Minor fixes and solution2.2.6.1_foldLeft

chapter02/worksheets/solution2.2.6.1.scala

@@ -4,8 +4,11 @@ signature and a sample test are:
 
 def fromPairs[A](xs: Seq[(A, A)]): Seq[A] = ???
 
+scala> fromPairs(List((1,2), (3,4)))
+res0: Seq[Int] = List(1, 2, 3, 4)
+
 scala> fromPairs(List((a,b), (c,<nothing>)))
-res1: Seq[(String, String)] = List("a", "b", "c", "<nothing>")
+res1: Seq[String] = List("a", "b", "c", "<nothing>")
 
 Hint: This can be done with foldLeft or with flatMap.
  */
@@ -18,11 +21,16 @@ def fromPairs[A](xs: Seq[(A, A)]): Seq[A] = {
 
 val result = fromPairs(List(("a","b"), ("c","<nothing>")))
 val expected = List("a", "b", "c", "<nothing>")
-
 assert(result == expected)
 
+val a: Seq[Int] = this.fromPairs(List((1, 2), (3, 4)))
+val c: Seq[Int] = List(1, 2, 3, 4)
+assert(a == c)
+
 // scala> :load solution2.2.6.1.scala
 // :load solution2.2.6.1.scala
 // def fromPairs[A](xs: Seq[(A, A)]): Seq[A]
 // val result: Seq[String] = List(a, b, c, <nothing>)
 // val expected: List[String] = List(a, b, c, <nothing>)
+// val a: Seq[Int] = List(1, 2, 3, 4)
+// val c: Seq[Int] = List(1, 2, 3, 4)

chapter02/worksheets/solution2.2.6.1_foldLeft.scala

@@ -0,0 +1,46 @@
+/**
+  Exercise 2.2.6.1
+
+  Implement a function fromPairs that performs the inverse
+  transformation to the toPairs function defined in Example 2.2.5.6.
+
+  The required type signature and a sample test are:
+
+  def fromPairs[A](xs: Seq[(A, A)]): Seq[A] = ???
+
+  scala> fromPairs(List((1,2), (3,4)))
+  res0: Seq[Int] = List(1, 2, 3, 4)
+
+  scala> fromPairs(List(("a","b"), ("c",<nothing>)))
+  res1: Seq[String] = List(a, b, c, <nothing>)
+
+  Hint: This can be done with foldLeft or with flatMap.
+  */
+
+object FromPairs {
+  def fromPairs[A](xs: Seq[(A, A)]): Seq[A] = {
+    // Type alias, for brevity.
+    type Acc = (Seq[A])
+    def init: Acc = Seq()
+    xs.foldLeft(init){ (x, y) => x :+ y._1 :+ y._2 }
+  }
+    def main(args: Array[String]) = {
+      val result: Seq[String] = this.fromPairs(List(("a","b"), ("c","<nothing>")))
+      val expected: Seq[String] = List("a", "b", "c", "<nothing>")
+      println("result: %s".format(result))
+        assert(result == expected)
+      val a: Seq[Int] = this.fromPairs(List((1, 2), (3, 4)))
+      val b: Seq[Int] = List(1, 2, 3, 4)
+      println("a: %s".format(a))
+        assert(a == b)
+    }
+}
+
+FromPairs.main(Array())
+
+// scala> :load solution2.2.6.1_foldLeft.scala
+// :load solution2.2.6.1_foldLeft.scala
+// result: List(a, b, c, <nothing>)
+// a: List(1, 2, 3, 4)
+// // defined object FromPairs
+