The argument of the to_s method

class Contact
  attr_writer :first_name, :middle_name, :last_name

  def first_name
    @first_name
  end

  def middle_name
    @middle_name
  end

  def last_name
    @last_name
  end

  def first_last
    first_name + " " + last_name
  end

  def last_first
    last_first = last_name
    last_first += ", "
    last_first += first_name
    if !@middle_name.nil?
      last_first += " "
      last_first += middle_name.slice(0, 1)
      last_first += "."
    end
    last_first
  end

  def full_name
    full_name = first_name
    if !@middle_name.nil?
      full_name += " "
      full_name += middle_name
    end
    full_name += ' '
    full_name += last_name
    full_name
  end

  def to_s(format = 'full_name')
    case format
    when 'full_name'
      full_name
    when 'last_first'
      last_first
    when 'first'
      first_name
    when 'last'
      last_name
    else
      first_last
    end
  end
end

jason = Contact.new
jason.first_name = "Jason"
jason.last_name = "Seifer"
puts jason.to_s
puts jason.to_s('full_name')
puts jason.to_s('last_first')

nick = Contact.new
nick.first_name = "Nick"
nick.middle_name = "A"
nick.last_name = "Pettit"
puts nick.to_s('first_last')