Undefined method `<<' for nil:NilClass (NoMethodError)

Question

I have an undefined method.

rb:31:in `add_song': undefined method `<<' for nil:NilClass (NoMethodError)

I do understand that @library[artist] gives nil , but I don't understand why and do not know how to fix it. Any advice?

module Promptable
  def prompt(message = "What music would you like to add", symbol = ":>")
  print message
  print symbol
  gets.chomp
  end
end


class Library
  attr_accessor :artist, :song
  def initialize
    @library = {}
  end

  def add_artist(artist)
    @library[artist] = []
  end

  def add_song(song)
    @library[artist] << song
  end

  def show
    puts @library
  end
end

class Artist
  attr_accessor :name, :song

  def initialize(artist)
    @name = artist[:name]
    @song = artist[:song]
  end

  def to_s
   "#{name}, #{song}"
  end
end


if __FILE__ == $PROGRAM_NAME
  include Promptable
  include Menu
  my_library = Library.new
  my_library.add_artist(Artist.new(:name => prompt("What it the artist name    ?")))
  my_library.add_song(Artist.new(:song => prompt("What is the song name ?")))
  my_library.show
end

Answer 1

You're calling add_artist with one instance of Artist and add_song with another. When you look up the artist's list of songs in add_song with @library[artist] you're using a hash key (the second instance of Artist ) which is not equivalent to the hash key under which you stored the list (the first instance of Artist ), so you're not getting the list back, but nil .

To use two different instances of Artist as equivalent hash keys, you'll need to decide when two instances of Artist should be equal and implement eql? and hash appropriately .