Scala - Check If object exist inside a list - scala

Question

I'm trying to write an algorithm to check if an object exists inside a list.
My case class is the following:
case class Person(id:Int, name:String, friends:List[Person] = Nil)
I've wrote it using this code:
#tailrec
final def find(id: Int, tree: List[Person]):Option[Person] = tree match {
case Nil => None
case (h :: t) => if(h.id == id) Option(h) else find(key, t ::: h.friends)
}
Is it a good approach? Use tail recursive and append another list on tail list? If not, what would be the best approach?

Answer 1

I ended up with the following implementation which basically tail recursive BFS:
#tailrec
def find(id: Int, level: List[Person], visited: Set[Person] = Set.empty): Option[Person] =
if (level.isEmpty) None
else {
// Try to find person on this level
val found = level.find(_.id == id).filterNot(visited.contains)
if (found.isDefined) found
else find(
id,
// Next level construction
level.flatMap(_.friends).distinct,
// Keep track of visited to handle cycles
visited ++ level
)
}
UPDATE: Also, I would suggest to add call by name to be able to play around with more test cases:
class Person(val id: Int, val name: String, friends: => List[Person]) {
def friendList = friends
override def toString = name
}
object Person {
def apply(id: Int, name: String, friends: => List[Person] = Nil) = new Person(id, name, friends)
}
In that case you can compose the example from the comments:
val john = Person(1, "john")
val russ = Person(2, "russ")
val bob = Person(3, "bob")
val lois = Person(5, "lois")
lazy val `eve friends` = List(john, anne)
lazy val `peter friends` = List(eve, bob, russ)
lazy val `anne friends` = List(peter, lois)
val eve = Person(7, "eve", `eve friends`)
val peter = Person(8, "peter", `peter friends`)
val anne: Person = Person(6, "anne", `anne friends`)
println(find(8, List(eve, peter, lois))) // result: Some(peter)