def splitted sep=','
      @data.map do |s|
        if s.nil?
          nil
        elsif s.respond_to? :split
          s.split sep
        else
          [s]
        end
      end
    end
Return an Array with the data splitted by a separator.
a=Daru::Vector.new(["a,b","c,d","a,b","d"])
a.splitted
=>
[["a","b"],["c","d"],["a","b"],["d"]]