Spark: FlatMapValues query

Question

I'm reading the Learning Spark book and couldn't understand the following pair rdd transformation.

rdd.flatMapValues(x => (x to 5))

It is applied on an rdd {(1,2),(3,4),(3,6)} and the output of the transformation is {(1,2),(1,3),(1,4),(1,5),(3,4),(3,5)}

Can someone please explain this.

Answer 1

flatMapValues method is a combination of flatMap and mapValues.

Let's start with the given rdd.

val sampleRDD = sc.parallelize(Array((1,2),(3,4),(3,6)))

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mapValues maps the values while keeping the keys.

For example, sampleRDD.mapValues(x => x to 5) returns

Array((1,Range(2, 3, 4, 5)), (3,Range(4, 5)), (3,Range()))

notice that for key-value pair (3, 6), it produces (3,Range()) since 6 to 5 produces an empty collection of values.

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flatMap "breaks down" collections into the elements of the collection. You can search for more accurate description of flatMap online like here and here.

For example,

given val rdd2 = sampleRDD.mapValues(x => x to 5), if we do rdd2.flatMap(x => x), you will get

Array((1,2),(1,3),(1,4),(1,5),(3,4),(3,5)).

That is, for every element in the collection in each key, we create a (key, element) pair.

Also notice that (3, Range()) does not produce any additional key element pair since the sequence is empty.

now combining flatMap and mapValues, you get flatMapValues.