How to handle categorical features with spark-ml?

Question

How do I handle categorical data with spark-ml and not spark-mllib ?

Thought the documentation is not very clear, it seems that classifiers e.g. RandomForestClassifier, LogisticRegression, have a featuresCol argument, which specifies the name of the column of features in the DataFrame, and a labelCol argument, which specifies the name of the column of labeled classes in the DataFrame.

Obviously I want to use more than one feature in my prediction, so I tried using the VectorAssembler to put all my features in a single vector under featuresCol.

However, the VectorAssembler only accepts numeric types, boolean type, and vector type (according to the Spark website), so I can't put strings in my features vector.

How should I proceed?

Answer 1

I just wanted to complete Holden's answer.

Since Spark 2.3.0,OneHotEncoder has been deprecated and it will be removed in 3.0.0. Please use OneHotEncoderEstimator instead.

In Scala:

import org.apache.spark.ml.Pipeline
import org.apache.spark.ml.feature.{OneHotEncoderEstimator, StringIndexer}

val df = Seq((0, "a", 1), (1, "b", 2), (2, "c", 3), (3, "a", 4), (4, "a", 4), (5, "c", 3)).toDF("id", "category1", "category2")

val indexer = new StringIndexer().setInputCol("category1").setOutputCol("category1Index")
val encoder = new OneHotEncoderEstimator()
  .setInputCols(Array(indexer.getOutputCol, "category2"))
  .setOutputCols(Array("category1Vec", "category2Vec"))

val pipeline = new Pipeline().setStages(Array(indexer, encoder))

pipeline.fit(df).transform(df).show
// +---+---------+---------+--------------+-------------+-------------+
// | id|category1|category2|category1Index| category1Vec| category2Vec|
// +---+---------+---------+--------------+-------------+-------------+
// |  0|        a|        1|           0.0|(2,[0],[1.0])|(4,[1],[1.0])|
// |  1|        b|        2|           2.0|    (2,[],[])|(4,[2],[1.0])|
// |  2|        c|        3|           1.0|(2,[1],[1.0])|(4,[3],[1.0])|
// |  3|        a|        4|           0.0|(2,[0],[1.0])|    (4,[],[])|
// |  4|        a|        4|           0.0|(2,[0],[1.0])|    (4,[],[])|
// |  5|        c|        3|           1.0|(2,[1],[1.0])|(4,[3],[1.0])|
// +---+---------+---------+--------------+-------------+-------------+

In Python:

from pyspark.ml import Pipeline
from pyspark.ml.feature import StringIndexer, OneHotEncoderEstimator

df = spark.createDataFrame([(0, "a", 1), (1, "b", 2), (2, "c", 3), (3, "a", 4), (4, "a", 4), (5, "c", 3)], ["id", "category1", "category2"])

indexer = StringIndexer(inputCol="category1", outputCol="category1Index")
inputs = [indexer.getOutputCol(), "category2"]
encoder = OneHotEncoderEstimator(inputCols=inputs, outputCols=["categoryVec1", "categoryVec2"])
pipeline = Pipeline(stages=[indexer, encoder])
pipeline.fit(df).transform(df).show()
# +---+---------+---------+--------------+-------------+-------------+
# | id|category1|category2|category1Index| categoryVec1| categoryVec2|
# +---+---------+---------+--------------+-------------+-------------+
# |  0|        a|        1|           0.0|(2,[0],[1.0])|(4,[1],[1.0])|
# |  1|        b|        2|           2.0|    (2,[],[])|(4,[2],[1.0])|
# |  2|        c|        3|           1.0|(2,[1],[1.0])|(4,[3],[1.0])|
# |  3|        a|        4|           0.0|(2,[0],[1.0])|    (4,[],[])|
# |  4|        a|        4|           0.0|(2,[0],[1.0])|    (4,[],[])|
# |  5|        c|        3|           1.0|(2,[1],[1.0])|(4,[3],[1.0])|
# +---+---------+---------+--------------+-------------+-------------+

Since Spark 1.4.0, MLLib also supplies OneHotEncoder feature, which maps a column of label indices to a column of binary vectors, with at most a single one-value.

This encoding allows algorithms which expect continuous features, such as Logistic Regression, to use categorical features

Let's consider the following DataFrame:

val df = Seq((0, "a"),(1, "b"),(2, "c"),(3, "a"),(4, "a"),(5, "c"))
            .toDF("id", "category")

The first step would be to create the indexed DataFrame with the StringIndexer:

import org.apache.spark.ml.feature.StringIndexer

val indexer = new StringIndexer()
                   .setInputCol("category")
                   .setOutputCol("categoryIndex")
                   .fit(df)

val indexed = indexer.transform(df)

indexed.show
// +---+--------+-------------+                                                    
// | id|category|categoryIndex|
// +---+--------+-------------+
// |  0|       a|          0.0|
// |  1|       b|          2.0|
// |  2|       c|          1.0|
// |  3|       a|          0.0|
// |  4|       a|          0.0|
// |  5|       c|          1.0|
// +---+--------+-------------+

You can then encode the categoryIndex with OneHotEncoder :

import org.apache.spark.ml.feature.OneHotEncoder

val encoder = new OneHotEncoder()
                   .setInputCol("categoryIndex")
                   .setOutputCol("categoryVec")

val encoded = encoder.transform(indexed)

encoded.select("id", "categoryVec").show
// +---+-------------+
// | id|  categoryVec|
// +---+-------------+
// |  0|(2,[0],[1.0])|
// |  1|    (2,[],[])|
// |  2|(2,[1],[1.0])|
// |  3|(2,[0],[1.0])|
// |  4|(2,[0],[1.0])|
// |  5|(2,[1],[1.0])|
// +---+-------------+