// hide all digits of a credit card number except the last 4. eg

// cc = "12345678901123456"
// hide_card_number(cc)
// [ "************3456" ]


  def hide_card_number(card_number)
    size = card_number.to_s.size - 1
    cc = "" 
    i = 0
    0.upto(size) do 
      if( i > size - 4 )
      else
        cc += "*" 
      end
      i += 1
    end
    cc = cc + card_number[size-3, size].to_s
    return cc 
  end