diff -r ed63dca8068a -r 4113d4d8cf62 main_marking2/danube_test3.scala
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/main_marking2/danube_test3.scala	Mon Jan 25 00:21:00 2021 +0000
@@ -0,0 +1,56 @@
+import CW7b._
+
+
+// first test 
+
+val ls1_urban = List(("1", "a"), ("2", "a"), ("1", "c"), ("2", "a"), ("1", "c"))
+
+val m_urban = groupById(ls1_urban, Map())
+
+assert(m_urban.getOrElse("1", Nil).count(_ == "c") == 2)
+assert(m_urban.getOrElse("1", Nil).count(_ == "a") == 1)
+assert(m_urban.getOrElse("2", Nil) == List("a", "a"))
+
+// second test
+
+def urban_gb(ratings: List[(String, String)]) = 
+  ratings.groupBy(_._1).view.mapValues(_.map(_._2).toSet).toMap 
+
+def urban_gb2(ratings: List[(String, String)]) = 
+  ratings.groupBy(_._1).view.mapValues(_.map(_._2)).toMap 
+
+def urban_ck(ratings: List[(String, String)]) =
+  urban_gb(ratings) == groupById(ratings, Map()).view.mapValues(_.toSet).toMap
+
+
+val ls2_urban = List(("1", "a"), ("1", "b"), ("2", "x"), ("3", "a"), ("2", "y"), ("3", "c"))
+
+assert(urban_ck(ls2_urban))
+
+// thrird test
+
+val ls3_urban = (11 to 1000 by 5).map(_.toString).toList
+val ls4_urban = ls3_urban zip ls3_urban.tail
+val ls5_urban = ls4_urban ::: ls4_urban.reverse
+
+assert(urban_ck(ls5_urban))
+
+
+/*
+import io.Source
+import scala.util._
+
+def urban_get_csv_file(name: String) : List[String] = {
+  val csv = Source.fromFile(name)
+  csv.mkString.split("\n").toList.drop(1)
+}
+
+def urban_process_ratings(lines: List[String]) : List[(String, String)] = {
+  for (cols <- lines.map(_.split(",").toList); 
+       if (cols(2).toFloat >= 4)) yield (cols(0), cols(1))  
+}
+
+val urban_ratings = urban_process_ratings(urban_get_csv_file("ratings.csv").take(1000))
+
+assert(urban_ck(urban_ratings))
+*/