scala function not called from lambda function - scala

Question

I am new to scala and just understanding how can I transform via Map call. The foo function is not getting called. What is that I am missing ?
import org.apache.spark.rdd.RDD
import org.apache.spark.SparkConf
import org.apache.spark.SparkContext
object Expt {
def main(args: Array[String]): Unit = {
var conf = new SparkConf().setAppName("Test").setMaster("local[*]")
val sc = new SparkContext(conf)
val a1 = sc.parallelize(List((1,"one"),(2,"two"),(3,"three"))
val a3 = a1.map(x => Expt.foo(x))
}
def foo(x: (Int,String)) : (Int,String) = {
println(x)
x
}
}

Answer 1

You don't execute any action so map is never evaluated. Also println in map usually won't have visible effect.

Answer 2

trigger computation using collect or foreach (do some action)
scala> val a1 = sc.parallelize(List((1,"one"),(2,"two"),(3,"three")))
a1: org.apache.spark.rdd.RDD[(Int, String)] = ParallelCollectionRDD[6] at parallelize at <console>:24
scala> val a3 = a1.map(x => foo(x))
a3: org.apache.spark.rdd.RDD[(Int, String)] = MapPartitionsRDD[7] at map at <console>:28
scala> a3.collect
(1,one)
(2,two)
(3,three)
res3: Array[(Int, String)] = Array((1,one), (2,two), (3,three))
scala> a3.foreach(_ => ())
(1,one)
(3,three)
(2,two)