How to make binary classication in Spark ML without StringIndexer

问题

I try to use Spark ML DecisionTreeClassifier in Pipeline without StringIndexer, because my feature is already indexed as (0.0; 1.0). DecisionTreeClassifier as label requires double values, so this code should work:

def trainDecisionTreeModel(training: RDD[LabeledPoint], sqlc: SQLContext): Unit = {
  import sqlc.implicits._
  val trainingDF = training.toDF()
  //format of this dataframe: [label: double, features: vector]

  val featureIndexer = new VectorIndexer()
    .setInputCol("features")
    .setOutputCol("indexedFeatures")
    .setMaxCategories(4)
    .fit(trainingDF)

  val dt = new DecisionTreeClassifier()
    .setLabelCol("label")
    .setFeaturesCol("indexedFeatures")


  val pipeline = new Pipeline()
    .setStages(Array(featureIndexer, dt))
  pipeline.fit(trainingDF)
}

But actually I get

java.lang.IllegalArgumentException:
DecisionTreeClassifier was given input with invalid label column label,
without the number of classes specified. See StringIndexer.

Of course I can just put StringIndexer and let him make it's work for my double "label" field, but I want to work with output rawPrediction column of DecisionTreeClassifier to get probability of 0.0 and 1.0 for each row like...

val predictions = model.transform(singletonDF) 
val zeroProbability = predictions.select("rawPrediction").asInstanceOf[Vector](0)
val oneProbability = predictions.select("rawPrediction").asInstanceOf[Vector](1)

If I put StringIndexer in Pipeline - I will not know indexes of my input labels "0.0" and "1.0" in rawPrediction vector, because String indexer will index by value's frequency, which could vary.

Please, help to prepare data for DecisionTreeClassifier without using StringIndexer or suggest some another way to get probability of my original labels (0.0; 1.0) for each row.

回答1:

You can always set required metadata manually:

import sqlContext.implicits._
import org.apache.spark.ml.attribute.NominalAttribute

val meta = NominalAttribute
  .defaultAttr
  .withName("label")
  .withValues("0.0", "1.0")
  .toMetadata

val dfWithMeta = df.withColumn("label", $"label".as("label", meta))
pipeline.fit(dfWithMeta)

来源：https://stackoverflow.com/questions/35922214/how-to-make-binary-classication-in-spark-ml-without-stringindexer