Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/4735#discussion_r27625124
--- Diff:
mllib/src/main/scala/org/apache/spark/ml/feature/LabelIndexer.scala ---
@@ -0,0 +1,126 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.ml.feature
+
+import org.apache.spark.SparkException
+import org.apache.spark.annotation.AlphaComponent
+import org.apache.spark.ml.{Estimator, Model}
+import org.apache.spark.ml.attribute.NominalAttribute
+import org.apache.spark.ml.param._
+import org.apache.spark.sql.DataFrame
+import org.apache.spark.sql.functions._
+import org.apache.spark.sql.types.{StringType, StructType}
+import org.apache.spark.util.collection.OpenHashMap
+
+/**
+ * Base trait for [[LabelIndexer]] and [[LabelIndexerModel]].
+ */
+private[feature] trait LabelIndexerBase extends Params with HasLabelCol
with HasOutputCol {
+
+ /** Validates and transforms the input schema. */
+ protected def validateAndTransformSchema(schema: StructType, paramMap:
ParamMap): StructType = {
+ val map = this.paramMap ++ paramMap
+ val labelType = schema(map(labelCol)).dataType
+ require(labelType == StringType, s"The label column must be
string-typed but got $labelType.")
+ val inputFields = schema.fields
+ val outputColName = map(outputCol)
+ require(inputFields.forall(_.name != outputColName),
+ s"Output column $outputColName already exists.")
+ val attr = NominalAttribute.defaultAttr.withName(map(outputCol))
+ val outputFields = inputFields :+ attr.toStructField()
+ StructType(outputFields)
+ }
+}
+
+/**
+ * :: AlphaComponent ::
+ * A label indexer that maps a string column of labels to an ML column of
label indices.
+ * The indices are in [0, numLabels), ordered by label frequencies.
+ * So the most frequent label gets index 0.
+ */
+@AlphaComponent
+class LabelIndexer extends Estimator[LabelIndexerModel] with
LabelIndexerBase {
+
+ /** @group setParam */
+ def setLabelCol(value: String): this.type = set(labelCol, value)
--- End diff --
I will rename `labelCol` to `inputCol`. Feel free to suggest better class
names. I'm okay with not calling it `LabelIndexer`. How about `StringToNominal`?
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