在创建时用for循环填充不可变映射

Question

我的地图是这样的:

val fields: Map[(Int, Int), Field]

，我想这样做:

val fields: Map[(Int, Int), Field] = 
Map(
   for(a <- 0 to 10)
   {
     (0, a) -> new Field(this, 0, a)
   }
)

而不是长长的复制/粘贴列表，如:

  (0, 0) -> new Field(this, 0, 0),
  (1, 0) -> new Field(this, 1, 0),
  (2, 0) -> new Field(this, 2, 0),
  (3, 0) -> new Field(this, 3, 0),
  (4, 0) -> new Field(this, 4, 0),
  (5, 0) -> new Field(this, 5, 0),
  (6, 0) -> new Field(this, 6, 0),
  (7, 0) -> new Field(this, 7, 0),
  (8, 0) -> new Field(this, 8, 0),
  (0, 1) -> new Field(this, 0, 1), ...

但是我得到

类型不匹配，期望:(NotInferedA, NotInferedB)，实际:Unit

为什么会这样，我该如何克服?

Answer 1

问题是for理解不返回任何东西。这里有两种不同的解决方法。我个人更喜欢第二个。

case class Field(map: Map[(Int, Int), Field], a: Int, b: Int)
val fields: Map[(Int, Int), Field] = 
Map(
   (for(a <- 0 to 10) yield (0, a) -> new Field(fields, 0, a)): _*
)
val fields: Map[(Int, Int), Field] =
  (0 to 10).map(a => (0, a) -> new Field(fields, 0, a)).toMap

编辑:

case class Field(board: Board, x: Int, y: Int)
class Board {
  val fields: Map[(Int, Int), Field] =
    (0 to 10).map(a => (0, a) -> new Field(this, 0, a)).toMap
}

class Board {
  val fields: Map[(Int, Int), Field] =
    (for(a <- 0 to 10; b <- 0 to 10)
        yield (a, b) -> new Field(this, a, b)).toMap
}