Using Ruby to Find Common Lines in X files

Question

Right now I have 3 files, but I'd like to do this in a way that I can add more later. Each file is a list of IDs, like this.

174535945 174538045 160515924 81712260 25241494

I'd like the output to be the items that appear in list 1 and list 2, list 2 and list 3, and those that occur in list 1 and list 2 and list 3.

Would the most ruby way to be create a hash with a key for each list, and then get all keys and test against all hashes, or is there a nice gem that would help with this?

Thanks,

Answer 1

Use sets:

require 'set'

list_1 = open(filename_1).read.split.to_set
list_2 = open(filename_2).read.split.to_set
list_3 = open(filename_3).read.split.to_set

puts list_1 & list_2
puts list_2 & list_3
puts list_1 & list_2 & list_3

Answer 2

I think your suggestion of using a hash is the way to go:

require 'set'

class Test
  def initialize()
    @lists = {174=>[1,2,3], 111=>[2,3,4], 160=>[2,4,5]}
  end

  def common_members(*keys)
    lists_as_sets = {}
    @lists.each_pair {|key,list| lists_as_sets[key] = list.to_set}
    common = lists_as_sets[keys.shift] # Set to key[0] and nil that element of keys
    keys.each {|k| common = common & lists_as_sets[k]} # Intersect the remaining keys, if any
    common.to_a
  end
end

t = Test.new
p t.common_members(174,111) # => [2,3]
p t.common_members(111,160) # => [2,4]
p t.common_members(174,111,160) # =>[2]