array.rb

class Array
  def join_with_oxford_comma(delimiter=", ")
    string = ""
    if self.length == 1
      string = self.first
    elsif self.length == 2
      string = self.join(" and ")
    else
      self.each_with_index do |elem,i|
        if i == 0
          string+= elem.to_s
        elsif i == self.length-1
          string+="#{delimiter}and #{elem}"
        else
          string+="#{delimiter}#{elem}"
        end
      end
    end
    string
  end
end