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Ruby Ruby Objects and Classes Variables and Methods Instance Variables and Local Variables

Julia Fitzer
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Julia Fitzer
Front End Web Development Techdegree Student 5,574 Points
Posted by Julia Fitzer
.a{fill-rule:evenodd;}techdegree
Julia Fitzer
Front End Web Development Techdegree Student 5,574 Points

instance variables within a method

Let's say you define a method within a class and you want to use an instance variable. Must you always include the @ ? That doesn't seem to be the case based on a quiz answer I gave that "passed". Or is this a matter of convention and good style?

For example, let's say the class has the instance variable @title and I want to write a method to return the title, both of these seem to work: 

def title
    title
end

def title
     @title
end

1 Answer

Kourosh Raeen
Kourosh Raeen
23,733 Points
Kourosh Raeen
Kourosh Raeen
23,733 Points

Hi Julia - Instance variables do need to start with @, otherwise they would be considered a local variable. I don't think the example you mentioned would work. This is what I tried:

class Book
  def initialize(author, title)
    @author = author
    @title = title
  end

  def title
    @title
  end
end

book = Book.new("David A. Black", "The Well-Grounded Rubyist")
puts book.title

The above code works and the title method returns the value of the instance variable @title. Now try this:

class Book
  def initialize(author, title)
    @author = author
    @title = title
  end

  def title
    title
  end
end

book = Book.new("David A. Black", "The Well-Grounded Rubyist")
puts book.title

This time you would get the error "Stack level too deep" because ruby thinks that the method title is calling itself, creating a recursion that will never stop.

Now, lets add another variable insdie the initialize method but this time without the @ symbol:

class Book
  def initialize(author, title)
    @author = author
    @title = title
    number_of_page = 538
  end

  def title
    @title
  end

  def pages
    number_of_page
  end
end

book = Book.new("David A. Black", "The Well-Grounded Rubyist")
puts book.title
puts book.pages

This time you get the error "undefined local variable or method number_of_pages", because number_of_pages is a local variable that is only accessible inside the initialize method not anywhere else. Ok, lets now try this:

class Book
  def initialize(author, title)
    @author = author
    @title = title
  end

  def title
    @title
  end

  def pages
    number_of_page = 538
    return "This book has #{number_of_page} pages."
  end
end

book = Book.new("David A. Black", "The Well-Grounded Rubyist")
puts book.title
puts book.pages

This time it works since we are using the local variable number_of_pages in the method where it is defined.

Hope this helps and happy coding!