Github user HyukjinKwon commented on a diff in the pull request: https://github.com/apache/spark/pull/17967#discussion_r117878731 --- Diff: mllib/src/main/scala/org/apache/spark/ml/feature/RFormula.scala --- @@ -37,6 +37,42 @@ import org.apache.spark.sql.types._ */ private[feature] trait RFormulaBase extends HasFeaturesCol with HasLabelCol { + /** + * Param for how to order categories of a string FEATURE column used by `StringIndexer`. + * The last category after ordering is dropped when encoding strings. + * Supported options: 'frequencyDesc', 'frequencyAsc', 'alphabetDesc', 'alphabetAsc'. + * The default value is 'frequencyDesc'. When the ordering is set to 'alphabetDesc', `RFormula` + * drops the same category as R when encoding strings. + * + * The options are explained using an example `'b', 'a', 'b', 'a', 'c', 'b'`: + * {{{ + * +-----------------+---------------------------------------+----------------------------------+ --- End diff -- I guess I am not supposed to make a decision call though. Please let me know @felixcheung and @yanboliang if you have any preference.
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