Optionally adding items to a Scala Map

问题

I am looking for an idiomatic solution to this problem.

I am building a val Scala (immutable) Map and would like to optionally add one or more items:

val aMap =
  Map(key1 -> value1,
      key2 -> value2,
      (if (condition) (key3 -> value3) else ???))

How can this be done without using a var? What should replace the ???? Is it better to use the + operator?

val aMap =
  Map(key1 -> value1,
      key2 -> value2) +
  (if (condition) (key3 -> value3) else ???))

One possible solution is:

val aMap =
  Map(key1 -> value1,
      key2 -> value2,
      (if (condition) (key3 -> value3) else (null, null))).filter {
        case (k, v) => k != null && v != null
      }

Is this the best way?

回答1:

How about something along the lines of

val optional = if(condition) Seq((key3 -> value3)) else Nil
val entities = Seq(key1 -> value1, key2 -> value2) ++ optional
val aMap = Map(entities:_*)

回答2:

Another possibility is to take advantage of the iterable nature of Option.

A non-empty value o:

scala> val o = Some('z' -> 3)
scala> (Seq('x' -> 1, 'y' -> 2) ++ o).toMap
res1: scala.collection.immutable.Map[Char,Int] = Map(x -> 1, y -> 2, z -> 3)

An empty value o:

scala> val o = None
scala> (Seq('x' -> 1, 'y' -> 2) ++ o).toMap
res2: scala.collection.immutable.Map[Char,Int] = Map(x -> 1, y -> 2)

回答3:

You can add directly to the map:

scala> val map = Map(1 -> 2, 3 -> 4)
scala> val some = Some(5 -> 6)
scala> val none = None
scala> val combinedMap = map ++ some ++ none
combinedMap: scala.collection.immutable.Map[Int,Int] = Map(1 -> 2, 3 -> 4, 5 -> 6)

来源：https://stackoverflow.com/questions/6048853/optionally-adding-items-to-a-scala-map