gsamaras gsamaras - 1 year ago 78
Python Question

Prepare my bigdata with Spark via Python

My 100m in size, quantized data:

(1424411938', [3885, 7898])
(3333333333', [3885, 7898])

Desired result:

(3885, [3333333333, 1424411938])
(7898, [3333333333, 1424411938])

So what I want, is to transform the data so that I group 3885 (for example) with all the
that have it). Here is what I did in :

def prepare(data):
result = []
for point_id, cluster in data:
for index, c in enumerate(cluster):
found = 0
for res in result:
if c == res[0]:
found = 1
if(found == 0):
result.append((c, []))
for res in result:
if c == res[0]:
return result

but when I
RDD with
, it seem to do what I want only in the current partition, thus return a bigger result than the desired.

For example, if the 1st record in the start was in the 1st partition and the 2nd in the 2nd, then I would get as a result:

(3885, [3333333333])
(7898, [3333333333])
(3885, [1424411938])
(7898, [1424411938])

How to modify my
to get the desired effect? Alternatively, how to process the result that
produces, so that I can get the desired result?

As you may already have noticed from the code, I do not care about speed at all.

Here is a way to create the data:

data = []
from random import randint
for i in xrange(0, 10):
data.append((randint(0, 100000000), (randint(0, 16000), randint(0, 16000))))
data = sc.parallelize(data)

Answer Source

You can use a bunch of basic pyspark transformations to achieve this.

>>> rdd = sc.parallelize([(1424411938, [3885, 7898]),(3333333333, [3885, 7898])])
>>> r = rdd.flatMap(lambda x: ((a,x[0]) for a in x[1]))

We used flatMap to have a key, value pair for every item in x[1] and we changed the data line format to (a, x[0]), the a here is every item in x[1]. To understand flatMap better you can look to the documentation.

>>> r2 = r.groupByKey().map(lambda x: (x[0],tuple(x[1])))

We just grouped all key, value pairs by their keys and used tuple function to convert iterable to tuple.

>>> r2.collect()
[(3885, (1424411938, 3333333333)), (7898, (1424411938, 3333333333))]

As you said you can use [:150] to have first 150 elements, I guess this would be proper usage:

r2 = r.groupByKey().map(lambda x: (x[0],tuple(x[1])[:150]))

I tried to be as explanatory as possible. I hope this helps.

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