Scala Language Pattern Matching Pattern Matching on a Seq
Example
To check for a precise number of elements in the collection
def f(ints: Seq[Int]): String = ints match {
case Seq() =>
"The Seq is empty !"
case Seq(first) =>
s"The seq has exactly one element : $first"
case Seq(first, second) =>
s"The seq has exactly two elements : $first, $second"
case s @ Seq(_, _, _) =>
s"s is a Seq of length three and looks like ${s}" // Note individual elements are not bound to their own names.
case s: Seq[Int] if s.length == 4 =>
s"s is a Seq of Ints of exactly length 4" // Again, individual elements are not bound to their own names.
case _ =>
"No match was found!"
}
To extract the first(s) element(s) and keeping the rest as a collection:
def f(ints: Seq[Int]): String = ints match {
case Seq(first, second, tail @ _*) =>
s"The seq has at least two elements : $first, $second. The rest of the Seq is $tail"
case Seq(first, tail @ _*) =>
s"The seq has at least one element : $first. The rest of the Seq is $tail"
// alternative syntax
// here of course this one will never match since it checks
// for the same thing as the one above
case first +: tail =>
s"The seq has at least one element : $first. The rest of the Seq is $tail"
case _ =>
"The seq didn't match any of the above, so it must be empty"
}
In general, any form that can be used to construct a sequence can be used to pattern match against an existing sequence.
Note that while using Nil and :: will work when pattern matching a Sequence, it does convert it to a List, and can have unexpected results. Constrain yourself to Seq( ...) and +: to avoid this.
Note that while using :: will not work for WrappedArray, Vector etc, see:
scala> def f(ints:Seq[Int]) = ints match {
| case h :: t => h
| case _ => "No match"
| }
f: (ints: Seq[Int])Any
scala> f(Array(1,2))
res0: Any = No match
And with +:
scala> def g(ints:Seq[Int]) = ints match {
| case h+:t => h
| case _ => "No match"
| }
g: (ints: Seq[Int])Any
scala> g(Array(1,2).toSeq)
res4: Any = 1
