How to create listBuffer in collect function

问题

I tought that List is enough but I need to add element to my list.

I've tried to put this in ListBuffer constructor but without result.

  var leavesValues: ListBuffer[Double] =
    leaves
      .collect { case leaf: Leaf => leaf.value.toDouble }
      .toList

Later on I'm going to add value to my list so my expected output is mutable list.

Solution of Raman Mishra

But what if I need to append single value to the end of leavesValues

1. I can reverse but it's not good enough

2. I can use ListBuffer like below but I believe that there is cleaner solution:

   val leavesValues: ListBuffer[Double] = ListBuffer()
   leavesValues.appendAll(leaves
     .collect { case leaf: Leaf => leaf.value.toDouble }
     .toList)
   

回答1:

  case class Leaf(value:String)

  val leaves = List(Leaf("5"), Leaf("6"), Leaf("7"), Leaf("8") ,Leaf("9") )

  val leavesValues: List[Double] =
    leaves
      .collect { case leaf: Leaf => leaf.value.toDouble }

  val value = Leaf("10").value.toDouble

  val answer = value :: leavesValues

  println(answer)

you can do it like this after getting the list of leavesValues you can prepand the value you want to add into the list.

来源：https://stackoverflow.com/questions/54034675/how-to-create-listbuffer-in-collect-function