Spark Union inside a loop gives void - scala

Question

I try to make a RDD from iterative union from another RDD inside a loop but the result works exclusively if i perform an action on the result RDD inside the loop.
var rdd : RDD[Int] = sc.emptyRDD
for ( i <- 1 to 5 ) {
val rdd1 = sc.parallelize(Array(1))
rdd = rdd ++ rdd1
}
// rdd.foreach(println) => void
for ( i <- 1 to 5 ) {
val rdd1 = sc.parallelize(Array(1))
rdd = rdd ++ rdd1
rdd.foreach(x=>x)
}
// rdd.foreach(println) => ( 1,1,1,1,1)
If I create rdd1 outside the loop everything works fine but not inside.
Does it exist a specific lightweight action to solve this problem ?

Answer 1

One thing to keep in mind is that when you apply the foreach action to your RDD, the action while apply on each individual worker. Therefore in the first case, if you check the stdout's of each executor, you will find the printed values from rdd. If you want these values to be printed to your console, you can aggregate the elements of an RDD (or a subset of them) at the driver, and then apply your function (e.g. rdd.collect.foreach(println), rdd.take(3).foreach(println), etc).