stock_stats

book_in_stock.rb

#---
# Our job is to take all the CSV files and work out how many of each title we have, 
# as well as the total list price of the books in stock.
# CSV files looks something like this:
# "Date",     "ISBN",         "Amount"
# "20080412", "9781934356104", 39.45
# "20080413", "9781934356166", 45.67
# Excerpted from "Programming Ruby"
#---
class BookInStock      
  
  attr_reader :isbn, :price
  
  def initialize(isbn, price)
    @isbn  = isbn
    @price = Float(price)
  end  

end

csv_reader.rb

require 'csv' 
require_relative 'book_in_stock'

class CsvReader
  
  def initialize
    @books_in_stock = []
  end
  
  def read_in_csv_data(csv_file_name)
    CSV.foreach(csv_file_name, headers: true) do |row|
      @books_in_stock << BookInStock.new(row["ISBN"], row["Amount"])
    end
  end  
  
  # later we'll see how to use inject to sum a collection
  def total_value_in_stock
    sum = 0.0        
    @books_in_stock.each {|book| sum += book.price}
    sum
  end  
  
  def number_of_each_isbn
    # ...
  end

end     

stock_stats.rb

require_relative 'csv_reader'

reader = CsvReader.new

ARGV.each do |csv_file_name|
  STDERR.puts "Processing #{csv_file_name}"
  reader.read_in_csv_data(csv_file_name)
end

puts "Total value = #{reader.total_value_in_stock}"

Let’s give ourselves a simple problem to solve. Let’s say that we’re running a secondhand bookstore. Every week, we do stock control. A gang of clerks uses portable bar-code scanners to record every book on our shelves. Each scanner generates a simple comma-separated value (CSV) file containing one row for each book scanned. The row contains (among other things) the book’s ISBN and price. An extract from one of these files looks something like this:

"Date", "ISBN", "Amount"

"20080412", "9781934356104", 39.45

"20080413", "9781934356166", 45.67

Our job is to take all the CSV files and work out how many of each title we have, as well as the total list price of the books in stock.

Whenever you’re designing OO systems, a good first step is to identify the things you’re dealing with. Typically each type of thing becomes a class in your final program, and the things themselves are instances of these classes.