Spark Dataframe of WrappedArray to Dataframe[Vector]

问题

I have a spark Dataframe df with the following schema:

root
 |-- features: array (nullable = true)
 |    |-- element: double (containsNull = false)

I would like to create a new Dataframe where each row will be a Vector of Doubles and expecting to get the following schema:

root
     |-- features: vector (nullable = true)

So far I have the following piece of code (influenced by this post: Converting Spark Dataframe(with WrappedArray) to RDD[labelPoint] in scala) but I fear something is wrong with it because it takes a very long time to compute even a reasonable amount of rows. Also, if there are too many rows the application will crash with a heap space exception.

val clustSet = df.rdd.map(r => {
          val arr = r.getAs[mutable.WrappedArray[Double]]("features")
          val features: Vector = Vectors.dense(arr.toArray)
          features
          }).map(Tuple1(_)).toDF()

I suspect that the instruction arr.toArray is not a good Spark practice in this case. Any clarification would be very helpful.

Thank you!

回答1:

It's because .rdd have to unserialize objects from internal in-memory format and it is very time consuming.

It's ok to use .toArray - you are operating on row level, not collecting everything to the driver node.

You can do this very easy with UDFs:

import org.apache.spark.ml.linalg._
val convertUDF = udf((array : Seq[Double]) => {
  Vectors.dense(array.toArray)
})
val withVector = dataset
  .withColumn("features", convertUDF('features))

Code is from this answer: Convert ArrayType(FloatType,false) to VectorUTD

However there author of the question didn't ask about differences

来源：https://stackoverflow.com/questions/44051530/spark-dataframe-of-wrappedarray-to-dataframevector

标签

scala

apache-spark

spark-dataframe