Github user sethah commented on a diff in the pull request:
https://github.com/apache/spark/pull/11601#discussion_r55550039
--- Diff: mllib/src/main/scala/org/apache/spark/ml/feature/Imputer.scala ---
@@ -0,0 +1,288 @@
+/*
+ * 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.hadoop.fs.Path
+
+import org.apache.spark.annotation.{Experimental, Since}
+import org.apache.spark.ml.{Estimator, Model}
+import org.apache.spark.ml.param.{DoubleParam, Param, ParamMap, Params}
+import org.apache.spark.ml.param.shared.{HasInputCol, HasOutputCol}
+import org.apache.spark.ml.util._
+import org.apache.spark.mllib.linalg._
+import org.apache.spark.sql.{DataFrame, Row}
+import org.apache.spark.sql.functions.{col, udf}
+import org.apache.spark.sql.types.{DoubleType, StructField, StructType}
+
+/**
+ * Params for [[Imputer]] and [[ImputerModel]].
+ */
+private[feature] trait ImputerParams extends Params with HasInputCol with
HasOutputCol {
+
+ /**
+ * The imputation strategy.
+ * If "mean", then replace missing values using the mean along the axis.
+ * If "median", then replace missing values using the median along the
axis.
+ * If "most", then replace missing using the most frequent value along
the axis.
+ * Default: mean
+ *
+ * @group param
+ */
+ val strategy: Param[String] = new Param(this, "strategy", "strategy for
imputation. " +
+ "If mean, then replace missing values using the mean along the axis." +
+ "If median, then replace missing values using the median along the
axis." +
+ "If most, then replace missing using the most frequent value along the
axis.")
--- End diff --
Could you add a param validation function since there are a limited number
of valid strategies? You can add an attribute like `val
supportedMissingValueStrategies = Set("mean", "median", "most")` to the
`Imputer` companion object like is done
[here](https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala#L50)
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