mapPartitions - How Does it Works
I am trying to understand about mapPartitions but i am still not sure how it works in the below example it create three partition val parallel = sc.parallelize(1 to 10, 3) and when we do below parallel.mapPartitions( x = List(x.next).iterator).collect it prints value Array[Int] = Array(1, 4, 7) Can some one please explain why it prints 1,4,7 only Thanks, -- View this message in context: http://apache-spark-user-list.1001560.n3.nabble.com/mapPartitions-How-Does-it-Works-tp22123.html Sent from the Apache Spark User List mailing list archive at Nabble.com. - To unsubscribe, e-mail: user-unsubscr...@spark.apache.org For additional commands, e-mail: user-h...@spark.apache.org
RE: mapPartitions - How Does it Works
Here is what I think: mapPartitions is for a specialized map that is called only once for each partition. The entire content of the respective partitions is available as a sequential stream of values via the input argument (Iterarator[T]). The combined result iterators are automatically converted into a new RDD. So in this case, the RDD (1,2,, 10) is split as 3 partitions, (1,2,3), (4,5,6), (7,8,9,10). For every partition, your function is the get the first element as x.next, using it to build a list, return the iterator from the List. So each partition will return (1), (4) and (7) as 3 iterator, then combine to one final RDD (1, 4, 7). Yong Date: Wed, 18 Mar 2015 10:19:34 -0700 From: ashish.us...@gmail.com To: user@spark.apache.org Subject: mapPartitions - How Does it Works I am trying to understand about mapPartitions but i am still not sure how it works in the below example it create three partition val parallel = sc.parallelize(1 to 10, 3) and when we do below parallel.mapPartitions( x = List(x.next).iterator).collect it prints value Array[Int] = Array(1, 4, 7) Can some one please explain why it prints 1,4,7 only Thanks, -- View this message in context: http://apache-spark-user-list.1001560.n3.nabble.com/mapPartitions-How-Does-it-Works-tp22123.html Sent from the Apache Spark User List mailing list archive at Nabble.com. - To unsubscribe, e-mail: user-unsubscr...@spark.apache.org For additional commands, e-mail: user-h...@spark.apache.org
Re: mapPartitions - How Does it Works
Map partitions works as follows : 1) For each partition of your RDD, it provides an iterator over the values within that partition 2) You then define a function that operates on that iterator Thus if you do the following: val parallel = sc.parallelize(1 to 10, 3) parallel.mapPartitions( x = x.map(s = s + 1)).collect You would get: res3: Array[Int] = Array(2, 3, 4, 5, 6, 7, 8, 9, 10, 11) In your example, x is not a pointer that traverses the iterator (e.g. With .next()) , it¹s literally the Iterable object itself. On 3/18/15, 10:19 AM, ashish.usoni ashish.us...@gmail.com wrote: I am trying to understand about mapPartitions but i am still not sure how it works in the below example it create three partition val parallel = sc.parallelize(1 to 10, 3) and when we do below parallel.mapPartitions( x = List(x.next).iterator).collect it prints value Array[Int] = Array(1, 4, 7) Can some one please explain why it prints 1,4,7 only Thanks, -- View this message in context: http://apache-spark-user-list.1001560.n3.nabble.com/mapPartitions-How-Does -it-Works-tp22123.html Sent from the Apache Spark User List mailing list archive at Nabble.com. - To unsubscribe, e-mail: user-unsubscr...@spark.apache.org For additional commands, e-mail: user-h...@spark.apache.org The information contained in this e-mail is confidential and/or proprietary to Capital One and/or its affiliates. The information transmitted herewith is intended only for use by the individual or entity to which it is addressed. If the reader of this message is not the intended recipient, you are hereby notified that any review, retransmission, dissemination, distribution, copying or other use of, or taking of any action in reliance upon this information is strictly prohibited. If you have received this communication in error, please contact the sender and delete the material from your computer. - To unsubscribe, e-mail: user-unsubscr...@spark.apache.org For additional commands, e-mail: user-h...@spark.apache.org
Re: mapPartitions - How Does it Works
List(x.next).iterator is giving you the first element from each partition, which would be 1, 4 and 7 respectively. On 3/18/15, 10:19 AM, ashish.usoni ashish.us...@gmail.com wrote: I am trying to understand about mapPartitions but i am still not sure how it works in the below example it create three partition val parallel = sc.parallelize(1 to 10, 3) and when we do below parallel.mapPartitions( x = List(x.next).iterator).collect it prints value Array[Int] = Array(1, 4, 7) Can some one please explain why it prints 1,4,7 only Thanks, -- View this message in context: http://apache-spark-user-list.1001560.n3.nabble.com/mapPartitions-How-Does -it-Works-tp22123.html Sent from the Apache Spark User List mailing list archive at Nabble.com. - To unsubscribe, e-mail: user-unsubscr...@spark.apache.org For additional commands, e-mail: user-h...@spark.apache.org - To unsubscribe, e-mail: user-unsubscr...@spark.apache.org For additional commands, e-mail: user-h...@spark.apache.org
Re: mapPartitions - How Does it Works
Unlike a map() wherein your task is acting on a row at a time, with mapPartitions(), the task is passed the entire content of the partition in an iterator. You can then return back another iterator as the output. I don't do scala, but from what I understand from your code snippet... The iterator x can return all the rows in the partition. But you are returning back after consuming the first row. Hence you see only 1,4,7 in your output. These are the first rows of each of your 3 partitions. Regards Sab On 18-Mar-2015 10:50 pm, ashish.usoni ashish.us...@gmail.com wrote: I am trying to understand about mapPartitions but i am still not sure how it works in the below example it create three partition val parallel = sc.parallelize(1 to 10, 3) and when we do below parallel.mapPartitions( x = List(x.next).iterator).collect it prints value Array[Int] = Array(1, 4, 7) Can some one please explain why it prints 1,4,7 only Thanks, -- View this message in context: http://apache-spark-user-list.1001560.n3.nabble.com/mapPartitions-How-Does-it-Works-tp22123.html Sent from the Apache Spark User List mailing list archive at Nabble.com. - To unsubscribe, e-mail: user-unsubscr...@spark.apache.org For additional commands, e-mail: user-h...@spark.apache.org
