scala,dictionary,collections,initialization,immutability , Fill immutable map with for loop upon creation


Fill immutable map with for loop upon creation

Question:

Tag: scala,dictionary,collections,initialization,immutability

I have this map that looks like this:

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

and I thought about doing something like:

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

instead of a long copy/paste list like:

  (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), ...

But I get

Type mismatch, expected: (NotInferedA, NotInferedB), actual: Unit

Why is this and how can I overcome this?


Answer:

The problem is that your for comprehension doesn't return anything. Here are two different solutions to your problem. I personally would prefer the second one.

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

edit:

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
}

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