Github user yanboliang commented on a diff in the pull request: https://github.com/apache/spark/pull/17967#discussion_r117524713 --- Diff: mllib/src/main/scala/org/apache/spark/ml/feature/RFormula.scala --- @@ -37,6 +37,31 @@ import org.apache.spark.sql.types._ */ private[feature] trait RFormulaBase extends HasFeaturesCol with HasLabelCol { + /** + * Param for how to order labels of string column. The first label after ordering is assigned + * an index of 0. + * Options are: + * - 'frequencyDesc': descending order by label frequency (most frequent label assigned 0) + * - 'frequencyAsc': ascending order by label frequency (least frequent label assigned 0) + * - 'alphabetDesc': descending alphabetical order + * - 'alphabetAsc': ascending alphabetical order + * Default is 'frequencyDesc'. + * When the ordering is set to 'alphabetDesc', `RFormula` drops the same category as R + * when encoding strings. + * + * @group param + */ + @Since("2.3.0") + final val stringOrderType: Param[String] = new Param(this, "stringOrderType", --- End diff -- What about ```orderTypeOfStringIndexer``` or any other suggestion? I think the param name ```stringOrderType``` in ```StringIndexer``` is clear enough, but ```RFormula``` involves lots of feature transformers across many steps, we should make users understand this param only takes effect on ```StringIndexer``` stage. And it's better to document more clear rather than copying corresponding section from ```StringIndexer```. cc @felixcheung
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