Merge pull request #69 from andreas-roehler/master

#63, solution2.2.6.3 without extra var

chapter02/worksheets/solution2.2.6.3_2.scala

@@ -0,0 +1,22 @@
+/** 
+  Exercise 2.2.6.3
+  Use foldLeft to implement the zipWithIndex method for sequences.
+
+  The required type signature and a sample test:
+
+  def zipWithIndex[A](xs: Seq[A]): Seq[(A, Int)] = ???
+
+  scala> zipWithIndex(Seq("a", "b", "c", "d"))
+  res0: Seq[(String, Int)] = List((a, 0), (b, 1), (c, 2), (d, 3))
+  */
+
+def zipWithIndex[A](xs: Seq[A]): Seq[(A, Int)] = {
+  type Acc = Seq[(A, Int)]
+  def init: Acc = Seq((xs.head, -1))
+  xs.foldLeft(init) { (x, y) => x :+ (y, x.last._2 + 1) }.tail
+}
+
+val expected: Seq[(String, Int)] = List(("a", 0), ("b", 1), ("c", 2), ("d", 3))
+val result = zipWithIndex(Seq("a", "b", "c", "d"))
+assert(result == expected) 
+