How do I populate a list of objects with new values

Question

Apologies: I'm well noob

I have an items class

class item(ind:Int,freq:Int,gap:Int){}

I have an ordered list of ints

val listVar = a.toList

where a is an array

I want a list of items called metrics where

ind is the (unique) integer freq is the number of times that ind appears in list gap is the minimum gap between ind and the number in the list before it so far I have:

def metrics = for {
  n <- 0 until 255 
  listVar filter (x == n) count > 0 
}
  yield  new item(n, (listVar filter == n).count,0)

It's crap and I know it - any clues?

I'm not sure why your question got down voted.. – huynhjl Dec 01 '11 at 15:40 — huynhjl, Dec 01 '11 at 15:40

Daniel C. Sobral · Accepted Answer · 2011-12-01T20:29:54.790

4

Well, some of it is easy:

val freqMap = listVar groupBy identity mapValues (_.size)

This gives you ind and freq. To get gap I'd use a fold:

val gapMap = listVar.sliding(2).foldLeft(Map[Int, Int]()) { 
  case (map, List(prev, ind)) =>
    map + (ind -> (map.getOrElse(ind, Int.MaxValue) min ind - prev))
}

Now you just need to unify them:

freqMap.keys.map( k => new item(k, freqMap(k), gapMap.getOrElse(k, 0)) )

edited Dec 01 '11 at 20:29

answered Dec 01 '11 at 15:31

Daniel C. Sobral

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`freqMap.keys.map( k => new item(k, freqMap(k), gapMap.getOrElse(k, 0)) )` – Paddy Carroll Dec 01 '11 at 19:44

score 2 · Answer 2 · answered Dec 01 '11 at 15:32

Ideally you want to traverse the list only once and in the course for each different Int, you want to increment a counter (the frequency) as well as keep track of the minimum gap.

You can use a case class to store the frequency and the minimum gap, the value stored will be immutable. Note that minGap may not be defined.

case class Metric(frequency: Int, minGap: Option[Int])

In the general case you can use a Map[Int, Metric] to lookup the Metric immutable object. Looking for the minimum gap is the harder part. To look for gap, you can use the sliding(2) method. It will traverse the list with a sliding window of size two allowing to compare each Int to its previous value so that you can compute the gap.

Finally you need to accumulate and update the information as you traverse the list. This can be done by folding each element of the list into your temporary result until you traverse the whole list and get the complete result.

Putting things together:

listVar.sliding(2).foldLeft(
  Map[Int, Metric]().withDefaultValue(Metric(0, None))
) {
  case (map, List(a, b)) =>
    val metric = map(b)
    val newGap = metric.minGap match {
      case None => math.abs(b - a)
      case Some(gap) => math.min(gap, math.abs(b - a))
    }
    val newMetric = Metric(metric.frequency + 1, Some(newGap))
    map + (b -> newMetric)
  case (map, List(a)) => 
    map + (a -> Metric(1, None))
  case (map, _) => 
    map
}

Result for listVar: List[Int] = List(2, 2, 4, 4, 0, 2, 2, 2, 4, 4)

scala.collection.immutable.Map[Int,Metric] = Map(2 -> Metric(4,Some(0)), 
4 -> Metric(4,Some(0)), 0 -> Metric(1,Some(4)))

You can then turn the result into your desired item class using map.toSeq.map((i, m) => new Item(i, m.frequency, m.minGap.getOrElse(-1))).

You can also create directly your Item object in the process, but I thought the code would be harder to read.

How do I populate a list of objects with new values

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