Scala: iterate over collection in random order without creating a copy

Question

Is there a way in Scala to obtain a stream/view/iterator for an immutable collection (eg List or Vector) which will traverse the collection in random order? As far as I understand Random.shuffle(coll) creates a copy, and it is not a memory-efficient way. Is Random.shuffle(coll.view) (or coll.stream or coll.iterator) a better approach? Does this create a noticeable CPU overhead? Or is there some more appropriate way?

Answer 1

Shuffling algorithms need to move randomly around in the collection and need to remember past choices somehow. The immutable collections are not very speedy with random access ( O(n) instead of O(1) for List , arguably O(1) for Vector but the constant factor is large).

Copying the collection, in comparison, is almost always the wise thing to do.

Answer 2

If collections copying is taken into account, consider shuffling an Array of indices to a Vector of interest, for instance like this,

val myVector: Vector[MyObject] = ...

val shuffledIdx = util.Random.shuffle(0 until myVector.size)

This copies a sequence of Int , likely lighter than dedicated objects. Then

shuffledIdx.map { idx => task (myVector(idx)) }

iterates / maps over each item in myVector in a random order.