diff -r 3350cc06804b -r ecec79b9ab25 progs/lecture1.scala
--- a/progs/lecture1.scala	Thu Nov 09 16:34:08 2017 +0000
+++ b/progs/lecture1.scala	Fri Nov 10 00:56:12 2017 +0000
@@ -13,7 +13,6 @@
 // (you cannot reassign values: z = 9 will give an error)
 
 
-
 // Hello World
 //=============
 
@@ -97,7 +96,7 @@
 "1,2,3,4,5".split(",").mkString("\n")
 "1,2,3,4,5".split(",3,").mkString("\n")
 
-// Types
+// Types (slide)
 //=======
 
 /* Scala is a strongly typed language
@@ -124,16 +123,16 @@
 println("""">\n<"""")
 
 /* in Java
-val lyrics = "Baa, Baa, Black Sheep \n" +
-             "Have you any wool? \n" +
-             "Yes, sir, yes sir \n" +
-             "Three bags full"
+val lyrics = "Sun dips down, the day has gone \n" +
+             "Witches, wolves and giants yawn \n" +
+             "Queen and dragon, troll and gnome \n" +
+             "Baddy buddies head for home"
 */ 
 
-val lyrics = """Baa, Baa, Black Sheep  
-                |Have you any wool?
-                |Yes, sir, yes sir
-                |Three bags full""".stripMargin
+val lyrics = """Sun dips down, the day has gone
+                |Witches, wolves and giants yawn
+                |Queen and dragon, troll and gnome
+                |Baddy buddies head for home""".stripMargin
 
 println(lyrics)
 
@@ -276,25 +275,73 @@
 for (n <- (1 to 10)) println(n)
 
 
-// concurrency (ONLY WORKS IN SCALA 2.11.8, not in SCALA 2.12.0)
+
+
+
+
+// concurrency (ONLY WORKS IN SCALA 2.11.8, not in SCALA 2.12)
+//             (would need to have this wrapped into a function, or
+//              REPL called with scala -Yrepl-class-based)
 for (n <- (1 to 10)) println(n)
 for (n <- (1 to 10).par) println(n)
 
 
 // for measuring time
-def time_needed[T](i: Int, code: => T) = {
+def time_needed[T](n: Int, code: => T) = {
   val start = System.nanoTime()
-  for (j <- 1 to i) code
+  for (i <- (0 to n)) code
   val end = System.nanoTime()
-  ((end - start) / i / 1.0e9) + " secs"
+  (end - start) / 1.0e9
 }
 
+
 val list = (1 to 1000000).toList
 time_needed(10, for (n <- list) yield n + 42)
 time_needed(10, for (n <- list.par) yield n + 42)
 
 
 
+// Problems with mutability and parallel computations
+//====================================================
+
+def count_intersection(A: Set[Int], B: Set[Int]) : Int = {
+  var count = 0
+  for (x <- A; if (B contains x)) count += 1 
+  count
+}
+
+val A = (1 to 1000).toSet
+val B = (1 to 1000 by 4).toSet
+
+count_intersection(A, B)
+
+// but do not try to add .par to the for-loop above
+
+
+//propper parallel version
+def count_intersection2(A: Set[Int], B: Set[Int]) : Int = 
+	A.par.count(x => B contains x)
+
+count_intersection2(A, B)
+
+
+//for measuring time
+def time_needed[T](n: Int, code: => T) = {
+  val start = System.nanoTime()
+  for (i <- (0 to n)) code
+  val end = System.nanoTime()
+  (end - start) / 1.0e9
+}
+
+val A = (1 to 1000000).toSet
+val B = (1 to 1000000 by 4).toSet
+
+time_needed(10, count_intersection(A, B))
+time_needed(10, count_intersection2(A, B))
+
+
+
+
 // Webpages
 //==========
 
@@ -302,10 +349,13 @@
 
 // obtaining a webpage
 val url = """https://nms.kcl.ac.uk/christian.urban/""" 
-val url = """http://api.postcodes.io/postcodes/CR84LQ""" 
 Source.fromURL(url)("ISO-8859-1").mkString
 
 
+// another example
+//val url = """http://api.postcodes.io/postcodes/CR84LQ""" 
+
+
 // a function for looking up constituency data
 def consty_lookup(pcode: String) : String = {
   val url = "http://api.postcodes.io/postcodes/" + pcode
@@ -350,7 +400,7 @@
 
 // naive version of crawl - searches until a given depth,
 // visits pages potentially more than once
-def crawl(url: String, n: Int): Unit = {
+def crawl(url: String, n: Int) : Unit = {
   if (n == 0) ()
   else {
     println(s"Visiting: $n $url")