Here's the more functional programming-friendly take on the
computation (but yeah this is the naive formula):
rdd.groupByKey.mapValues { mcs =>
val values = mcs.map(_.foo.toDouble)
val n = values.count
val sum = values.sum
val sumSquares = values.map(x => x * x).sum
math.sqrt(n * sumSquares - sum * sum) / n
}
This gives you a bunch of (key,stdev). I think you want to compute
this RDD and *then* do something to save it if you like. Sure, that
could be collecting it locally and saving to a DB. Or you could use
foreach to do something remotely for every key-value pair. More
efficient would be to mapPartitions and do something to a whole
partition of key-value pairs at a time.
On Fri, Aug 1, 2014 at 9:56 PM, kriskalish <[email protected]> wrote:
> So if I do something like this, spark handles the parallelization and
> recombination of sum and count on the cluster automatically? I started
> peeking into the source and see that foreach does submit a job to the
> cluster, but it looked like the inner function needed to return something to
> work properly.
>
> val grouped = rdd.groupByKey()
> grouped.foreach{ x =>
> val iterable = x._2
> var sum = 0.0
> var count = 0
> iterable.foreach{ y =>
> sum = sum + y.foo
> count = count + 1
> }
> val mean = sum/count;
> // save mean to database...
> }
>
>
>
>
> --
> View this message in context:
> http://apache-spark-user-list.1001560.n3.nabble.com/Computing-mean-and-standard-deviation-by-key-tp11192p11207.html
> Sent from the Apache Spark User List mailing list archive at Nabble.com.
