Converting Map[String, Seq[Int]] to Seq[Seq[Int]]

Question

I am trying to convert data that looks like this:

val inputData =
  Seq(("STUDY1", "Follow-up", 1),
    ("STUDY1", "Off Study", 2),
    ("STUDY1", "Screening", 3),
    ("STUDY1", "Treatment", 4),
    ("STUDY2", "Follow-up", 5),
    ("STUDY2", "Off Study", 6),
    ("STUDY2", "Screening", 7),
    ("STUDY2", "Treatment", 8));

into data that looks like this:

val desiredData =
  Seq(Seq(1,2,3,4),
    Seq(5,6,7,8));

The closest that I've gotten is with this:

val result: Map[String, Seq[Int]] =
  data.groupBy(i => i._1)
    .mapValues(j => j.map(k => k._3))
    .mapValues(_.toArray)

result.values.toSeq

This yields:

res0: Seq[Seq[Int]] = Stream(WrappedArray(1, 2, 3, 4), ?)

That last question mark is throwing me for a loop.

EDIT Future internet travelers who land here: my code actually did work ... my confusion stemmed from understanding what the ? was all about. Answers from folks down below helped me see that mapValues did lazy evaluation, and that the ? simply implies that.

Note that your desiredData is slightly misleading. I assume you wanted Seq(Seq(1,2,3,4), Seq(5,6,7,8)). (What you currently have is of type: Seq[(Int, Int, Int, Int)] and not Seq[Seq[Int]]) — gzm0
– gzm0, Commented Jan 26, 2016 at 3:31
Hold on, what do you mean by "throwing me for a loop"? Is this actually looping or do you just not understand what ? means in this context? I tried this and it doesn't loop (on 2.11.7). So ? essentially means that the tail of the stream is lazily evaluated and not evaluated here (this is to prevent infinite loops in case of infinite streams). — gzm0
– gzm0, Commented Jan 26, 2016 at 3:54
@gzm0 sorry for the confusion. my choice of words wasn't great. what I mean was that I didn't understand the ? mean in this context. — Alan C.
– Alan C., Commented Jan 26, 2016 at 4:04
Btw, the above code works. I am not sure what the question is here? — marios
– marios, Commented Jan 26, 2016 at 4:18

gzm0 · Accepted Answer · 2016-01-26 03:50:05Z

6

mapValues on Map is lazy (unlike any other method on a default Map). So that might be the issue there. Have you tried:

data.groupBy(_._1).map(_._2.map(_._3).toArray)

Note that the toArray is completely optional here.

edited Jan 26, 2016 at 3:50

answered Jan 26, 2016 at 3:29

gzm0

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Luong Ba Linh · Accepted Answer · 2016-01-26 03:48:37Z

3

val result: List[Seq[Int]] =
  data.groupBy(_._1).mapValues(_.map(_._3)).values.toList

edited Jan 26, 2016 at 3:48

answered Jan 26, 2016 at 3:47

Luong Ba Linh

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1 Comment

gzm0 Over a year ago

Can you explain why this one works and the OP's doesn't?

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