// get_prices(List("GOOG", "AAPL"), 2010 to 2012)
val urban_prices = List(List(Some(311.349976), Some(20.544939)), 
                        List(Some(300.222351), Some(31.638695)), 
                        List(Some(330.555054), Some(39.478039)))


assert(get_deltas(urban_prices) == List(List(Some(-0.03573992567129673), Some(0.539975124774038)), 
                                        List(Some(0.10103412653643493), Some(0.24777709700099845))))


//get_prices(List("BIDU"), 2004 to 2008)
val urban_prices2 = List(List(None), List(None), 
                         List(Some(6.35)), List(Some(12.241)), 
                         List(Some(38.188)))


assert(get_deltas(urban_prices2) == List(List(None), List(None), 
                                        List(Some(0.9277165354330709)), 
                                        List(Some(2.119679764725104))))