

Fork and clone this code-along, then run your tests with learn test to get an overview of what we are trying to do. Follow along to get the tests to pass.

What Is a Local Variable?

We've been working with variables for a while now. For example:

what_time_is_it = "Lab time"

The code above sets a variable, what_time_is_it, equal to the string "Lab time". Now we can use that variable to read and operate on that string.

what_time_is_it #=> "Lab time"

what_time_is_it.upcase #=> "LAB TIME"

The what_time_is_it 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 cannot be accessed by another method:

def method_a
  my_local_variable = "I can only be used in method_a"
  puts my_local_variable
end

def method_b
  puts my_local_variable
end

method_b
# NameError (undefined local variable or method `my_local_variable' for main:Object)

The scope of a local variable is that it is only available within the 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.

Local Variables in Instance Methods

Let's say we have a class called Dog that is responsible for producing individual dog objects. We want each dog instance to be able to have a name and to show its name. So we need to write two methods: one to set the dog's name and one to get the dog's name.

Open dog.rb inside the lib folder and add the following two methods to the Dog class:

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 assigns that value to a variable, this_dogs_name. The second method is responsible for reporting the name. Our two methods therefore are responsible for "setting" and "getting" an individual dog's name.

We want the methods to act as mechanisms to expose data from inside of the object to the outside world; in other words, we want to be able to do this:

lassie = Dog.new
lassie.name = "Lassie"

lassie.name #=> "Lassie"

In the code above, we are creating a new Dog instance and assigning it a name then asking it to return its name.

In your dog.rb file add the above code. Run learn test again. You should see output that includes something like this:

$ learn test
Failure/Error: this_dogs_name

NameError:
  undefined local variable or method `this_dogs_name' for #<Dog:0x00007fa7909a3078>

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, which means that it cannot be accessed outside of the method in which it is defined.

To solve this problem, we'll need a new kind of variable: an instance variable.

Implementing Instance Variables

We define an instance variable by prefacing the variable name with an @ symbol.

Instance variables are bound to an instance of a class. That means that when we set the value of an attribute on an instance of a class, the value we set is associated with that particular instance. This makes intuitive sense: a dog's name belongs to that specific dog, not to Dogs in general.

Instance variables hold information about an instance, i.e., they describe attributes or properties of the instance, such as its name or breed. They 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.

Inside dog.rb, 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

Run learn test again; your tests should now be passing. By changing the local variable this_dogs_name to an instance variable @this_dogs_name, we've effectively changed its scope: instead of only being available to the method it's defined in, it's now available to all instance methods defined within the class.

Defining Getter and Setter Methods

In Object-Oriented Ruby, we want to be able to set attributes on the instances of our classes. To do this, we create setter methods, which take an argument and set the instance variable equal to that value:

class Dog
  # setter method
  def name=(value)
    # assign the @name instance variable a value of whatever is passed as an argument
    @name = value
  end
end

odie = Dog.new

# call the Dog#name= method with a value of "Odie"
odie.name = "Odie"
# this is equivalent to calling odie.name=("Odie")

We also want those objects to "know" the values of their attributes. So we also create a getter method that will return the value of the instance variable:

class Dog
  # setter method
  def name=(value)
    @name = value
  end

  # getter method
  def name
    # access the data from the @name instance variable and return it
    @name
  end
end

odie = Dog.new
odie.name = "Odie"

# call the Dog#name method
puts odie.name
# "Odie"

By using the @ in front of the variable name, the variable that's created inside the setter method is available inside the getter method (and inside any other methods in our class).

Note that this is quite different from the way we assign attributes to objects in JavaScript, which doesn't require instance variables or getter/setter methods:

class Dog {}

const odie = new Dog();
odie.name = "Odie";
console.log(odie.name);
// "Odie"

Remember, unlike JavaScript, whenever we use dot notation in Ruby, we're calling a method on an object.

Conclusion

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. If we define objects as the combination of data and behavior, then instance variables are where we store the data, and instance methods define the behavior.

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.

njambi-muhoro/phase-3-ruby-oo-basics-instance-variables-lab