[SPARK-20619][ML] StringIndexer supports multiple ways to order label

mllib/src/main/scala/org/apache/spark/ml/feature/StringIndexer.scala

@@ -59,6 +59,29 @@ private[feature] trait StringIndexerBase extends Params with HasInputCol with Ha
   @Since("1.6.0")
   def getHandleInvalid: String = $(handleInvalid)
 
+  /**
+   * Param for how to order labels of string column. The first label after ordering is assigned
+   * an index of 0.
+   * Options are:
+   *   - 'frequencyDesc': descending order by label frequency (most frequent label assigned 0)
+   *   - 'frequencyAsc': ascending order by label frequency (least frequent label assigned 0)
+   *   - 'alphabetDesc': descending alphabetical order
+   *   - 'alphabetAsc': ascending alphabetical order
+   * Default is 'frequencyDesc'.
+   *
+   * @group param
+   */
+  @Since("2.3.0")
+  final val stringOrderType: Param[String] = new Param(this, "stringOrderType",
+    "how to order labels of string column. " +
+    "The first label after ordering is assigned an index of 0. " +
+    s"Supported options: ${StringIndexer.supportedStringOrderType.mkString(", ")}.",
+    ParamValidators.inArray(StringIndexer.supportedStringOrderType))
+
+  /** @group getParam */
+  @Since("2.3.0")
+  def getStringOrderType: String = $(stringOrderType)
+
   /** Validates and transforms the input schema. */
   protected def validateAndTransformSchema(schema: StructType): StructType = {
     val inputColName = $(inputCol)
@@ -79,8 +102,9 @@ private[feature] trait StringIndexerBase extends Params with HasInputCol with Ha
 /**
  * A label indexer that maps a string column of labels to an ML column of label indices.
  * If the input column is numeric, we cast it to string and index the string values.
- * The indices are in [0, numLabels), ordered by label frequencies.
- * So the most frequent label gets index 0.
+ * The indices are in [0, numLabels). By default, this is ordered by label frequencies
+ * so the most frequent label gets index 0. The ordering behavior is controlled by
+ * setting `stringOrderType`.
  *
  * @see `IndexToString` for the inverse transformation
  */
@@ -96,6 +120,11 @@ class StringIndexer @Since("1.4.0") (
   @Since("1.6.0")
   def setHandleInvalid(value: String): this.type = set(handleInvalid, value)
 
+  /** @group setParam */
+  @Since("2.3.0")
+  def setStringOrderType(value: String): this.type = set(stringOrderType, value)
+  setDefault(stringOrderType, StringIndexer.frequencyDesc)
+
   /** @group setParam */
   @Since("1.4.0")
   def setInputCol(value: String): this.type = set(inputCol, value)
@@ -107,11 +136,17 @@ class StringIndexer @Since("1.4.0") (
   @Since("2.0.0")
   override def fit(dataset: Dataset[_]): StringIndexerModel = {
     transformSchema(dataset.schema, logging = true)
-    val counts = dataset.na.drop(Array($(inputCol))).select(col($(inputCol)).cast(StringType))
-      .rdd
-      .map(_.getString(0))
-      .countByValue()
-    val labels = counts.toSeq.sortBy(-_._2).map(_._1).toArray
+    val values = dataset.na.drop(Array($(inputCol)))
+      .select(col($(inputCol)).cast(StringType))
+      .rdd.map(_.getString(0))
+    val labels = $(stringOrderType) match {
+      case StringIndexer.frequencyDesc => values.countByValue().toSeq.sortBy(-_._2)
+        .map(_._1).toArray
+      case StringIndexer.frequencyAsc => values.countByValue().toSeq.sortBy(_._2)
+        .map(_._1).toArray
+      case StringIndexer.alphabetDesc => values.distinct.collect.sortWith(_ > _)
+      case StringIndexer.alphabetAsc => values.distinct.collect.sortWith(_ < _)
+    }
     copyValues(new StringIndexerModel(uid, labels).setParent(this))
   }
 
@@ -131,6 +166,12 @@ object StringIndexer extends DefaultParamsReadable[StringIndexer] {
   private[feature] val KEEP_INVALID: String = "keep"
   private[feature] val supportedHandleInvalids: Array[String] =
     Array(SKIP_INVALID, ERROR_INVALID, KEEP_INVALID)
+  private[feature] val frequencyDesc: String = "frequencyDesc"
+  private[feature] val frequencyAsc: String = "frequencyAsc"
+  private[feature] val alphabetDesc: String = "alphabetDesc"
+  private[feature] val alphabetAsc: String = "alphabetAsc"
+  private[feature] val supportedStringOrderType: Array[String] =
+    Array(frequencyDesc, frequencyAsc, alphabetDesc, alphabetAsc)
 
   @Since("1.6.0")
   override def load(path: String): StringIndexer = super.load(path)

mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala

@@ -291,4 +291,27 @@ class StringIndexerSuite
       NominalAttribute.decodeStructField(transformed.schema("labelIndex"), preserveName = true)
     assert(attrs.name.nonEmpty && attrs.name.get === "labelIndex")
   }
+
+  test("StringIndexer order types") {
+    val data = Seq((0, "b"), (1, "b"), (2, "c"), (3, "a"), (4, "a"), (5, "b"))
+    val df = data.toDF("id", "label")
+    val indexer = new StringIndexer()
+      .setInputCol("label")
+      .setOutputCol("labelIndex")
+
+    val expected = Seq(Set((0, 0.0), (1, 0.0), (2, 2.0), (3, 1.0), (4, 1.0), (5, 0.0)),
+      Set((0, 2.0), (1, 2.0), (2, 0.0), (3, 1.0), (4, 1.0), (5, 2.0)),
+      Set((0, 1.0), (1, 1.0), (2, 0.0), (3, 2.0), (4, 2.0), (5, 1.0)),
+      Set((0, 1.0), (1, 1.0), (2, 2.0), (3, 0.0), (4, 0.0), (5, 1.0)))
+
+    var idx = 0
+    for (orderType <- StringIndexer.supportedStringOrderType) {
+      val transformed = indexer.setStringOrderType(orderType).fit(df).transform(df)
+      val output = transformed.select("id", "labelIndex").rdd.map { r =>
+        (r.getInt(0), r.getDouble(1))
+      }.collect().toSet
+      assert(output === expected(idx))
+      idx += 1
+    }
+  }
 }