import scala.collection.mutable.Map
var overlapByY: Map[Int, Int] = (0 to records.length).map(a => (a, 0)).toMap
Gives the error
polymorphic expression cannot be instantiated to expected type;
[error] found : Map (in scala.collection.immutable)
[error] required: Map[Int,Int] (in scala.collection.mutable)
[error] var overlapByY: Map[Int, Int] = (0 to 8).map(a => (a, 0)).toMap
[error] ^
toMap explicitly results in an immutable Map (even though the implementation probably uses a mutable Map internally...)
There are a few ways to go about this. I would tend to use foldLeft:
import scala.collection.mutable
(0 to records.length) // note that to is inclusive, 0 until records.length might actually be what you want...
.foldLeft(mutable.Map.empty[Int, Int]) { (map, elem) =>
map += elem -> 0
}
This would be equivalent to the even more imperative:
{
val map = mutable.Map.empty[Int, Int]
(0 to records.length).foreach { elem => map += elem -> 0 }
map
}
toMap returns an immutable.Map so you can't use that if you want a mutable.Map. Use the to method with the mutable.Map object as parameter:
val overlapByY: mutable.Map[Int, Int] =
(0 to records.length).map(a => (a, 0)) to mutable.Map
Since it's mutable, you can make overlapByY a val, and still add elements to it:
overlapByY += ( 1 -> 2)
For older versions of Scala, the old way of doing this was by passing the result of the map as a repeated-parameter using _* to the apply method of the mutable.Map:
val overlapByY2 =
mutable.Map((0 to records.length).map(a => (a, 0)): _*)
mutable package: import scala.collection.mutable for this, not the whole mutable.Map.
var.