Spark Scala: How to convert Dataframe[vector] to DataFrame[f1:Double, …, fn: Double)]

Question

I just used Standard Scaler to normalize my features for a ML application. After selecting the scaled features, I want to convert this back to a dataframe of Doubles, though the length of my vectors are arbitrary. I know how to do it for a specific 3 features by using

myDF.map{case Row(v: Vector) => (v(0), v(1), v(2))}.toDF("f1", "f2", "f3")

but not for an arbitrary amount of features. Is there an easy way to do this?

Example:

val testDF = sc.parallelize(List(Vectors.dense(5D, 6D, 7D), Vectors.dense(8D, 9D, 10D), Vectors.dense(11D, 12D, 13D))).map(Tuple1(_)).toDF("scaledFeatures")
val myColumnNames = List("f1", "f2", "f3")
// val finalDF = DataFrame[f1: Double, f2: Double, f3: Double] 

EDIT

I found out how to unpack to column names when creating the dataframe, but still am having trouble converting a vector to a sequence needed to create the dataframe:

finalDF = testDF.map{case Row(v: Vector) => v.toArray.toSeq /* <= this errors */}.toDF(List("f1", "f2", "f3"): _*)

Answer 1

One possible approach is something similar to this

import org.apache.spark.sql.functions.udf
import org.apache.spark.mllib.linalg.Vector

// Get size of the vector
val n = testDF.first.getAs[org.apache.spark.mllib.linalg.Vector](0).size

// Simple helper to convert vector to array<double> 
val vecToSeq = udf((v: Vector) => v.toArray)

// Prepare a list of columns to create
val exprs = (0 until n).map(i => $"_tmp".getItem(i).alias(s"f$i"))

testDF.select(vecToSeq($"scaledFeatures").alias("_tmp")).select(exprs:_*)

If you know a list of columns upfront you can simplify this a little:

val cols: Seq[String] = ???
val exprs = cols.zipWithIndex.map{ case (c, i) => $"_tmp".getItem(i).alias(c) }

For Python equivalent see How to split Vector into columns - using PySpark.

Answer 2

Alternate solution that evovled couple of days ago: Import the VectorDisassembler into your project (as long as it's not merged into Spark), now:

import org.apache.spark.ml.feature.VectorAssembler
import org.apache.spark.ml.linalg.Vectors

val dataset = spark.createDataFrame(
  Seq((0, 1.2, 1.3), (1, 2.2, 2.3), (2, 3.2, 3.3))
).toDF("id", "val1", "val2")


val assembler = new VectorAssembler()
  .setInputCols(Array("val1", "val2"))
  .setOutputCol("vectorCol")

val output = assembler.transform(dataset)
output.show()
/*
+---+----+----+---------+
| id|val1|val2|vectorCol|
+---+----+----+---------+
|  0| 1.2| 1.3|[1.2,1.3]|
|  1| 2.2| 2.3|[2.2,2.3]|
|  2| 3.2| 3.3|[3.2,3.3]|
+---+----+----+---------+*/

val disassembler = new org.apache.spark.ml.feature.VectorDisassembler()
  .setInputCol("vectorCol")
disassembler.transform(output).show()
/*
+---+----+----+---------+----+----+
| id|val1|val2|vectorCol|val1|val2|
+---+----+----+---------+----+----+
|  0| 1.2| 1.3|[1.2,1.3]| 1.2| 1.3|
|  1| 2.2| 2.3|[2.2,2.3]| 2.2| 2.3|
|  2| 3.2| 3.3|[3.2,3.3]| 3.2| 3.3|
+---+----+----+---------+----+----+*/

Answer 3

Please try VectorSlicer :

import org.apache.spark.ml.feature.VectorAssembler
import org.apache.spark.ml.linalg.Vectors

val dataset = spark.createDataFrame(
  Seq((1, 0.2, 0.8), (2, 0.1, 0.9), (3, 0.3, 0.7))
).toDF("id", "negative_logit", "positive_logit")


val assembler = new VectorAssembler()
  .setInputCols(Array("negative_logit", "positive_logit"))
  .setOutputCol("prediction")

val output = assembler.transform(dataset)
output.show()
/*
+---+--------------+--------------+----------+
| id|negative_logit|positive_logit|prediction|
+---+--------------+--------------+----------+
|  1|           0.2|           0.8| [0.2,0.8]|
|  2|           0.1|           0.9| [0.1,0.9]|
|  3|           0.3|           0.7| [0.3,0.7]|
+---+--------------+--------------+----------+
*/

val slicer = new VectorSlicer()
.setInputCol("prediction")
.setIndices(Array(1))
.setOutputCol("positive_prediction")

val posi_output = slicer.transform(output)
posi_output.show()

/*
+---+--------------+--------------+----------+-------------------+
| id|negative_logit|positive_logit|prediction|positive_prediction|
+---+--------------+--------------+----------+-------------------+
|  1|           0.2|           0.8| [0.2,0.8]|              [0.8]|
|  2|           0.1|           0.9| [0.1,0.9]|              [0.9]|
|  3|           0.3|           0.7| [0.3,0.7]|              [0.7]|
+---+--------------+--------------+----------+-------------------+
*/