Sequence with Streams in Scala

Question

Suppose there is a sequence a[i] = f(a[i-1], a[i-2], ... a[i-k]). How would you code it using streams in Scala?

Answer 1

It will be possible to generalize it for any k, using an array for a and another k parameter, and having, f.i., the function with a rest... parameter.

def next(a1:Any, ..., ak:Any, f: (Any, ..., Any) => Any):Stream[Any] {
  val n = f(a1, ..., ak)
  Stream.cons(n, next(a2, ..., n, f))
}

val myStream = next(init1, ..., initk)

in order to have the 1000th do next.drop(1000)

An Update to show how this could be done with varargs. Beware that there is no arity check for the passed function:

object Test extends App {

def next(a:Seq[Long], f: (Long*) => Long): Stream[Long] = {
  val v = f(a: _*)
  Stream.cons(v, next(a.tail ++ Array(v), f))
}

def init(firsts:Seq[Long], rest:Seq[Long], f: (Long*) => Long):Stream[Long] = {
  rest match {
    case Nil => next(firsts, f)
    case x :: xs => Stream.cons(x,init(firsts, xs, f))
  }
}

def sum(a:Long*):Long = {
  a.sum
}

val myStream = init(Seq[Long](1,1,1), Seq[Long](1,1,1), sum)


myStream.take(12).foreach(println)

}