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- The [pyspark documentation]() doesn't include an example for the aggregateByKey RDD method. I didn't find any nice examples online, so I wrote my own.
- Here's what the documetation does say:
- `aggregateByKey(self, zeroValue, seqFunc, combFunc, numPartitions=None)`
- > Aggregate the values of each key, using given combine functions and a neutral "zero value". This function can return a different result type, U, than the type of the values in this RDD, V. Thus, we need one operation for merging a V into a U and one operation for merging two U's, The former operation is used for merging values within a partition, and the latter is used for merging values between partitions. To avoid memory allocation, both of these functions are allowed to modify and return their first argument instead of creating a new U.
- `reduceByKey` and `aggregateByKey` are much more efficient than `groupByKey` and should be used for aggregations as much as possible.
- In the example below, I create an RDD that is a short list of characters. My functions will aggregate the functions together with concatenation. I added brackets to the two types of concatenation to help give you an idea of what `aggregateByKey` is doing.
- ```
- Welcome to
- ____ __
- / __/__ ___ _____/ /__
- _\ \/ _ \/ _ `/ __/ '_/
- /__ / .__/\_,_/_/ /_/\_\ version 1.1.0
- /_/
- Using Python version 2.7.5 (default, Mar 9 2014 22:15:05)
- SparkContext available as sc.
- In [1]: # Create rdd that is a list of characters
- In [2]: sc.parallelize(list("aaaaabbbbcccdd")) \
- ...: .map(lambda letter: (letter, {"value": letter})) \
- ...: .aggregateByKey(
- ...: # Value to start aggregation (passed as s to `lambda s, d`)
- ...: "start",
- ...: # Function to join final data type (string) and rdd data type
- ...: lambda s, d: "[ %s %s ]" % (s, d["value"]),
- ...: # Function to join two final data types.
- ...: lambda s1, s2: "{ %s %s }" % (s1, s2),
- ...: ) \
- ...: .collect()
- Out[2]:
- [('a', '{ { [ start a ] [ [ start a ] a ] } [ [ start a ] a ] }'),
- ('c', '{ [ start c ] [ [ start c ] c ] }'),
- ('b', '{ { [ [ start b ] b ] [ start b ] } [ start b ] }'),
- ('d', '[ [ start d ] d ]')]
- ```
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