OOP OOP - 18 days ago 7
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

List[List[Option[Double]]]
.

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:

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


But I want to return something like this:

List(
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

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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