# File lib/ai4r/data/data_set.rb, line 166
      def build_domain(attr)
        index = get_index(attr)
        if @data_items.first[index].is_a?(Numeric)
          return [Statistics.min(self, index), Statistics.max(self, index)]
        else
          return @data_items.inject(Set.new){|domain, x| domain << x[index]}
        end
      end