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package examples

/** Illustrate the use of pattern matching in Scala. */
object patterns {
  /** We need an abstract base class for trees. Subclasses with 
* the 'case' modifier can be used in pattern matching expressions
* to deconstruct trees.
abstract class Tree case class Branch(left: Tree, right: Tree) extends Tree case class Leaf(x: Int) extends Tree /** Case classes have an implicit constructor methods which allows
* to create objects withouth the 'new' keyword. It saves some typing
* and makes code clearer.
val tree1 = Branch(Branch(Leaf(1), Leaf(2)), Branch(Leaf(3), Leaf(4))) /** Return the sum of numbers found in leaves.
* 'match' is a generalization of 'switch' in C-like languages
* Patterns consist of case class constructors (which can
* be nested), and lower case variables which are
* bound to the values with which the class has been constructed.
def sumLeaves(t: Tree): Int = t match { case Branch(l, r) => sumLeaves(l) + sumLeaves(r) case Leaf(x) => x } /** This illustrates the use of Option types. Since the
* method is not known in advance to find 'x', the
* return type is an Option. Options have two possible
* values, either 'Some' or 'None'. It is a type-safe
* way around 'null' values.
def find[A, B](it: Iterator[(A, B)], x: A): Option[B] = { var result: Option[B] = None while (it.hasNext && result == None) { val Pair(x1, y) = if (x == x1) result = Some(y) } result } def printFinds[A](xs: List[(A, String)], x: A) = find(xs.elements, x) match { case Some(y) => println(y) case None => println("no match") } def main(args: Array[String]) { println("sum of leafs=" + sumLeaves(tree1)) printFinds(List((3, "three"), (4, "four")), 4) } }

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