More exercises from Chapter 4.

src/main/java/fpinscala/chapter4/Exercise_4_02.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2014 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package fpinscala.chapter4;
+
+import fpinscala.chapter3.Cons;
+
+public class Exercise_4_02
+{
+    public static Option<Double> variance(Cons<Double> list)
+    {
+        return mean(list).flatMap(m -> mean(list.map(x -> Math.pow(x - m, 2))));
+    }
+
+    private static Option<Double> mean(Cons<Double> list)
+    {
+        if (list.isEmpty())
+            return Option.empty();
+        return Option.unit(sum(list) / length(list));
+    }
+
+    private static double sum(Cons<Double> list)
+    {
+        if (list.isEmpty())
+            return 0.0;
+        return list.head + sum(list.tail);
+    }
+
+    private static long length(Cons<Double> list)
+    {
+        if (list.isEmpty())
+            return 0;
+        return 1 + length(list.tail);
+    }
+
+    public static void main(String[] args)
+    {
+        System.out.println(variance(Cons.of(1.0, 3.0, 3.0, 1.0)));
+    }
+}