Merge branch 'master' into feature/test-with-scala-3

chapter02/worksheets/solution2.2.6.1_flatMap.scala

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+/**
+  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)))
+  res1: Seq[Int] = List(1, 2, 3, 4)
+
+  scala> fromPairs(List((a,b), (c,<nothing>)))
+  res1: Seq[(String, 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] = {
+    xs.flatMap{ case (x1, x2) => List(x1, x2) }
+  }
+  def main(args: Array[String]) = {
+    val result: Seq[String] = 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] = 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_flatMap.scala
+// :load solution2.2.6.1_flatMap.scala
+// result: List(a, b, c, <nothing>)
+// a: List(1, 2, 3, 4)
+// // defined object FromPairs
+