# File lib/ai4r/clusterers/single_linkage.rb, line 89
      def create_distance_matrix(data_set)
        @distance_matrix = Array.new(data_set.data_items.length-1) {|index| Array.new(index+1)}      
        data_set.data_items.each_with_index do |a, i|
          i.times do |j|
            b = data_set.data_items[j]
            @distance_matrix[i-1][j] = @distance_function.call(a, b)
          end
        end
      end