solution2.2.6.2 provided

Signed-off-by: Andreas Roehler <[email protected]>

chapter02/worksheets/solution2.2.6.2.scala

@@ -0,0 +1,35 @@
+/** 
+  Exercise 2.2.6.2
+  Implement the flatten method for sequences by using foldLeft.
+
+  The required type signature and a sample test are:
+
+  def flatten[A](xxs: Seq[Seq[A]]): Seq[A] = ???
+
+  scala> flatten(Seq(Seq(1, 2, 3), Seq(), Seq(4)))
+  res0: Seq[Int] = List(1, 2, 3, 4)
+
+  scala> flatten(Seq(Seq("a","b"), Seq(), (Seq("c","<nothing>"))))
+  val res1: Seq[String] = List(a, b, c, <nothing>)
+  */
+
+def flatten[A](xxs: Seq[Seq[A]]): Seq[A] = { 
+  var init: Seq[A] = Seq()
+  xxs.foldLeft(init){ (x, y) => y.foreach( k => (init = init :+ k)); init}
+}
+
+val result = flatten(Seq(Seq(1, 2, 3), Seq(), Seq(4)))
+val expected: Seq[Int] = List(1, 2, 3, 4)
+assert(result == expected)
+
+val a = flatten(Seq(Seq("a","b"), Seq(), (Seq("c","<nothing>"))))
+val b: Seq[String] = List("a", "b", "c", "<nothing>")
+assert(a == b)
+
+// scala> :load solution2.2.6.2.scala
+// :load solution2.2.6.2.scala
+// def flatten[A](xxs: Seq[Seq[A]]): Seq[A]
+// val result: Seq[Int] = List(1, 2, 3, 4)
+// val expected: Seq[Int] = List(1, 2, 3, 4)
+// val a: Seq[String] = List(a, b, c, <nothing>)
+// val b: Seq[String] = List(a, b, c, <nothing>)