What is the difference between `@title` and `title` in Ruby on Rails?

General Discussions
1

What is the difference between @title and title in Ruby on Rails?

Both can be used as variable names, but how do they differ? Additionally, how do instance variables (@variable), local variables (variable), and global variables ($variable ruby) compare? When should I use @ or $ in my Rails application?

2

Explaining the Difference Between @title and title : In Ruby on Rails, @title and title refer to different types of variables:

  1. @title is an instance variable.

It belongs to an instance of a class, meaning it can be accessed across multiple methods in that specific instance.

Instance variables are typically used in Rails controllers and views. For example, in a controller, you might set @title = “Welcome” to pass data from the controller to the view, where it can be used to display the title of a page.

class PagesController < ApplicationController
  def show
    @title = "About Us"
  end
end
  1. title is a local variable.

Local variables are only accessible within the method or block where they are defined. They are not visible to other methods or parts of the program.

You would use local variables to store temporary data that doesn’t need to persist outside of a specific scope.

def display_title
  title = "Hello World"  # Local variable
  puts title
end
3

Now, let’s take a look at instance variables, local variables, and global variables in more detail:

  1. Instance variables (@variable):

These variables belong to an object (or instance of a class). They persist across method calls within that object. They are often used to store data that needs to be accessed or modified throughout the lifecycle of the object.

Passing data from controllers to views via instance variables. Example: @user = User.find(1)

  1. Local variables (variable):

Local variables are only accessible within the scope where they are defined (e.g., inside a method or block). Once the scope ends, the variable goes out of scope and is no longer accessible. Example: title = "Hello" inside a method or loop.

  1. Global variables ($variable):

Global variables are accessible throughout the entire Ruby program, no matter where they are declared. They can be dangerous to use, especially in large applications, because they can lead to unexpected behavior due to their global scope.

Best practice is to avoid global variables unless absolutely necessary. Example: $global_var = "I am accessible everywhere!"

4

To sum it up:

Use @ for instance variables when you want to store data specific to an object (for controllers, views, or models in Rails).

Use local variables for data that only needs to be used within a specific method or block. This keeps things more modular and prevents unnecessary data from leaking outside the scope.

Avoid $ for global variables. They should generally be avoided in Rails applications because they make the code harder to reason about.

If you find yourself needing global variables, you might want to reconsider your design approach. Instead, consider passing state through objects, methods, or classes.