Github user yanboliang commented on a diff in the pull request:
https://github.com/apache/spark/pull/17967#discussion_r117753728
--- 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'`:
+ * {{{
+ *
+-----------------+---------------------------------------+----------------------------------+
+ * | Option | Category mapped to 0 by StringIndexer | Category
dropped by RFormula |
+ *
+-----------------+---------------------------------------+----------------------------------+
+ * | 'frequencyDesc' | most frequent category ('b') | least
frequent category ('c') |
+ * | 'frequencyAsc' | least frequent category ('c') | most
frequent category ('b') |
+ * | 'alphabetDesc' | first alphabetical category ('a') | last
alphabetical category ('c') |
+ * | 'alphabetAsc' | last alphabetical category ('c') | first
alphabetical category ('a')|
--- End diff --
I think ```ascending``` is going up i.e. A-Z, ```descending``` is going
down i.e. Z-A. So in this example, first alphabetical category is "c" if order
by ```alphabetDesc```, first alphabetical category is "a" if order by
```alphabetAsc```. Please correct me if I have misunderstand.
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