combineByKey(createCombiner, mergeValue, mergeCombiners, numPartitions=None, partitionFunc=<function portable_hash>)¶
Generic function to combine the elements for each key using a custom set of aggregation functions.
Turns an RDD[(K, V)] into a result of type RDD[(K, C)], for a “combined type” C.
Users provide three functions:
createCombiner, which turns a V into a C (e.g., creates a one-element list)
mergeValue, to merge a V into a C (e.g., adds it to the end of a list)
mergeCombiners, to combine two C’s into a single one (e.g., merges the lists)
To avoid memory allocation, both mergeValue and mergeCombiners are allowed to modify and return their first argument instead of creating a new C.
In addition, users can control the partitioning of the output RDD.
- V and C can be different – for example, one might group an RDD of type
(Int, Int) into an RDD of type (Int, List[Int]).
>>> x = sc.parallelize([("a", 1), ("b", 1), ("a", 2)]) >>> def to_list(a): ... return [a] ... >>> def append(a, b): ... a.append(b) ... return a ... >>> def extend(a, b): ... a.extend(b) ... return a ... >>> sorted(x.combineByKey(to_list, append, extend).collect()) [('a', [1, 2]), ('b', )]