Scala 计算某个object在一个序列中出现的数量

Question

I am new to Scala and i am trying to create a function that calculates the amount a certain object appears in a sequence. So in this case we have a sequence with 1 Hippo and 3 Tigers in it. I want the amount of Tigers in the sequence. So the outcome of the function amountOfTigers should be an integer: 3. I want to make use of pattern matching and recursion to solve this. But i dont really know how to do this.

sealed trait Animal

case class Hippo(name: String, age: Int) extends Animal
case class Tiger(name: String, age: Int) extends Animal

def amountOfTigers(animals: Seq[Animal]): Int = animals match {
    case head +: tail => if (head.isInstanceOf[Tiger]) println(head); amountOfTigers(tail)
  }

val data = Seq[Animal](
    Hippo("Mino", 4),
    Tiger("Justin", 1),
    Tiger("Jason", 20),
    Tiger("Sloop", 10)
  )
  amountOfTigers(data)

println is used for testing purposes. The output i am getting right now is: Tiger(Justin,1) Tiger(Jason,20) Tiger(Sloop,10)

I want as a result the amount of Tigers in a sequence. So in this case its 3.

Answer 1

If you want to avoid recursion (which I personally suggest in this case - cause it's just an ordinary array of data), I suggest you use a loop, this way:

  def amountOfTigers(animals: Seq[Animal]): Int = animals.count(_.isInstanceOf[Tiger])

If you insist using recursion, I suggest using @tailrec as Christian hinted:

def amountOfTigersRec(animals: Seq[Animal]): Int = {
    @tailrec
    def rec_fun(tail: Seq[Animal], count: Int): Int = tail match {
      case Tiger(_, _) +: tail => rec_fun(tail, count + 1)
      case _ +: tail => rec_fun(tail, count)
      case Nil => count
    }

    rec_fun(animals, 0)
  }

Answer 2

This is an example:

sealed trait Animal

case class Hippo(name: String, age: Int) extends Animal
case class Tiger(name: String, age: Int) extends Animal

def amountOfTigers(animals: Seq[Animal]): Int = animals match {
  case Seq() => 0
  case Tiger(_, _) +: tail => amountOfTigers(tail) + 1
  case head +: tail => amountOfTigers(tail)
}

val data = Seq[Animal](
    Hippo("Mino", 4),
    Tiger("Justin", 1),
    Tiger("Jason", 20),
    Tiger("Sloop", 10)
  )
  
print(amountOfTigers(data))

Things you might want to check out:

• How to pattern match in Scala: https://docs.scala-lang.org/tour/pattern-matching.html as you rarely use isInstanceOf in Scala
• For recursive functions, usually you start with thinking about the trivial case. Here: An empty seq has zero tigers.
• Note that this example isn't very efficient, as it isn't tail recursive. Tail recursion is optimized into a loop by the Scala compiler. See eg https://www.geeksforgeeks.org/tail-recursion-in-scala/ for more details about this.