Github user sethah commented on a diff in the pull request:
https://github.com/apache/spark/pull/18513#discussion_r128786226
--- Diff:
mllib/src/main/scala/org/apache/spark/ml/feature/FeatureHasher.scala ---
@@ -0,0 +1,189 @@
+/*
+ * 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.Since
+import org.apache.spark.ml.Transformer
+import org.apache.spark.ml.attribute.AttributeGroup
+import org.apache.spark.ml.linalg.Vectors
+import org.apache.spark.ml.param.{IntParam, ParamMap, ParamValidators}
+import org.apache.spark.ml.param.shared.{HasInputCols, HasOutputCol}
+import org.apache.spark.ml.util.{DefaultParamsReadable,
DefaultParamsWritable, Identifiable, SchemaUtils}
+import org.apache.spark.mllib.feature.{HashingTF => OldHashingTF}
+import org.apache.spark.sql.{DataFrame, Dataset, Row}
+import org.apache.spark.sql.functions._
+import org.apache.spark.sql.types._
+import org.apache.spark.util.Utils
+import org.apache.spark.util.collection.OpenHashMap
+
+/**
+ * Feature hashing projects a set of categorical or numerical features
into a feature vector of
+ * specified dimension (typically substantially smaller than that of the
original feature
+ * space). This is done using the hashing trick
(https://en.wikipedia.org/wiki/Feature_hashing)
+ * to map features to indices in the feature vector.
+ *
+ * The [[FeatureHasher]] transformer operates on multiple columns. Each
column may be numeric
+ * (representing a real feature) or string (representing a categorical
feature). Boolean columns
--- End diff --
It might be good to make the behavior for each type of column clearer here.
Specifically for numeric columns that are meant to be categories. Something
like:
````scala
/**
* Behavior
* -Numeric columns: For numeric features, the hash value of the column
name is used to map the
* feature value to its index in the feature vector.
Numeric features are never
* treated as categorical, even when they are integers.
You must convert
* categorical columns to strings first.
* -String columns: ...
* -Boolean columns: ...
*/
````
Anyway, this is a very minor suggestion and I think it's also ok to leave
as is.
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