How to Sum a part of a list in RDD

问题:

I have an RDD, and I would like to sum a part of the list.

(key, element2 + element3)
(1, List(2.0, 3.0, 4.0, 5.0)), (2, List(1.0, -1.0, -2.0, -3.0))

output should look like this,

(1, 7.0), (2, -3.0)

Thanks

回答1:

You can map and indexing on the second part:

yourRddOfTuples.map(tuple => {val list = tuple._2; list(1) + list(2)})

Update after your comment, convert it to Vector:

yourRddOfTuples.map(tuple => {val vs = tuple._2.toVector; vs(1) + vs(2)})

Or if you do not want to use conversions:

yourRddOfTuples.map(_._2.drop(1).take(2).sum)

This skips the first element (.drop(1)) from the second element of the tuple (.map(_._2), takes the next two (.take(2)) (might be less if you have less) and sums them (.sum).

回答2:

You can map the key-list pair to obtain the 2nd and 3rd list elements as follows:

val rdd = sc.parallelize(Seq(
  (1, List(2.0, 3.0, 4.0, 5.0)),
  (2, List(1.0, -1.0, -2.0, -3.0))
))

rdd.map{ case (k, l) => (k, l(1) + l(2)) }.collect
// res1: Array[(Int, Double)] = Array((1,7.0), (2,-3.0))