I think the main concern is this would require scanning the data twice, and maybe the user should be aware of it ...
On Thu, Jun 5, 2014 at 10:29 AM, Andrew Ash <and...@andrewash.com> wrote: > I have a use case that would greatly benefit from RDDs having a .scanLeft() > method. Are the project developers interested in adding this to the public > API? > > > Looking through past message traffic, this has come up a few times. The > recommendation from the list before has been to implement a parallel prefix > scan. > > http://comments.gmane.org/gmane.comp.lang.scala.spark.user/1880 > https://groups.google.com/forum/#!topic/spark-users/ts-FdB50ltY > > The algorithm Reynold sketched in the first link leads to this working > implementation: > > val vector = sc.parallelize(1 to 20, 3) > > val sums = 0 +: vector.mapPartitionsWithIndex{ case(partition, iter) => > Iterator(iter.sum) }.collect.scanLeft(0)(_+_).drop(1) > > val prefixScan = vector.mapPartitionsWithIndex { case(partition, iter) => > val base = sums(partition) > println(partition, base) > iter.scanLeft(base)(_+_).drop(1) > }.collect > > > I'd love to have that replaced with this: > > val vector = sc.parallelize(1 to 20, 3) > val cumSum: RDD[Int] = vector.scanLeft(0)(_+_) > > > Any thoughts on whether this contribution would be accepted? What pitfalls > exist that I should be thinking about? > > Thanks! > Andrew >
