How to filter Scala Map List values

Question

I'm learning Scala and attempting to filter Map[String, List[Double]] such that within each sliding window of size 2 if the values are less than .1 then the key values map is returned.

For this data:

 val data  : Map[String, List[Double]] = Map(
      "a" -> List(0.086, 0.086, 0.398, -0.312, 0.312, 0.312, 0.312, 0.312, 0.312, 0.312),
      "b" -> List(-0.119, -0.119, 1.007, 1.201, 1.201, 1.201, -1.201, 1.201, 1.201, 1.201))

The Maps: "

"a" -> List(0.086, 0.086)
"b" -> List(-0.119, -0.119)

should be returned as both elements of the List are < .1

I create a sliding window of size 2 :

val sliding = data.map(l => l._2.sliding(2).toList)

sliding contains:

List(List(List(0.086, 0.086), List(0.086, 0.398), List(0.398, -0.312), List(-0.312, 0.312), List(0.312, 0.312), List(0.312, 0.312), List(0.312, 0.312), List(0.312, 0.312), List(0.312, 0.312)), List(List(-0.119, -0.119), List(-0.119, 1.007), List(1.007, 1.201), List(1.201, 1.201), List(1.201, 1.201), List(1.201, -1.201), List(-1.201, 1.201), List(1.201, 1.201), List(1.201, 1.201)))

But using sliding in this way means I've dropped the key's and I'm unsure how to filter this new data structure.

How do filter the values from data? I think I'm over complicating the solution with my use of sliding and that I could using pattern matching instead ?

Update:

Using below code satisfies the condition and produces expected output:

val data  : Map[String, List[Double]] = Map(
  "a" -> List(0.086, -0.398, -0.398, -0.312, 0.312, 0.312, 0.312, 0.312, 0.312, 0.312),
  "b" -> List(-0.119, -0.119, 1.007, 1.201, 1.201, 1.201, -1.201, 1.201, -1.201, -1.201)
)
val result = data.map {
  case (key, value) => key -> value.sliding(2).filter(_.forall(_ < .1)).toList
}.filter {
  case (_, values) => values.nonEmpty
}
println("result "+result)

Which prints:

Map(a -> List(List(0.086, -0.398), List(-0.398, -0.398), List(-0.398, -0.312)), b -> List(List(-0.119, -0.119), List(-1.201, -1.201)))

Ivan Kurchenko · Accepted Answer · 2021-03-01 10:33:32Z

2

If I understood task correctly, then you can go with something like:

val data  : Map[String, List[Double]] = Map(
      "a" -> List(0.086, 0.086, 0.398, -0.312, 0.312, 0.312, 0.312, 0.312, 0.312, 0.312),
      "b" -> List(-0.119, -0.119, 1.007, 1.201, 1.201, 1.201, -1.201, 1.201, 1.201, 1.201),
      "c" -> List(1.201, 1.3)
)

val result = data.map {
  case (key, value) => key -> value.sliding(2).filter(_.forall(_ < 0.1)).toList.flatten
}.filter {
  case (_, values) => values.nonEmpty
}

println(result.mkString("\n"))

Which will print out:

a -> List(0.086, 0.086)
b -> List(-0.119, -0.119)

Scatie: https://scastie.scala-lang.org/nW4ez389T7ysXfVYEVQJUg

edited Mar 1, 2021 at 10:33

answered Mar 1, 2021 at 10:00

Ivan Kurchenko

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6 Comments

blue-sky Over a year ago

thanks, b -> List(-0.119, -0.119) & a -> List(0.086, 0.086) should be returned as both these values are < .1, your filtering values < 1 not .1 , I've updated question to include the "a" map

Ivan Kurchenko Over a year ago

@blue-sky Yes, excuse me - I missed that do. Answer updated.

blue-sky Over a year ago

if I change 0.398 to -0.398 (the third element in "a" map) then Map(a -> List(0.086, 0.086, 0.086, -0.398, -0.398, -0.312), b -> List(-0.119, -0.119)) is returned, instead a -> List(0.086, 0.086) b -> List(List(-0.119, -0.119), List(-0.398, -0.312)) should be returned, I should have clarified this but I did not realise until I tested your code..

blue-sky Over a year ago

I just needed to remove flatten it seems, updated Scastie: scastie.scala-lang.org/EFzULvhCRi2lFNyURhwgMg

Ivan Kurchenko Over a year ago

@blue-sky Sorry, I'm afraid I'm not following you: (0.086, -0.398) - both numbers satisfies condition to be < 0.1 why they should not be present?

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