Initialize and Instance Variables

We had a look at the initialize method in the previous post and saw how it assigns values to instance variables via arguments when a class is instantiated. Below is a review of how the Book class uses the initialize method to assign the attributes title and author to any new book instance.

class Book

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

end

book_1 = Book.new("The Algebraist", "Iain M. Banks")

Now the @title in this instance is assigned to “The Algebraist”, and the @author in this instance is “Iain M. Banks”. Any other methods within Book can use methods to act upon these instance variables.

Getting and Setting

If we want to be able to return the title or author of the book_1 we must write a “getter” method. A getter method accesses and presents attributes. This can be thought of as reading attributes. A getter method can be written like:

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

	def get_title
		@title
	end
end

book_1 = Book.new("The Algebraist", "Iain M. Banks")
book_1.get_title #=> "The Algebraist"

You can see how get_title’s purpose is simply to return the instance variable. Also notice how an instance variable can be passed around all methods within the class, where the methods are called upon instances.

To change the title of an instance, or set it, we can create a setter method. Setting an attribute is akin to writing an attribute. Here is an example.

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

	def get_title
		@title
	end

	def set_title=(new_title)
		@title = new_title
	end
end

book_1 = Book.new("The Algebraist", "Iain M. Banks")
book_1.get_title #=> "The Algebraist"
book_1.set_title("The Wasp Factory")
book_1.get_title #=> "The Wasp Factory"

Here the set_title method is using an argument that the instance is passed to reset the instance variable.

Attribute Readers and Writers

We can see how getter methods are like readers and setter methods are like writers. It also seems that potentially writing two methods for every attribute that we want to read and write is quite long and tedious. Luckily Ruby has some helpful alternatives, called attribute readers and writers. If we want to only read attributes we use the keyword attr_reader, if we want to write we can use attr_writerand if we want to be able to do both then attr_reader will be the keyword to use. Here are some examples of how these keywords are used:

class Book

attr_accessor :title
attr_reader :author
attr_writer :year

	def initialize(title, author, year)
		@title = title
		@author = author
		@year = year
	end

end

book_1 = Book.new("The Violent Bear It Awry", "Flannery O'Connor", 1959)

book_1.title #=> "The Violent Bear It Awry"
book_1.title("The Violent Bear It Away")
book_1.title #=> "The Violent Bear It Away"

book_1.author #=> "Flannery O'Connor"

book_1.year(1960)

See how attr_accessor allows us to read and write the title of book_1, attr_reader allows us to only read the author of book_1 and attr_writer allows us to only write the year (of publication) for book_1 but not to view it.

There you have it!

P.S. “The Violent Bear It Away” by Flannery O’Connor is a real-life instance of Book I would highly recommend.