map and flatMap (chapter 19)

Author: tobyweston

src/main/scala/s4j/scala/chapter19/FlatMapExample.scala

@@ -0,0 +1,22 @@
+package s4j.scala.chapter19
+
+import java.util.Calendar
+
+object FlatMapExample extends App {
+
+  def ageEitherSide(birthYear: Int): List[Int] = {
+    val today = Calendar.getInstance.get(Calendar.YEAR)
+    List(today - 1 - birthYear, today - birthYear, today + 1 - birthYear)
+  }
+
+  val birthdays = List(1990, 1977, 1984)
+
+  println(birthdays.map(ageEitherSide))
+  // outputs:
+  // List(List(26, 27, 28), List(39, 40, 41), List(32, 33, 34))
+
+  println(birthdays.flatMap(ageEitherSide))
+  // outputs:
+  // List(24, 25, 26, 37, 38, 39, 30, 31, 32)
+
+}

src/main/scala/s4j/scala/chapter19/FlatMappable.scala

@@ -0,0 +1,34 @@
+package s4j.scala.chapter19
+
+class FlatMappable[A](elements: A*) {
+
+  def flatMap[B](f: A => List[B]): List[B] = {
+    val result = collection.mutable.MutableList[B]()
+    elements.foreach {
+      f.apply(_).foreach {
+        result += _
+      }
+    }
+    result.toList
+  }
+}
+
+
+package flatmap {
+
+  object Example extends App {
+
+    def oddNumbersTo(end: Int): List[Int] = {
+      val odds = collection.mutable.MutableList[Int]()
+      for (i <- 0 to end) {
+        if (i % 2 != 0) odds += i
+      }
+      odds.toList
+    }
+
+    val mappable = new FlatMappable(1, 2, 10)
+    val result = mappable.flatMap(oddNumbersTo)
+    println(result)
+  }
+
+}

src/main/scala/s4j/scala/chapter19/MapExample.scala

@@ -0,0 +1,22 @@
+package s4j.scala.chapter19
+
+import java.util.Calendar
+
+object MapExample {
+
+  def age(birthYear: Int) = {
+    val currentYear = Calendar.getInstance.get(Calendar.YEAR)
+    currentYear - birthYear
+  }
+
+  val birthdays = List(1990, 1977, 1984, 1961, 1973)
+
+  birthdays.map(age)  // = List(25, 38, 31, 54, 42)
+
+  // written in-line
+  birthdays.map(year => Calendar.getInstance.get(Calendar.YEAR) - year)
+
+  // with shorthand parameter _
+  birthdays.map(Calendar.getInstance.get(Calendar.YEAR) - _)
+
+}

src/main/scala/s4j/scala/chapter19/Mappable.scala

@@ -0,0 +1,49 @@
+package s4j.scala.chapter19
+
+class Mappable[A](elements: List[A]) {
+
+  def map[B](f: A => B): List[B] = {
+    val result = collection.mutable.MutableList[B]()
+    elements.foreach {
+      result += f(_)
+    }
+    result.toList
+  }
+
+  // recursive version (with a nested def)
+  def recur_map[B](f: A => B): List[B] = {
+    def recur(head: A, tail: List[A]): List[B] = {
+      tail match {
+        case Nil => List(f(head))
+        case _   => f(head) +: recur(tail.head, tail.tail)
+      }
+    }
+
+    recur(elements.head, elements.tail)
+  }
+
+  // tail recursive version
+  def tail_recur_map[B](f: A => B): List[B] = {
+    def recur(accumulator: List[B], elements: List[A]): List[B] = {
+      elements match {
+        case Nil          => accumulator
+        case head :: tail => recur(accumulator :+ f(head), tail)
+      }
+    }
+
+    recur(List[B](), elements)
+  }
+}
+
+package map {
+
+  object Example extends App {
+    val numbers = List(1, 2, 54, 4, 12, 43, 54, 23, 34)
+    val mappable = new Mappable(numbers)
+
+    println(mappable.map(_ * 2))
+    println(mappable.recur_map(_ * 2))
+    println(mappable.tail_recur_map(_ * 2))
+  }
+
+}