How to Use Instance Variables in Ruby: Simple Guide

In Ruby, an instance variable starts with @ and belongs to an object, storing data unique to that object. You use it inside instance methods to keep track of object-specific information accessible across instance methods.

Syntax

An instance variable in Ruby is written with an @ prefix followed by the variable name. It is used inside instance methods to hold data specific to each object instance.

• @variable_name: Defines or accesses an instance variable.
• Used inside instance methods to store or retrieve object-specific data.

ruby

class Person
  def initialize(name)
    @name = name  # instance variable
  end

  def greet
    "Hello, my name is #{@name}."
  end
end

Example

This example shows how to set and use an instance variable @name inside a Ruby class. Each object stores its own name, and the greet method uses that instance variable to return a personalized message.

ruby

class Person
  def initialize(name)
    @name = name
  end

  def greet
    "Hello, my name is #{@name}."
  end
end

person1 = Person.new("Alice")
person2 = Person.new("Bob")

puts person1.greet
puts person2.greet

Output

Hello, my name is Alice.
Hello, my name is Bob.

Common Pitfalls

One common mistake is trying to access an instance variable outside the object or before it is set, which returns nil. Another is confusing instance variables with local variables (no @), which are limited to the method scope.

Always use @ to refer to instance variables inside instance methods.

ruby

class Person
  def initialize(name)
    name = name  # wrong: local variable, not instance variable
  end

  def greet
    "Hello, my name is #{name}."  # error: 'name' undefined here
  end
end

# Correct way:
class Person
  def initialize(name)
    @name = name  # instance variable
  end

  def greet
    "Hello, my name is #{@name}."
  end
end

Quick Reference

• @variable: Instance variable, unique to each object.
• Defined inside instance methods like initialize.
• Accessible by all instance methods of the object.
• Not accessible directly outside the object.

Key Takeaways

Instance variables start with @ and store data unique to each object.

Use instance variables inside instance methods to keep object-specific state.

Do not confuse instance variables with local variables; always use @ for instance variables.

Instance variables are accessible across instance methods but not outside the object.

Initialize instance variables typically in the initialize method for each object.