pattern matching. the match statement c and java have a switch statement which uses a small integer...
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Pattern Matching
The match statement
C and Java have a switch statement which uses a small integer value to choose among alternatives
In Java 7, it is now possible to use switch with Strings switch is rarely used in Java programs
The Scala “equivalent” is the match expression scala> val n = 4n: Int = 4
scala> n match { | case 1 => "one" | case 2 => "two" | case _ => "many" | }res0: String = many
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Matching on values scala> object Obj
defined module Obj scala> val d = 5.0
d: Double = 5.0 scala> val lst = List(1, 2, 3)
lst: List[Int] = List(1, 2, 3) scala> def valMatch(x: Any) = x match {
| case 5 => println("Int") | case 5.0 => println("Double") | case "abc" => println("String") | case List(1, 2, 3) => println("List") | case Obj => println("User-defined Obj") | case _ => println("None of the above") | }valMatch: (x: Any)Unit
scala> valMatch(5)Int
scala> valMatch(Obj)User-defined Obj
scala> valMatch("abc")String
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Matching on types scala> def whatKind(x: Any) = x match {
| case s: String => s"This is the string [$s]" | case d: Double => s"$d is a Double" | case i: Int => s"$i is an Int" | case x => s"I don't know what $x is" | }whatKind: (x: Any)String
scala> whatKind(5.0)res3: String = 5.0 is a Double
scala> whatKind(List(1, 2, 3))res4: String = I don't know what List(1, 2, 3) is
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Matching with guards scala> def oddOrEven(n: Any) = n match {
| case n: Int if n % 2 == 0 => "Even integer" | case n: Int => "Odd integer" | case x => x + " is something else" | }oddOrEven: (n: Any)String
scala> oddOrEven(5)res18: String = Odd integer
scala> oddOrEven(6)res19: String = Even integer
scala> oddOrEven(7.0)res20: String = 7.0 is something else
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Matching on user-defined types For user-defined classes, you can match on the type
scala> class Person(val name: String)defined class Person
scala> val dave = new Person("Dave")dave: Person = Person@234bb715
scala> dave match { | case n: Person => "ok" | case _ => "nope!" | }res2: String = ok
But you can’t match on the particular object of a plain (non-case) class scala> dave match {
| case Person("Dave") => "ok" | case _ => "nope!" | }<console>:14: error: not found: value Person
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Matching on objects of case classes scala> val dave = new Person("Dave")
dave: Person = Person(Dave)
scala> val beth = new Person("Beth")beth: Person = Person(Beth)
scala> def sayHi(p: Person) = p match { | case Person("Dave") => "Oh, it's just Dave." | case Person(name) => s"Hi, $name!" | }sayHi: (p: Person)String
scala> sayHi(dave)res7: String = Oh, it's just Dave.
scala> sayHi(beth)res8: String = Hi, Beth!
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Matching on exceptions
Scala’s try-catch-finally is similar to Java’s, but the catch clauses are case expressions
try { val f = new FileReader("input.txt")} catch { case ex: FileNotFoundException => { println("Missing file exception") } case ex: IOException => { println("IO Exception") }} finally { println("Exiting finally...")}
Source: http://www.tutorialspoint.com/scala/scala_exception_handling.htm 8
Matching on optional values
The Option type is used frequently in Scala Scala’s None is used in places where Java might use null
scala> val scores = List(78, 43, 82, 67, 55)scores: List[Int] = List(78, 43, 82, 67, 55)
scala> scores find (_ > 90)res0: Option[Int] = None
scala> scores find (_ < 70)res1: Option[Int] = Some(43)
scala> (scores find (_ < 30)) match { | case None => "Everyone is passing" | case Some(n) => n + " isn't very good" | }res2: String = Everyone is passing
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Matching in assignments scala> val jean = new Person("Jean", 23)
jean: Person = Person(Jean,23)
scala> jean match { | case Person(n, a) => s"$n is $a years old" | }res5: String = Jean is 23 years old
scala> val Person(n, a) = jeann: String = Jeana: Int = 23
scala> val list = List(1, 2, 3, 4, 5)list: List[Int] = List(1, 2, 3, 4, 5)
scala> val h :: hh :: t = listh: Int = 1hh: Int = 2t: List[Int] = List(3, 4, 5)
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Matching in for expressions scala> val capitals = Map("France" -> "Paris", "Japan" ->
"Tokyo")capitals: scala.collection.immutable.Map[String,String] = Map(France -> Paris, Japan -> Tokyo)
scala> for ((country, city) <- capitals) { | println(s"The capital of $country is $city.") | }The capital of France is Paris.The capital of Japan is Tokyo.
Values that don’t match are simply filtered out scala> val scores = List(("John", 90), ("Mary", 100),
("Bill", 95), ("Jane", 100))scores: List[(String, Int)] = List((John,90), (Mary,100), (Bill,95), (Jane,100))
scala> for ((name, 100) <- scores) println(name)MaryJane
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Patterns as partial functions
A sequence of cases is a partial function, and may be used anywhere a function literal may be used
scala> val factorial: Int => Int = { | case 1 => 1 | case n => n * factorial(n - 1) | }factorial: Int => Int = <function1>=
scala> factorial(5)res14: Int = 120
scala> (1 to 10) map { | case n if n % 2 == 0 => n / 2 | case n => 3 * n + 1 | }res16: scala.collection.immutable.IndexedSeq[Int] = Vector(4, 1, 10, 2, 16, 3, 22, 4, 28, 5)
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Failing to match
It’s an error if no case in a pattern match is satisfied There are two basic choices for the last case:
case _ will match anything, and you don’t care what case variable will match anything, and you can use the value
of the variable in the body of the case
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isDefinedAt A Map is a partial function from keys to values The method isDefinedAt determines whether a partial
function is defined for a particular input value scala> val scores = List(("John", 90), ("Mary", 100),
("Bill", 95), ("Jane", 100))scores: List[(String, Int)] = List((John,90), (Mary,100), (Bill,95), (Jane,100))
scala> val mapScores = scores.toMapmapScores: scala.collection.immutable.Map[String,Int] = Map(John -> 90, Mary -> 100, Bill -> 95, Jane -> 100)
scala> mapScores.isDefinedAt("William")res1: Boolean = false
scala> mapScores.isDefinedAt("Bill")res2: Boolean = true 14
collect collect is a partial function that takes an iterator and returns a new iterator
over only defined values
def collect[B](pf: PartialFunction[A, B]): Iterator[B]
scala> val scores = Map(("John", 90), ("Mary", 100), ("Bill", 95), ("Jane", 100))scores: scala.collection.immutable.Map[String,Int] = Map(John -> 90, Mary -> 100, Bill -> 95, Jane -> 100)
scala> val names = List("John", "Frank", "Jane", "Joe")names: List[String] = List(John, Frank, Jane, Joe)
scala> names collect scoresres10: List[Int] = List(90, 100)
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Sealed classes
A class is sealed if (1) it is declared with the keyword sealed, and (2) all subclasses are declared on the same file
This allows Scala to check for missing cases scala> sealed class Person // omitting responses to save space
scala> class Man extends Personscala> class Woman extends Personscala> class Child extends Person
scala> val p: Person = new Womanp: Person = Woman@12efc0ee
scala> p match { | case m: Man => "male" | case f: Woman => "female" | }<console>:12: warning: match may not be exhaustive.It would fail on the following inputs: Child(), Person() p match { ^res5: String = female
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@ and _*
In pattern matching, val @ pattern_part captures the value of the pattern part in the value
In matching a sequence, _* matches the remainder of the sequence
scala> list match { | case a @ List(b @ _, c @ _*) => s"After $b in $a comes $c" | }res7: String = After 1 in List(1, 2, 3, 4, 5) comes List(2, 3, 4, 5)
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The End
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