Question

What would be the best way in Scala to do the following code in Java in proper functional way?

LinkedList<Item> itemsInRange = new LinkedList<Item>();
for (int y = 0; y < height(); y++) {
    for (int x = 0; x < width(); x++) {
        Item it = myMap.getItemAt(cy + y, cx + x);
        if (it != null)
            itemsInRange.add(it);
    }
}

// iterate over itemsInRange later

Over course, it can be translated directly into Scala in imperative way:

val itemsInRange = new ListBuffer[Item]
for (y <- 0 until height) {
    for (x <- 0 until width) {
        val it = tileMap.getItemAt(cy + x, cx + x)
        if (!it.isEmpty)
            itemsInRange.append(it.get)
    }
}

But I’d like to do it in proper, functional way.

I presume that there should be map operation over some sort of 2D range. Ideally, map would execute a function that would get x and y as input parameters and output Option[Item]. After that I’ll get something like Iterable[Option[Item]] and flatten over it will yield Iterable[Item]. If I’m right, the only missing piece of a puzzle is doing that map operation on 2D ranges in some way.

Answer 1

You can do this all in one nice step:

def items[A](w: Int, h: Int)(at: ((Int, Int)) => Option[A]): IndexedSeq[A] =
  for {
    x <- 0 until w
    y <- 0 until h
    i <- at(x, y)
  } yield i

Now say for example we have this representation of symbols on a four-by-four board:

val tileMap = Map(
  (0, 0) -> 'a,
  (1, 0) -> 'b,
  (3, 2) -> 'c
)

We just write:

scala> items(4, 4)(tileMap.get)
res0: IndexedSeq[Symbol] = Vector('a, 'b, 'c)

Which I think is what you want.