task3.rb

# Represents a book with
# - an isbn number, `isbn`,
# - and price of the book as a floating-point number, `price`,
# as attributes.
#
# The constructor should accept
# - the ISBN number (a string) as the first argument
# - and price as second argument,
# and should raise ArgumentError (one of Ruby's built-in exception types)
# if the ISBN number is the empty string
# or if the price is less than or equal to zero.
#
# Include the proper getters and setters for these attributes.
# Include a method `price_as_string`
# that returns the price of the book
# with a leading dollar sign and trailing zeros,
# that is, a price of 20 should display as "$20.00"
# and a price of 33.8 should display as "$33.80".
class BookInStock
  
  attr_accessor :isbn, :price
  
  def initialize(isbn, price)
    @isbn, @price = isbn.to_s, price.to_f.round(2)
    raise ArgumentError if (@isbn.empty? || @price <= 0)
  end  

  def price_as_string
    price_2f = '%.2f' % @price
    "$#{price_2f}"
  end  
end