OOP OOP - 1 year ago 72
Scala Question

How to create List of List of Option[Double] In for loop Scala

I'm pretty sure this question might be duplicated, but I haven't come across the answer for this. Please pardon my ignorant knowledge of Scala. I'm pretty newbie.

My aim is to loop over two lists (with different length) and return


My code so far:

def getDoubles(symbol: String, year: Int): Option[Double] = {
return Some(140.0)

// this method loops over list of strings and range of time. returns list of list of Option[Double]
def checkList(CSV: List[String], time: Range): List[List[Option[Double]]] = {
// this variable is whats going to be returned at the end
var listOfLists = List[List[Option[Double]]]()
// So we are going to loop over our list and our time, and link each element in our list to all the elements in our time range
for {
i < -CSV
j < -time
// the method getDoubles is pretty long, so i just made another version simplfying my question.
// get doubles bascially returns Option of doubles
yield (listOfLists = getDoubles(j, i)::listOfLists)
return listOfLists

The above code, when I call it with more sophisticated data, it returns:


But I want to return something like this:

List(Some(313.062468), Some(27.847252)),
List(Some(301.873641), Some(42.884065)),
List(Some(332.373186), Some(53.509768))

How can I do this?

Answer Source

You don't need to use any mutable variables for this. First of all, you if you need a nested list, you need a nested for. Then in yield you should write how looks each element of the collection produced by this for. It's not a loop body, you're not supposed to do any mutations there. The whole for-expression is the resulting collection. Check Scala FAQ on "How does yield work?".

def checkList(csv: List[String], time: Range): List[List[Option[Double]]] = {
  for {
    symbol <- csv
  } yield {
    for {
      year <- time.toList
    } yield getDoubles(symbol, year)

For-comprehension is just a syntax sugar for a combination of map, flatMap andfilter. In this case writing it with map is more concise and very straightforward:

def checkList(csv: List[String], time: Range): List[List[Option[Double]]] = {
  csv map { symbol =>
    time map { year =>
      getDoubles(symbol, year)

See also "What is Scala's yield?" question.

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