How to print elements of particular RDD partition in Spark?

Question

How to print the elements of a particular partition, say 5th, alone?

val distData = sc.parallelize(1 to 50, 10)

Answer 1

Using Spark/Scala:

val data = 1 to 50
val distData = sc.parallelize(data,10)
distData.mapPartitionsWithIndex( (index: Int, it: Iterator[Int]) =>it.toList.map(x => if (index ==5) {println(x)}).iterator).collect

produces:

26
27
28
29
30

Answer 2

Assume you do this just for test purpose, then use glom(). See Spark documentation: https://spark.apache.org/docs/1.6.0/api/python/pyspark.html#pyspark.RDD.glom

>>> rdd = sc.parallelize([1, 2, 3, 4], 2)
>>> rdd.glom().collect()
[[1, 2], [3, 4]]
>>> rdd.glom().collect()[1]
[3, 4]

Edit: Example in Scala:

scala> val distData = sc.parallelize(1 to 50, 10)
scala> distData.glom().collect()(4)
res2: Array[Int] = Array(21, 22, 23, 24, 25)

Answer 3

you could possible use a counter against foreachPartition() API to achieve it.

Here is a Java program that prints content of each partition JavaSparkContext context = new JavaSparkContext(conf);

    JavaRDD<Integer> myArray = context.parallelize(Arrays.asList(1,2,3,4,5,6,7,8,9));
    JavaRDD<Integer> partitionedArray = myArray.repartition(2);

    System.out.println("partitioned array size is " + partitionedArray.count());
    partitionedArray.foreachPartition(new VoidFunction<Iterator<Integer>>() {

        public void call(Iterator<Integer> arg0) throws Exception {

            while(arg0.hasNext()) {
                System.out.println(arg0.next());
            }

        }
    });