Variable in Ruby language

Variable is a label to give name to the memory location, which hold any data to be used by the program. The data can be of any data type including integer, boolean, string, array, hash or else.

Suppose, you want to use some name, then in Ruby(or in any language), you have to use variable to do that.

name = "foo"

In above code, variable with name name is created to identify the value of some name foo. During the program, variable name will refer to foo data.

# print name of some person
puts "Name of the person is #{name}"

# output: Name of the person is foo

You can see in above code how name variable is used.

We have following categories of variable depending on their use.

Local Variable

A local variable name starts with a lowercase letter or underscore(_). It is only accessible or have its scope within the block where it is initialized. Once the code block completes, the variable has no scope i.e after that it is not accessible.

When uninitialized local variables are called, they are interpreted as call to a method that has no arguments.

def print_name
  name = "foo"        # name is local variable and only accessible in print_name
  puts name
end

print_name()
# puts name           # calling variable name here will give an error

Instance Variable

An instance variable begins with @ sign. It belongs to one instance of the class and can be accessed from any methods of that instance. Uninitialized instance variables have the value nil.

class RubyStudent
  def initialize(id, name, addr)
    @student_id = id
    @student_name = name
    @student_addr = addr
  end

  def display_details()
    puts "Student ID: #{@student_id}"
    puts "Student Name: #{@student_name}"
    puts "Student Address: #{@student_addr}"
  end
end

# Create Students
stud1 = RubyStudent.new(1001, "Foo", "Some Address One")
stud2 = RubyStudent.new(1002, "Bar", "Some Address Two")

stud1.display_details()
stud2.display_details()

# Output
Student ID: 1001
Student Name: Foo
Student Address: Some Address One
Student ID: 1002
Student Name: Bar
Student Address: Some Address Two

Class variable

A class variable name starts with @@ sign. They need to be initialized before use. A class variable belongs to the whole class and can be accessible from anywhere inside the class. If the value will be changed at one instance, it will be changed at every instance.

A class variable is shared by all the descendents of the class. An uninitialized class variable will result in an error.

class RubyStudent
  @@no_of_students = 0

  def initialize(name)
    @student_name = name
    @@no_of_students += 1
  end

  # some other codes

  def total_students()
    puts "Total number of students: #{@@no_of_students}"
  end
end

# Create Students
stud1 = RubyStudent.new("Foo")
stud2 = RubyStudent.new("Bar")
stud3 = RubyStudent.new("Baz")

stud1.total_students()
stud2.total_students()
stud3.total_students()

# Output
Total number of students: 3
Total number of students: 3
Total number of students: 3

Global Variable

A global variable name starts with a $ sign. Its scope is globally, means it can be accessed from any where in a program. An uninitialized global variable will have a nil value.

$global_guru = "Guru"

class RubyGuru
  def print_global
    puts "Global guru in RubyGuru class is #{$global_guru}"
  end
end

class RubyStudent
  def print_global
    puts "Global guru in RubyStudent class is #{$global_guru}"
  end
end

teacher = RubyGuru.new
teacher.print_global
student = RubyStudent.new
student.print_global

# Output
Global guru in RubyGuru class is Guru
Global guru in RubyStudent class is Guru

Constant in Ruby

A constant has a name starting with an uppercase character. It should be assigned a value at most once.

Constants may be defined within classes, but unlike instance variables, they are accessible outside the class.

A_CONST = 10
# output 10
A_CONST = 20
# output
warning: already initialized constant A_CONST
warning: previous definition of A_CONST was here

puts A_CONST
# output 20

So, in above case, the value of constant A_CONST changes after second assignment with only warning.

Key points

• Constants defined within a class or module may be accessed anywhere within the class or module.
• Outside the class or module, they may be accessed using the scope operator, :: prefixed by an expression that returns the appropriate class or module.
• Constants defined outside any class or module may be accessed as it is or by using the scope operator with no prefix.
• Constants may not be defined in methods.
• Constants may be added to existing classes and modules from the outside by using the class or module name and the scope operator before the constant name.

Following program shows all aspect as mentioned in the Key points section.

OUTER_CONST = 99

class Const
  def get_const
    CONST
  end
  CONST = OUTER_CONST + 1
end

puts Const.new.get_const      # 100
puts Const::CONST             # 100
puts ::OUTER_CONST            # 99
puts Const::NEW_CONST = 123   # 123

Ruby Pseudo Variables

These are special type of variables but behave like constants as we cannot assign any value to these variables.

Ruby has following Pseudo variables:

• self: The receiver object of the current method.
• true: Representing true value
• false: Representing false value
• nil: Representing nothing
• __FILE__: The name of the current Ruby program.
• __LINE__: The current line number in the Ruby program.