solution2.5.2.6.scala provided

.gitignore

@@ -2,8 +2,10 @@
 *RCS*
 *.class
 *.log
+chapter02/worksheets/solution2.5.2.5_foldLeft.scala
 
 # virtual machine crash logs, see http://www.java.com/en/download/help/error_hotspot.xml
+auspack/
 befehle.org
 hs_err_pid*
 target

chapter02/worksheets/solution2.5.2.6.scala

@@ -0,0 +1,43 @@
+/** 
+  Exercise 2.5.2.6
+
+  In a sorted integer array where no values are repeated, find all
+  pairs of values whose sum equals a given number 𝑛.
+  Use tail recursion.
+
+  A type signature and a sample test:
+
+  def pairs(goal: Int, xs: Array[Int]): Set[(Int, Int)] = ???
+
+  scala> pairs(10, Array(1, 2, 4, 5, 6, 8))
+  res0: Set[(Int, Int)] = Set((2,8), (4,6), (5,5))
+  */
+
+def pairsIntern(goal: Int, xs: Array[Int], res: Seq[(Int, Int)] = Seq((0, 0))): Seq[(Int, Int)] = { 
+  if (xs.isEmpty) res.reverse.tail
+  else {
+    val a = xs.filter(_ + xs.head == goal)
+    if (a.isEmpty)
+      pairsIntern(goal, xs.tail, res)
+    else {
+      val b = (xs.head, a(0))
+      val c = b +: res
+      pairsIntern(goal, xs.tail, c)
+    }
+  }
+}
+
+def pairs(goal: Int, xs: Array[Int]): Set[(Int, Int)] = {
+  pairsIntern(goal, xs).toSet
+}
+
+val expected: Set[(Int, Int)] = Set((2,8), (4,6), (5,5)) 
+val result = pairs(10, Array(1, 2, 4, 5, 6, 8))
+assert(result == expected) 
+
+// scala> :load solution2.5.2.6.scala
+// :load solution2.5.2.6.scala
+// def pairsIntern(goal: Int, xs: Array[Int], res: Seq[(Int, Int)]): Seq[(Int, Int)]
+// def pairs(goal: Int, xs: Array[Int]): Set[(Int, Int)]
+// val expected: Set[(Int, Int)] = Set((2,8), (4,6), (5,5))
+// val result: Set[(Int, Int)] = Set((2,8), (4,6), (5,5))