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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