Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/5500#discussion_r28827821
--- Diff:
mllib/src/main/scala/org/apache/spark/ml/feature/OneHotEncoder.scala ---
@@ -0,0 +1,109 @@
+/*
+ * 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.annotation.AlphaComponent
+import org.apache.spark.ml.UnaryTransformer
+import org.apache.spark.ml.attribute.NominalAttribute
+import org.apache.spark.mllib.linalg.{Vector, Vectors, VectorUDT}
+import org.apache.spark.ml.param._
+import org.apache.spark.ml.param.shared.{HasInputCol, HasOutputCol}
+import org.apache.spark.ml.util.SchemaUtils
+import org.apache.spark.sql.types.{DataType, DoubleType, StructType}
+
+/**
+ * A one-hot encoder that maps a column of label indices to a column of
binary vectors, with
+ * at most a single one-value. By default, the binary vector has an
element for each category, so
+ * with 5 categories, an input value of 2.0 would map to an output vector
of
+ * (0.0, 0.0, 1.0, 0.0, 0.0). If includeFirst is set to false, the first
category is omitted, so the
+ * output vector for the previous example would be (0.0, 1.0, 0.0, 0.0)
and an input value
+ * of 0.0 would map to a vector of all zeros. Omitting the first category
enables the vector
+ * columns to be independent.
--- End diff --
Without the first category the rest could be still dependent. We should say
"Including the first category would make the vector columns linearly dependent
because they sum up to one."
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