Class Variables and Methods Lab

Objectives

1. Use class variables to keep track of data pertaining to a class.

2. Define class methods to expose data pertaining to a class.

Overview

In this lab, we'll be dealing with a Song class. The Song class can produce individual songs. Each song has a name, an artist and a genre. We need our Song class to be able to keep track of the number of songs that it creates.

We need our Song class to be able to show us all of the artists of existing songs:

Song.artists
  # => ["Jay-Z", "Drake", "Beyonce"]

We need our Song class to be able to show us all of the genres of existing songs:

Song.genres
  # => ["Rap", "Pop"]

We also need our Song class to be able to keep track of the number of songs of each genre it creates.

In other words, calling:

Should return something like this;

{"rap" => 5, "rock" => 1, "country" => 3}

Lastly, we want our Song class to reveal to us the number of songs each artist is responsible for.

Song.artist_count
  # => {"Beyonce" => 17, "Jay-Z" => 40}

We'll accomplish this with the use of class variables and methods.

Instructions

• Define your Song class such that an individual song is initialized with a name, artist and genre.

• There should be an attr_accessor for those three attributes.

ninety_nine_problems = Song.new("99 Problems", "Jay-Z", "rap")

ninety_nine_problems.name
  # => "99 Problems"

ninety_nine_problems.artist
  # => "Jay-Z"

ninety_nine_problems.genre
  # => "rap"

• Create a class variable, @@count. We will use this variable to keep track of the number of new songs that are created from the Song class. Set this variable equal to 0.

• At what point should we increment our @@count of songs? Whenever a new song is created. Your #initialize method should use the @@count variable and increment the value of that variable by 1.

• Write a class method, .count, that returns the total number of songs created.

• Write a class method, .genres that returns an array of all of the genres of existing songs. This array should contain only unique genres––no duplicates! Think about what you'll need to do to get this method working.

  • You'll need a class variable, let's call it @@genres, that is equal to an empty array.

  • When should you add genres to the array? Whenever a new song is created. Your #initialize method should add the genre of the song being created to the @@genres array. All genres should be added to the array. Control for duplicates when you code your .genres class method, not when you add genres to the original @@genres array. We will want to know how many songs of each genre have been created. We'll revisit that job a little later on.

• Write a class method, .artists, that returns an array of all of the artists of the existing songs. This array should only contain unique artists––no repeats! Once again think about what you need to do to implement this behavior.

  • You'll need a class variable, let's call it @@artists, that is equal to an empty array.

  • When should you add artists to this array? Whenever a new song is intialized. Your #initialize method should add artists to the @@artists array. All artists should be added to the array. Control for duplicates when you code your .artists class method, not when you add artists to the original @@artists array. We will want to know how many songs each have been assigned to each artist. We'll revisit that job a little later on when we write our .artist_count method.

• Write a class method, .genre_count, that returns a hash in which the keys are the names of each genre. Each genre name key should point to a value that is the number of songs that have that genre.

Song.genre_count
  # => {"rap" => 5, "rock" => 1, "country" => 3}

This manner of displaying numerical data is called a histogram. How will you create your histogram? You will need to iterate over the @@genres array and populate a hash with the key/value pairs. You will need to check to see if the hash already contains a key of a particular genre. If so, increment the value of that key by one, otherwise, create a new key/value pair.

• Write a class method, .artist_count, that returns a histogram similar to the one above, but for artists rather than genres.

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