foldLeft or foldRight equivalent in Spark?

Question

In Spark's RDDs and DStreams we have the 'reduce' function for transforming an entire RDD into one element. However the reduce function takes (T,T) => T However if we want to reduce a List in Scala we can use foldLeft or foldRight which takes type (B)( (B,A) => B) This is very useful because you start folding with a type other then what is in your list.

Is there a way in Spark to do something similar? Where I can start with a value that is of different type then the elements in the RDD itself

Answer 1

Use aggregate instead of reduce. It allows you also to specify a "zero" value of type B and a function like the one you want: (B,A) => B. Do note that you also need to merge separate aggregations done on separate executors, so a (B, B) => B function is also required.

Alternatively, if you want this aggregation as a side effect, an option is to use an accumulator. In particular, the accumulable type allows for the result type to be of a different type than the accumulating type.

Also, if you even need to do the same with a key-value RDD, use aggregateByKey.