Sample run of chapter's interpreter examples
Traversable(1, 2, 3) Iterable("x", "y", "z") Map("x" -> 24, "y" -> 25, "z" -> 26) Set(Color.Red, Color.Green, Color.Blue) SortedSet("hello", "world") Buffer(x, y, z) IndexedSeq(1.0, 2.0) LinearSeq(a, b, c)
List(1, 2, 3) HashMap("x" -> 24, "y" -> 25, "z" -> 26)
scala> List(1, 2, 3) map (_ + 1) res0: List[Int] = List(2, 3, 4) scala> Set(1, 2, 3) map (_ * 2) res1: scala.collection.immutable.Set[Int] = Set(2, 4, 6)
def foreach[U](f: Elem => U)
def foreach[U](f: Elem => U): Unit = { val it = iterator while (it.hasNext) f(it.next()) }
scala> val xs = List(1, 2, 3, 4, 5) xs: List[Int] = List(1, 2, 3, 4, 5) scala> val git = xs grouped 3 git: Iterator[List[Int]] = non-empty iterator scala> git.next() res2: List[Int] = List(1, 2, 3) scala> git.next() res3: List[Int] = List(4, 5) scala> val sit = xs sliding 3 sit: Iterator[List[Int]] = non-empty iterator scala> sit.next() res4: List[Int] = List(1, 2, 3) scala> sit.next() res5: List[Int] = List(2, 3, 4) scala> sit.next() res6: List[Int] = List(3, 4, 5)
sealed abstract class Tree case class Branch(left: Tree, right: Tree) extends Tree case class Node(elem: Int) extends Tree
sealed abstract class Tree extends Traversable[Int] { def foreach[U](f: Int => U) = this match { case Node(elem) => f(elem) case Branch(l, r) => l foreach f; r foreach f } }
sealed abstract class Tree extends Iterable[Int] { def iterator: Iterator[Int] = this match { case Node(elem) => Iterator.single(elem) case Branch(l, r) => l.iterator ++ r.iterator } }
scala> val fruit = Set("apple", "orange", "peach", "banana") fruit: scala.collection.immutable.Set[String] = Set(apple, orange, peach, banana) scala> fruit("peach") res7: Boolean = true scala> fruit("potato") res8: Boolean = false
scala> var s = Set(1, 2, 3) s: scala.collection.immutable.Set[Int] = Set(1, 2, 3) scala> s += 4; s -= 2 scala> s res10: scala.collection.immutable.Set[Int] = Set(1, 3, 4)
scala> val s = collection.mutable.Set(1, 2, 3) s: scala.collection.mutable.Set[Int] = Set(1, 2, 3) scala> s += 4 res11: s.type = Set(1, 2, 3, 4) scala> s -= 2 res12: s.type = Set(1, 3, 4)
def get(key): Option[Value]
scala> def f(x: String) = { | println("taking my time."); Thread.sleep(100) | x.reverse } f: (x: String)String
scala> val cache = collection.mutable.Map[String, String]() cache: scala.collection.mutable.Map[String,String] = Map()
scala> def cachedF(s: String) = cache.getOrElseUpdate(s, f(s)) cachedF: (s: String)String scala> cachedF("abc") taking my time. res16: String = cba scala> cachedF("abc") res17: String = cba
def cachedF(arg: String) = cache get arg match { case Some(result) => result case None => val result = f(arg) cache(arg) = result result }
scala> val str = 1 #:: 2 #:: 3 #:: Stream.empty str: scala.collection.immutable.Stream[Int] = Stream(1, ?)
scala> def fibFrom(a: Int, b: Int): Stream[Int] = | a #:: fibFrom(b, a + b) fibFrom: (a: Int, b: Int)Stream[Int]
scala> val fibs = fibFrom(1, 1).take(7) fibs: scala.collection.immutable.Stream[Int] = Stream(1, ?) scala> fibs.toList res23: List[Int] = List(1, 1, 2, 3, 5, 8, 13)
scala> val vec = scala.collection.immutable.Vector.empty vec: scala.collection.immutable.Vector[Nothing] = Vector() scala> val vec2 = vec :+ 1 :+ 2 vec2: scala.collection.immutable.Vector[Int] = Vector(1, 2) scala> val vec3 = 100 +: vec2 vec3: scala.collection.immutable.Vector[Int] = Vector(100, 1, 2) scala> vec3(0) res24: Int = 100
scala> val vec = Vector(1, 2, 3) vec: scala.collection.immutable.Vector[Int] = Vector(1, 2, 3) scala> vec updated (2, 4) res25: scala.collection.immutable.Vector[Int] = Vector(1, 2, 4) scala> vec res26: scala.collection.immutable.Vector[Int] = Vector(1, 2, 3)
scala> collection.immutable.IndexedSeq(1, 2, 3) res27: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3)
scala> val stack = scala.collection.immutable.Stack.empty stack: scala.collection.immutable.Stack[Nothing] = Stack() scala> val hasOne = stack.push(1) hasOne: scala.collection.immutable.Stack[Int] = Stack(1) scala> stack res28: scala.collection.immutable.Stack[Nothing] = Stack() scala> hasOne.top res29: Int = 1 scala> hasOne.pop res30: scala.collection.immutable.Stack[Int] = Stack()
scala> val empty = scala.collection.immutable.Queue[Int]() empty: scala.collection.immutable.Queue[Int] = Queue()
scala> val has1 = empty.enqueue(1) has1: scala.collection.immutable.Queue[Int] = Queue(1)
scala> val has123 = has1.enqueue(List(2, 3)) has123: scala.collection.immutable.Queue[Int] = Queue(1, 2, 3)
scala> val (element, has23) = has123.dequeue element: Int = 1 has23: scala.collection.immutable.Queue[Int] = Queue(2, 3)
scala> 1 to 3 res31: scala.collection.immutable.Range.Inclusive = Range(1, 2, 3) scala> 5 to 14 by 3 res32: scala.collection.immutable.Range = Range(5, 8, 11, 14)
scala> 1 until 3 res33: scala.collection.immutable.Range = Range(1, 2)
scala> val set = collection.immutable.TreeSet.empty[Int] set: scala.collection.immutable.TreeSet[Int] = TreeSet() scala> set + 1 + 3 + 3 res34: scala.collection.immutable.TreeSet[Int] = TreeSet(1, 3)
scala> val bits = scala.collection.immutable.BitSet.empty bits: scala.collection.immutable.BitSet = BitSet() scala> val moreBits = bits + 3 + 4 + 4 moreBits: scala.collection.immutable.BitSet = BitSet(3, 4) scala> moreBits(3) res35: Boolean = true scala> moreBits(0) res36: Boolean = false
scala> val map = collection.immutable.ListMap( | 1 -> "one", 2 -> "two") map: scala.collection.immutable.ListMap[Int,String] = Map(1 -> one, 2 -> two) scala> map(2) res37: String = "two"
scala> val buf = collection.mutable.ArrayBuffer.empty[Int] buf: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer() scala> buf += 1 res38: buf.type = ArrayBuffer(1) scala> buf += 10 res39: buf.type = ArrayBuffer(1, 10) scala> buf.toArray res40: Array[Int] = Array(1, 10)
scala> val buf = collection.mutable.ListBuffer.empty[Int] buf: scala.collection.mutable.ListBuffer[Int] = ListBuffer() scala> buf += 1 res41: buf.type = ListBuffer(1) scala> buf += 10 res42: buf.type = ListBuffer(1, 10) scala> buf.toList res43: List[Int] = List(1, 10)
scala> val buf = new StringBuilder buf: StringBuilder = scala> buf += 'a' res44: buf.type = a scala> buf ++= "bcdef" res45: buf.type = abcdef scala> buf.toString res46: String = abcdef
scala> val queue = new scala.collection.mutable.Queue[String] queue: scala.collection.mutable.Queue[String] = Queue() scala> queue += "a" res47: queue.type = Queue(a) scala> queue ++= List("b", "c") res48: queue.type = Queue(a, b, c) scala> queue res49: scala.collection.mutable.Queue[String] = Queue(a, b, c) scala> queue.dequeue res50: String = a scala> queue res51: scala.collection.mutable.Queue[String] = Queue(b, c)
scala> val stack = new scala.collection.mutable.Stack[Int] stack: scala.collection.mutable.Stack[Int] = Stack() scala> stack.push(1) res52: stack.type = Stack(1) scala> stack res53: scala.collection.mutable.Stack[Int] = Stack(1) scala> stack.push(2) res54: stack.type = Stack(2, 1) scala> stack res55: scala.collection.mutable.Stack[Int] = Stack(2, 1) scala> stack.top res56: Int = 2 scala> stack res57: scala.collection.mutable.Stack[Int] = Stack(2, 1) scala> stack.pop res58: Int = 2 scala> stack res59: scala.collection.mutable.Stack[Int] = Stack(1)
scala> val map = collection.mutable.HashMap.empty[Int,String] map: scala.collection.mutable.HashMap[Int,String] = Map() scala> map += (1 -> "make a web site") res60: map.type = Map(1 -> make a web site) scala> map += (3 -> "profit!") res61: map.type = Map(1 -> make a web site, 3 -> profit!) scala> map(1) res62: String = make a web site scala> map contains 2 res63: Boolean = false
scala> val bits = scala.collection.mutable.BitSet.empty bits: scala.collection.mutable.BitSet = BitSet() scala> bits += 1 res64: bits.type = BitSet(1) scala> bits += 3 res65: bits.type = BitSet(1, 3) scala> bits res66: scala.collection.mutable.BitSet = BitSet(1, 3)
scala> val a1 = Array(1, 2, 3) a1: Array[Int] = Array(1, 2, 3) scala> val a2 = a1 map (_ * 3) a2: Array[Int] = Array(3, 6, 9) scala> val a3 = a2 filter (_ % 2 != 0) a3: Array[Int] = Array(3, 9) scala> a3.reverse res1: Array[Int] = Array(9, 3)
scala> val seq: Seq[Int] = a1 seq: Seq[Int] = WrappedArray(1, 2, 3) scala> val a4: Array[Int] = seq.toArray a4: Array[Int] = Array(1, 2, 3) scala> a1 eq a4 res2: Boolean = true
scala> val seq: Seq[Int] = a1 seq: Seq[Int] = WrappedArray(1, 2, 3) scala> seq.reverse res2: Seq[Int] = WrappedArray(3, 2, 1) scala> val ops: collection.mutable.ArrayOps[Int] = a1 ops: scala.collection.mutable.ArrayOps[Int] = [I(1, 2, 3) scala> ops.reverse res3: Array[Int] = Array(3, 2, 1)
scala> a1.reverse res4: Array[Int] = Array(3, 2, 1)
scala> intArrayOps(a1).reverse res5: Array[Int] = Array(3, 2, 1)
// This is wrong! def evenElems[T](xs: Vector[T]): Array[T] = { val arr = new Array[T]((xs.length + 1) / 2) for (i <- 0 until xs.length by 2) arr(i / 2) = xs(i) arr }
error: cannot find class tag for element type T val arr = new Array[T]((arr.length + 1) / 2) ^
// This works import scala.reflect.ClassTag def evenElems[T: ClassTag](xs: Vector[T]): Array[T] = { val arr = new Array[T]((xs.length + 1) / 2) for (i <- 0 until xs.length by 2) arr(i / 2) = xs(i) arr }
scala> evenElems(Vector(1, 2, 3, 4, 5)) res6: Array[Int] = Array(1, 3, 5) scala> evenElems(Vector("this", "is", "a", "test", "run")) res7: Array[java.lang.String] = Array(this, a, run)
scala> def wrap[U](xs: Vector[U]) = evenElems(xs) <console>:9: error: No ClassTag available for U def wrap[U](xs: Vector[U]) = evenElems(xs) ^
scala> def wrap[U: ClassTag](xs: Vector[U]) = evenElems(xs) wrap: [U](xs: Vector[U])(implicit evidence$1: scala.reflect.ClassTag[U])Array[U]
scala> val str = "hello" str: java.lang.String = hello scala> str.reverse res6: String = olleh scala> str.map(_.toUpper) res7: String = HELLO scala> str drop 3 res8: String = lo scala> str slice (1, 4) res9: String = ell scala> val s: Seq[Char] = str s: Seq[Char] = WrappedString(h, e, l, l, o)
scala> import collection.mutable.{HashMap, ArrayBuffer} import collection.mutable.{HashMap, ArrayBuffer} scala> val buf = ArrayBuffer(1, 2, 3) buf: scala.collection.mutable.ArrayBuffer[Int] = ArrayBuffer(1, 2, 3) scala> val map = HashMap(buf -> 3) map: scala.collection.mutable.HashMap[scala.collection. mutable.ArrayBuffer[Int],Int] = Map((ArrayBuffer(1, 2, 3),3))
scala> map(buf) res13: Int = 3 scala> buf(0) += 1 scala> map(buf) java.util.NoSuchElementException: key not found: ArrayBuffer(2, 2, 3)
def lazyMap[T, U](coll: Iterable[T], f: T => U) = new Iterable[U] { def iterator = coll.iterator map f }
scala> val v = Vector(1 to 10: _*) v: scala.collection.immutable.Vector[Int] = Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) scala> v map (_ + 1) map (_ * 2) res5: scala.collection.immutable.Vector[Int] = Vector(4, 6, 8, 10, 12, 14, 16, 18, 20, 22)
scala> (v.view map (_ + 1) map (_ * 2)).force res12: Seq[Int] = Vector(4, 6, 8, 10, 12, 14, 16, 18, 20, 22)
scala> val vv = v.view vv: scala.collection.SeqView[Int,Vector[Int]] = SeqView(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
scala> vv map (_ + 1) res13: scala.collection.SeqView[Int,Seq[_]] = SeqViewM(...)
scala> res13 map (_ * 2) res14: scala.collection.SeqView[Int,Seq[_]] = SeqViewMM(...)
scala> res14.force res15: Seq[Int] = Vector(4, 6, 8, 10, 12, 14, 16, 18, 20, 22)
def isPalindrome(x: String) = x == x.reverse def findPalindrome(s: Seq[String]) = s find isPalindrome
findPalindrome(words take 1000000)
findPalindrome(words.view take 1000000)
scala> val arr = (0 to 9).toArray arr: Array[Int] = Array(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
scala> val subarr = arr.view.slice(3, 6) subarr: scala.collection.mutable.IndexedSeqView[ Int,Array[Int]] = IndexedSeqViewS(...)
scala> def negate(xs: collection.mutable.Seq[Int]) = | for (i <- 0 until xs.length) xs(i) = -xs(i) negate: (xs: scala.collection.mutable.Seq[Int])Unit
scala> negate(subarr) scala> arr res4: Array[Int] = Array(0, 1, 2, -3, -4, -5, 6, 7, 8, 9)
val actors = for (i <- 1 to 10) yield actor { ... }
val actors = (1 to 10) map (i => actor { ... })
val actors = for (i <- (1 to 10).view) yield actor { ... }
while (it.hasNext) println(it.next())
it foreach println
for (elem <- it) println(elem)
scala> val it = Iterator("a", "number", "of", "words") it: Iterator[java.lang.String] = non-empty iterator scala> it.map(_.length) res1: Iterator[Int] = non-empty iterator scala> res1 foreach println 1 6 2 5 scala> it.next() java.util.NoSuchElementException: next on empty iterator
scala> val it = Iterator("a", "number", "of", "words") it: Iterator[java.lang.String] = non-empty iterator scala> it dropWhile (_.length < 2) res4: Iterator[java.lang.String] = non-empty iterator scala> it.next() res5: java.lang.String = number
val (it1, it2) = it.duplicate
// This won't work def skipEmptyWordsNOT(it: Iterator[String]) = { while (it.next().isEmpty) {} }
def skipEmptyWords(it: BufferedIterator[String]) = while (it.head.isEmpty) { it.next() }
scala> val it = Iterator(1, 2, 3, 4) it: Iterator[Int] = non-empty iterator scala> val bit = it.buffered bit: java.lang.Object with scala.collection. BufferedIterator[Int] = non-empty iterator scala> bit.head res10: Int = 1 scala> bit.next() res11: Int = 1 scala> bit.next() res11: Int = 2
Traversable() // An empty traversable object List() // The empty list List(1.0, 2.0) // A list with elements 1.0, 2.0 Vector(1.0, 2.0) // A vector with elements 1.0, 2.0 Iterator(1, 2, 3) // An iterator returning three integers. Set(dog, cat, bird) // A set of three animals HashSet(dog, cat, bird) // A hash set of the same animals Map('a' -> 7, 'b' -> 0) // A map from characters to integers
List.apply(1.0, 2.0)
scala> List(1, 2, 3) res17: List[Int] = List(1, 2, 3) scala> Traversable(1, 2, 3) res18: Traversable[Int] = List(1, 2, 3) scala> mutable.Traversable(1, 2, 3) res19: scala.collection.mutable.Traversable[Int] = ArrayBuffer(1, 2, 3)
Iterator \Leftrightarrow java.util.Iterator Iterator \Leftrightarrow java.util.Enumeration Iterable \Leftrightarrow java.lang.Iterable Iterable \Leftrightarrow java.util.Collection mutable.Buffer \Leftrightarrow java.util.List mutable.Set \Leftrightarrow java.util.Set mutable.Map \Leftrightarrow java.util.Map
scala> import collection.JavaConversions._ import collection.JavaConversions._
scala> import collection.mutable._ import collection.mutable._ scala> val jul: java.util.List[Int] = ArrayBuffer(1, 2, 3) jul: java.util.List[Int] = [1, 2, 3] scala> val buf: Seq[Int] = jul buf: scala.collection.mutable.Seq[Int] = ArrayBuffer(1, 2, 3) scala> val m: java.util.Map[String, Int] = HashMap("abc" -> 1, "hello" -> 2) m: java.util.Map[String,Int] = {hello=2, abc=1}
Seq \Rightarrow java.util.List mutable.Seq \Rightarrow java.util.List Set \Rightarrow java.util.Set Map \Rightarrow java.util.Map
scala> val jul: java.util.List[Int] = List(1, 2, 3) jul: java.util.List[Int] = [1, 2, 3] scala> jul.add(7) java.lang.UnsupportedOperationException at java.util.AbstractList.add(AbstractList.java:131)
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