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Suppose I have the following class hierarchy:
trait A; class A1 extends A; class A2 extends A
Now I need to filter A1 instances in List[A]. I use either pattern matching or isInstanceOf.
as.filter(cond(_){case _: A1 => true}) // use pattern matching
as.filter(_.isInstanceOf[A1]) // use isInstanceOf
Does it work the same ? What would you prefer ?
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Scala's pattern matching statement is most useful for matching on algebraic types expressed via case classes. Scala also allows the definition of patterns independently of case classes, using unapply methods in extractor objects.
The match method takes a number of cases as an argument. Each alternative takes a pattern and one or more expressions that will be performed if the pattern matches.
case _ => does not check for the type, so it would match anything (similar to default in Java). case _ : ByteType matches only an instance of ByteType . It is the same like case x : ByteType , just without binding the casted matched object to a name x .
Using if expressions in case statements i match { case a if 0 to 9 contains a => println("0-9 range: " + a) case b if 10 to 19 contains b => println("10-19 range: " + a) case c if 20 to 29 contains c => println("20-29 range: " + a) case _ => println("Hmmm...") }
Why don't you use collect? That has the added benefit that the returned list will be of the right type (List[A1] instead of List[A])
val a1s = as.collect { case x:A1 => x }
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