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import scala.collection.mutable._
def indexes(a: Array[String], m: Map[String,Int]) = a.flatMap(m.get(_))
def 
 
indexes1(a: Array[String], m: Map[String,Int]) = a.map(m.get(_))
val am = Array("Tom", "Fred", "Harry")
val m1 = Map("Dick" -> 4)

呼叫這倆個方法：

val inamm=indexes(am,m1)
inamm.foreach(print(_))
println
val inamm1=indexes1(am,m1)
inamm1.foreach(print(_))

輸出結果：

3
Some(3)NoneNone

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很好奇為什麼能過濾掉None，請教了scala中國群裡的kerr、林晴、wxk、小星心等人，總結如下：

(1) m.get(_)的返回型別是Option[Int]，這個好理解；原始碼中Option.class中有這樣的隱式轉換：

implicit def option2Iterable[A](xo: Option[A]): Iterable[A] = xo.toList

toList的定義如下：

def toList: List[A] =
  if (isEmpty) List() else new ::(this.get, Nil)

def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[Repr, B, That]): That = {
  def builder = bf(repr) // extracted to keep method size under 35 bytes, so that it can be JIT-inlined
val b = builder
  for (x <- this) b ++= f(x).seq
  b.result
}

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trait CanBuildFrom[-From, -Elem, +To] {

  /** Creates a new builder on request of a collection.
   *  @param from  the collection requesting the builder to be created.
   *  @return a builder for collections of type `To` with element type `Elem`.
   *          The collections framework usually arranges things so
   *          that the created builder will build the same kind of collection
   *          as `from`.
   */
def apply(from: From): Builder[Elem, To]

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import generic._

/** The base trait of all builders.
 *  A builder lets one construct a collection incrementally, by adding
 *  elements to the builder with `+=` and then converting to the required
 *  collection type with `result`.
 *
 *  @tparam  Elem  the type of elements that get added to the builder.
 *  @tparam  To    the type of collection that it produced.
 *
 *  @since 2.8
 */
trait Builder[-Elem, +To] extends Growable[Elem] {

  /** Adds a single element to the builder.
   *  @param elem the element to be added.
   *  @return the builder itself.
   */

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