@ variables in Ruby on Rails

Question

What's the difference between @title and title? Since both of them can be variable names. Also, how do I decide which kind of variable I should use? With @ or not?

Solution 1

title is a local variable. They only exists within its scope (current block)

@title is an instance variable - and is available to all methods within the class.

You can read more here: http://strugglingwithruby.blogspot.dk/2010/03/variables.html

In Ruby on Rails - declaring your variables in your controller as instance variables (@title) makes them available to your view.

Solution 2

Use @title in your controllers when you want your variable to be available in your views.

The explanation is that @title is an instance variable while title is a local variable. Rails makes instance variables from controllers available to views because the template code (erb, haml, etc) is executed within the scope of the current controller instance.

Solution 3

A tutorial about What is Variable Scope? presents some details quite well, just enclose the related here.

+------------------+----------------------+
| Name Begins With |    Variable Scope    |
+------------------+----------------------+
| $                | A global variable    |
| @                | An instance variable |
| [a-z] or _       | A local variable     |
| [A-Z]            | A constant           |
| @@               | A class variable     |
+------------------+----------------------+

Solution 4

The difference is in the scope of the variable. The @version is available to all methods of the class instance.

The short answer, if you're in the controller and you need to make the variable available to the view then use @variable.

For a much longer answer try this: http://www.ruby-doc.org/docs/ProgrammingRuby/html/tut_classes.html

Solution 5

@variables are called instance variables in ruby. Which means you can access these variables in ANY METHOD inside the class. [Across all methods in the class]

Variables without the @ symbol are called local variables, which means you can access these local variables within THAT DECLARED METHOD only. Limited to the local scope.

Example of Instance Variables:

class Customer
  def initialize(id, name, addr)
    @cust_id = id
    @cust_name = name
    @cust_addr = addr
  end

  def display_details
    puts "Customer id #{@cust_id}"
    puts "Customer name #{@cust_name}"
    puts "Customer address #{@cust_addr}"
  end
end

In the above example @cust_id, @cust_name, @cust_addr are accessed in another method within the class. But the same thing would not be accessible with local variables.

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