Merge pull request #64 from andreas-roehler/master

solution2.5.2.1.scala provided

chapter02/worksheets/solution2.5.2.1.scala

@@ -0,0 +1,54 @@
+/** 
+  Exercise 2.5.2.1
+  Define a function dsq that computes the sum of squared digits of
+  a given integer;
+
+  for example, dsq(123) = 14
+  (see Example 2.5.1.6).
+
+  Generalize dsq to take as an argument a function f: Int => Int
+
+  replacing the squaring operation.
+
+  The required type signature and a sample test:
+
+  def digitsFSum(x: Int)(f: Int => Int): Int = ???
+
+  scala> digitsFSum(123){ x => x * x }
+
+  res0: Int = 14
+
+  scala> digitsFSum(123){ x => x * x * x }
+  res1: Int = 36
+  */
+
+def digitsOf(n: Int): Seq[Int] = {
+  Stream.iterate(n) { nk => nk / 10 }
+    .takeWhile { nk => nk != 0 }
+    .map { nk => nk % 10 }
+    .toList.reverse
+}
+
+def digitsFSum(x: Int)(f: Int => Int): Int =
+  digitsOf(x).map{ x => f(x)}.foldLeft(0)(_+ _)
+
+val expected: Int =  14
+val result =  digitsFSum(123){ x => x * x }
+assert(result == expected) 
+
+// scala> :load solution2.5.2.1.scala
+// :load solution2.5.2.1.scala
+// there was 1 deprecation warning; re-run with -deprecation for details
+// 1 warning found
+// def digitsOf(n: Int): Seq[Int]
+// def digitsFSum(x: Int)(f: Int => Int): Int
+// val expected: Int = 14
+// val result: Int = 14
+
+val a =  36
+val b =  digitsFSum(123){ x => x * x * x }
+assert(a == b)
+
+// val a: Int = 36
+// val b: Int = 36
+