Variables and constants are locations which hold data to be used by a program. Constants are meant to declare values that we expect to be unchanged throughout our program. On the other hand, we use variables to declare values that we expect to change throughout our program.

Variables

There are four types of variables in ruby. They are

1. Global Variables
2. Local Variables
3. Class Variables
4. Instance Variables

Global Variables

Global Variables start with a dollar sign. They are assigned globally within the program and can also be accessed from anywhere within the program.

In the example below, $favorite_season is a global variable and can be accessed by the print_favorite_season method.

$favorite_season = "Summer"

def print_favorite_season
    puts "My favorite season is #{$favorite_season}."
end

print_favorite_season #=> "My favorite season is Summer."

In the example below, $least_favorite_season is a global variable that is not initialized (is not set to a value). In ruby, this returns nil instead of throwing an error.

$least_favorite_season 

puts $least_favorite_season.inspect #=> `nil`

Local Variables

Local Variables start with a lowercase or an underscore. Any local variable defined inside of a method is not available outside of that specific method. In the example below, we are able to access favorite_season only inside the method it is defined in but not outside of print_favorite_season.

def print_favorite_season
    favorite_season = "Summer"
    puts "My favorite season is #{favorite_season}."
end

print_favorite_season #=> "My favorite season is Summer."

favorite_season #=> undefined local variable or method 

If a local variable is defined outside of a specific method, then it is also not accessible inside of the method. So in the example below, the method print_favorite_season is not able to access the local variable favorite_season.

favorite_season = "Summer"

def print_favorite_season
    puts "My favorite season is #{favorite_season}."
end

print_favorite_season #=> undefined local variable or method 

In ruby, if a local variable is not initialized, it will returns nil instead of throwing an error. Here is an example.

def _print_local_var
    uninitialized_local_var = 
    puts uninitialized_local_var.inspect
end

_print_local_var #=> nil

Class Variables

Class Variables start with two at symbols. They are initialized inside the class before they can be used within the program. Once initialized, they are accessible anywhere within the program where the class they are related to is being used.

In the example below, we see the class variable @@num_of_todo_items being accessed globally and locally within the context of the single class that it was assigned to.

class Todo
    @@num_of_todo_items = 0

    def initialize
        @@num_of_todo_items += 1
    end

    def self.count
        puts @@num_of_todo_items
    end

end

to_do1 = Todo.new
to_do2 = Todo.new 

Todo.count #=> 2

If a class variable is uninitialized, then it will result in an error.

class Test
    @@uninitialized_class_var
    puts @@uninitialized_class_var
end

#=> uninitialized class variable @@uninitialized_class_var

Instance Variables

Instance Variables start with an at symbol. They are similar to class variables when it comes to scope. What makes them different is that their values are local to the instances of a certain class or object.

In the example below, we are defining an instance variable called @todo_item and setting it to an instance of the Todo class called todo_item. We are able to access @todo_item in a new method called display_todo_item.

class Todo

    @@num_of_todo_items = 0

    def initialize (todo_item)
        @todo_item = todo_item
    end

    def display_todo_item
        puts @todo_item
    end
end

to_do1 = Todo.new("clean house")
to_do2 = Todo.new("practice Ruby")

to_do1.display_todo_item #=> "clean house"
to_do2.display_todo_item #=> "practice Ruby"

Ruby does not require us to initialize instance variables and the instance variables will have a nil value when uninitialized. Here is an example.

class TestInstanceVar
    @uninitialized_instance_var
    puts @uninitialized_instance_var.inspect #=> nil
end

Constants

Constants are defined using all capital letters, are accessible globally, and are used to define values we expect to be constant (not change). If we try to re-assign the values of constants, ruby does not throw errors. However, it will give us a warning saying we are trying to re-assign the values of a constant.

SEASONS = ["spring", "summer", "fall", "winter"]
puts SEASONS #=> ["spring", "summer", "fall", "winter"]

SEASONS = "replacing the seasons array with a string"
puts SEASONS 
#=> warning: already initialized constant SEASONS 
#=> "replacing the seasons array with a string"

Similar to class variables, referencing an uninitialized constant produces an error.

SEASON

puts SEASON.inspect #=> uninitialized constant SEASON

Summary

Here is a quick way to identify constants and the different kinds of variables.

$global_variable

local_variable

_also_local_variable

@@class_variable

@instance_variable

CONSTANT