2d scala array iteration

Question

I have a 2d array of type boolean (not important) It is easy to iterate over the array in non-functional style. How to do it FP style?

var matrix = Array.ofDim[Boolean](5, 5) 

for ex, I would like to iterate through all the rows for a given column and return a list of int that would match a specific function. Example: for column 3, iterate through rows 1 to 5 to return 4, 5 if the cell at (4, 3), (5, 3) match a specif function. Thx v much

def getChildren(nodeId: Int) : List[Int] = {
    info("getChildren("+nodeId+")")

    var list = List[Int]()
    val nodeIndex = id2indexMap(nodeId)

    for (rowIndex <- 0 until matrix.size) {
      val elem = matrix(rowIndex)(nodeIndex)
      if (elem) {
        println("Row Index = " + rowIndex)
        list = rowIndex :: list
      }
    }

    list
  }

Answer 1

What about

(1 to 5) filter {i => predicate(matrix(i)(3))}

where predicate is your function?

Note that initialized with (5,5) indexes goes from 0 to 4.

Update: based on your example

def getChildren(nodeId: Int) : List[Int] = {
  info("getChildren("+nodeId+")")
  val nodeIndex = id2indexMap(nodeId)

  val result = (0  until matrix.size).filter(matrix(_)(nodeIndex)).toList
  result.forEach(println)
  result
}

You may move the print in the fiter if you want too, and reverse the list if you want it exactly as in your example

Answer 2

If you're not comfortable with filters and zips, you can stick with the for-comprehension but use it in a more functional way:

for {
  rowIndex <- matrix.indices
  if matrix(rowIndex)(nodeIndex)
} yield { 
  println("Row Index = " + rowIndex)
  rowIndex
}

yield builds a new collection from the results of the for-comprehension, so this expression evaluates to the collection you want to return. seq.indices is a method equivalent to 0 until seq.size. The curly braces allow you to span multiple lines without semicolons, but you can make it in-line if you want:

for (rowIndex <- matrix.indices; if matrix(rowIndex)(nodeIndex)) yield rowIndex

Should probably also mention that normally if you're iterating through an Array you won't need to refer to the indices at all. You'd do something like

for {
  row  <- matrix 
  elem <- row
} yield f(elem)

but your use-case is a bit unusual in that it requires the indices of the elements, which you shouldn't normally be concerned with (using array indices is essentially a quick and dirty hack to pair a data element with a number). If you want to capture and use the notion of position you might be better off using a Map[Int, Boolean] or a case class with such a field.