Scala Tutorial - Scala Pattern Matching

Pattern matching allows us to make a programmatic choice between multiple conditions.

Example

object Main extends App {
    def printNum(int: Int) {
     int match {
         case 0 => println("Zero")
         case 1 => println("One")
         case _ => println("more than one")
     }
    }
    printNum(0)
    printNum(1)
    printNum(2)
}

The last case with the underscore _ is wildcard. It matches anything not defined in the cases above it.

The following code illustrates the example of calculating Fibonacci numbers.

def fibonacci(in: Int): Int = in match {
 case 0 => 0
 case 1 => 1
 case n => fibonacci(n - 1) + fibonacci(n - 2)
}

Scala allows guards to be placed in patterns to test for particular conditions that cannot be tested in the pattern declaration itself.

Thus, we can write our Fibonacci calculator to return 0 if a negative number is passed in as in the following examples.

def fib2(in: Int): Int = in match {
 case n if n <= 0 => 0
 case 1 => 1
 case n => fib2(n - 1) + fib2(n - 2)
}

Matching Any Type

Let's consider a list of Any type of element, containing a String, a Double, an Int, and a Char.

object Main extends App {
    val anyList= List(1, "A", 2, 2.5, 'a')

    for (m <- anyList) {
        m match {
            case i: Int => println("Integer: " + i)
            case s: String => println("String: " + s)
            case f: Double => println("Double: " + f)
            case other => println("other: " + other)
        }
    }
}

Testing Data Types

The following method tests an incoming Object to see whether it's a String, an Integer, or something else.

def test2(in: Any) = in match {
    case s: String => "String, length "+s.length
    case i: Int if i > 0 => "Natural Int"
    case i: Int => "Another Int"
    case a: AnyRef => a.getClass.getName
    case _ => "null"
}