A hybrid of map and fold?

Question

Is there a function in Scala, which is a hybrid of map and fold? I'd like not to use a temporary var variable. I want to return a list and at the same time be able to have an accumulator.

update:

I'd like to do this

  def findMaxSubarray(source: Seq[Int]) = {
    var lastSum = 0
    val maxes = source map { x =>
      lastSum += x
      if (lastSum < 0) lastSum = 0
      lastSum
    }

    maxes.max
  }

without using scalaz and var, only using standard scala library.

Related question: http://stackoverflow.com/questions/12261157/what-is-proper-monad-or-sequence-comprehension-to-both-map-and-carry-state-acros — Ionuț G. Stan, Mar 04 '14 at 07:48

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

3

For updated question

You could use scanLeft (or scan) like this:

val seq = Stream.continually(util.Random.nextInt(200) - 100).take(100)
val maxes = seq.scan(0){ (lastSum, i) => (lastSum + i) max 0 }
maxes.max
// Int = 547

It produces collection of all subresults of foldLeft

For initial question

There is such method in scalaz: foldMap.

(0 to 2).toList.foldMap{_ + 1}
// Int = 6

It works for monoids, zero is used as initial value:

(0 to 2).map{_ + 1}.fold(Monoid[Int].zero){_ |+| _}

There is no such method in standard scala library, but you could use view before map to avoid intermediate collection creation:

(0 to 2).view.map{_ + 1}.fold(0){_ + _}

edited Jun 20 '20 at 09:12

Community

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answered Mar 04 '14 at 07:23

senia

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what about the standard scala library? – Incerteza Mar 04 '14 at 07:26
but `fold(0){_ + _}` will return a single value, whereas I need a list. – Incerteza Mar 04 '14 at 07:33
@Alex A fold takes some collection and returns a single value. What do you expect from your fold? Sounds like you're just describing a map. – Jean-Philippe Pellet Mar 04 '14 at 07:36
@Alex: please, add initial data and desired result to your question. – senia Mar 04 '14 at 07:36
1

I think he needs a stateful map, but doesn't want to use a var outside the mapping function. – Ionuț G. Stan Mar 04 '14 at 07:41
@Jean-PhilippePellet: I guess I got it now: `map{i => val r = i + 1, tmp += r; r}`. Without reiterating over collection. – senia Mar 04 '14 at 07:41
@Alex: In general you should use `map` and then `fold` unless you are sure that you have a bottleneck (and you have proved this with benchmarks). – senia Mar 04 '14 at 07:44
@Jean-PhilippePellet: it's a typo. `tmp` should be `var`. – senia Mar 04 '14 at 07:45

A hybrid of map and fold?

1 Answers1

For updated question

For initial question