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PySpark RDD | countByKey method

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PySpark RDD
schedule Jul 1, 2022
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PySpark RDD's countByKey(~) method groups by the key of the elements in a pair RDD, and counts each group.

Parameters

This method does not take in any parameter.

Return Value

A DefaultDict[key,int].

Examples

Consider the following PySpark pair RDD:

rdd = sc.parallelize([("a",5),("a",1),("b",2),("c",4)])
rdd.collect()
[('a', 5), ('a', 1), ('b', 2), ('c', 4)]

Here, we are using the parallelize(~) method to create a RDD.

Getting the count of each group in PySpark Pair RDD

To group by the key, and get the count of each group:

rdd.countByKey()
defaultdict(int, {'a': 2, 'b': 1, 'c': 1})

Here, the returned value is DefaultDict, which is basically a dictionary in which accessing values that do not exist in the dictionary will return a 0 instead of throwing an error.

You can access the count of a key just as you would for an ordinary dictionary:

counts = rdd.countByKey()
counts["a"]
2

Accesing counts of keys that do not exist will return 0:

counts = rdd.countByKey()
counts["z"]
0
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Published by Isshin Inada
Edited by 0 others
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