Attach metadata to vector column in Spark

问题:

Context: I have a data frame with two columns: label, and features.

org.apache.spark.sql.DataFrame = [label: int, features: vector]

Where features is a mllib.linalg.VectorUDT of numeric type built using VectorAssembler.

Question: Is there a way to assign a schema to the features vector? I want to keep track of the name of each feature.

Tried so far:

val defaultAttr = NumericAttribute.defaultAttr
val attrs = Array("feat1", "feat2", "feat3").map(defaultAttr.withName)
val attrGroup = new AttributeGroup("userFeatures", attrs.asInstanceOf[Array[Attribute]])

---

scala> attrGroup.toMetadata 
res197: org.apache.spark.sql.types.Metadata = {"ml_attr":{"attrs":{"numeric":[{"idx":0,"name":"f1"},{"idx":1,"name":"f2"},{"idx":2,"name":"f3"}]},"num_attrs":3}}

But was not sure how to apply this to an existing data frame.

回答1:

There at lest two options:

1. On existing DataFrame you can use as method with metadata argument:

   import org.apache.spark.ml.attribute._
   
   val rdd = sc.parallelize(Seq(
     (1, Vectors.dense(1.0, 2.0, 3.0))
   ))
   val df = rdd.toDF("label", "features")
   
   df.withColumn("features", $"features".as("_", attrGroup.toMetadata))
   

2. When you create new DataFrame convert AttributeGroup toStructField and use it as a schema for a given column:

   import org.apache.spark.sql.types.{StructType, StructField, IntegerType}
   
   val schema = StructType(Array(
     StructField("label", IntegerType, false),
     attrGroup.toStructField()
   ))
   
   spark.createDataFrame(
     rdd.map(row => Row.fromSeq(row.productIterator.toSeq)),
     schema)
   

If vector column has been created using VectorAssembler column metadata describing parent columns should be already attached.

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

val raw = sc.parallelize(Seq(
  (1, 1.0, 2.0, 3.0)
)).toDF("id", "feat1", "feat2", "feat3")

val assembler = new VectorAssembler()
  .setInputCols(Array("feat1", "feat2", "feat3"))
  .setOutputCol("features")

val dfWithMeta = assembler.transform(raw).select($"id", $"features")
dfWithMeta.schema.fields(1).metadata

// org.apache.spark.sql.types.Metadata = {"ml_attr":{"attrs":{"numeric":[
//   {"idx":0,"name":"feat1"},{"idx":1,"name":"feat2"},
//   {"idx":2,"name":"feat3"}]},"num_attrs":3}

Vector fields are not directly accessible using dot syntax (like $features.feat1) but can used by specialized tools like VectorSlicer:

import org.apache.spark.ml.feature.VectorSlicer

val slicer = new VectorSlicer()
  .setInputCol("features")
  .setOutputCol("featuresSubset")
  .setNames(Array("feat1", "feat3"))

slicer.transform(dfWithMeta).show
// +---+-------------+--------------+
// | id|     features|featuresSubset|
// +---+-------------+--------------+
// |  1|[1.0,2.0,3.0]|     [1.0,3.0]|
// +---+-------------+--------------+

For PySpark see How can I declare a Column as a categorical feature in a DataFrame for use in ml