csv_2_list.rb

=begin
@date 23 July 2015
@author: pietro.passarelli@gmail.com

Given a CSV with two coulmns that have duplicate fields, 
this script compares the two lists and returns a list of the duplicates,
and prints it out as text file.

run it as `ruby csv_2_list.rb name_of_the_csv_file_.csv`
=end

require 'csv'
filename = ARGV.first
#csv_file = CSV.read(filename)

# 3 arrays, one for the first list, one for the second and one for the duplicates
list_one = []
list_two =[]
duplicates =[]

# iterate through the csv elements to put colum one in list_one and column two of the csv in list_two array
CSV.foreach(filename) do |r|
	list_one << r[0]
	if r[1] != nil 
		list_two << r[1]
	end 

end


puts "### Identifiying Duplicates ###"
# using built in method & on two arrays we can create a new array that only contains the duplciates
# this mehtod is highly optimised and can handle very long lists.
duplicates = list_one & list_two

# outputing duplicates to screen
puts duplicates.size 

# Writing duplicates to file, one per line
File.open("duplicates.txt", 'w') do |file| 
	# looping through duplicates array
	duplicates.each do |d|
		# writing duplicate item to file
		file.write(d)
		# adding a new line before the next one
		file.write("\n") 
	end 
end