Pulling a value from one CSV based on a value in another

Question

I am trying to figure out the best way to pull a value from a CSV file called lookup.csv based on a value in master.csv, and then save the new file as

Answer 1

I'm using 1.9, where FasterCSV is available as CSV in the standard lib. First I'd create a lookup hash out of lookup.csv:

cities = Hash[CSV.read('lookup.csv', :col_sep => ' | ').to_a[1..-1]]

If the file is very big, you might want to iterate over it with CSV.foreach and build the hash row by row:

cities = {}
CSV.foreach('lookup.csv', :col_sep => ' | ', :headers => true, :return_headers => false) do |line|
  cities[line['City']] = line['City ID']  
end  

Then iterate over master.csv, do a lookup of the city in the hash and write that to output.csv:

CSV.open('output.csv', "w", :headers => ['First Name', 'Last Name', 'City ID'], :write_headers => true) do |output|
  CSV.foreach('master.csv', :col_sep => ' | ', :headers => true, :return_headers => false) do |line|
    output << [line['First Name'], line['Last Name'], cities[line['City']]]
  end  
end