gistfile1.rb

# setup my filenames
sent_filename     = ARGV[0]
billed_filename   = ARGV[1]
to_bill_filename  = sent_filename.split('.')[0]+'_send_again.csv'

# create my arrays
sent_array = []
File.open(sent_filename, 'r').each_line { |line| sent_array << line }

return_array = []
File.open(billed_filename, 'r').each_line { |line| return_array << line }

billed_array  = []
to_bill_array = []

# compare sent and billed and create new array of who was billed
sent_array[1..-1].each do |transaction|           # I skip the first line because its auth info
  if transaction != nil                           # make sure we don't include a blank line
    transaction_parts = transaction.split(',')    # split the transaction line into its parts
    transaction_id = transaction_parts[16].to_i   # part 16 has our unique id, and an integer
    return_array.each do |return_item|            # loop thru the returns
      return_id = return_item.split(',')[4].to_i  # split the return item into its parts
      if return_id == transaction_id              # compare the return id with transaction id
        billed_array << transaction               # add transaction to billed array
      end
    end
  end
end

# create output file minus the intersection of the sent_array and billed_array
output = File.open(to_bill_filename, "w")
output << (( sent_array | billed_array ) - ( sent_array & billed_array ))
output.close