Github user jkbradley commented on a diff in the pull request:
https://github.com/apache/spark/pull/10166#discussion_r46906261
--- Diff: docs/ml-features.md ---
@@ -951,9 +951,157 @@ indexed.show()
</div>
</div>
+
+## IndexToString
+
+Symmetrically to `StringIndexer`, `IndexToString` maps a column of label
indices
+back to a column containing the original labels as strings. The common use
case
+is to produce indices from labels with `StringIndexer`, train a model with
those
+indices and retrieve the original labels from the column of predicted
indices
+with `IndexToString`. However, you are free to supply your own labels.
+
+**Examples**
+
+Building on the `StringIndexer` example, let's assume we have the following
+DataFrame with columns `id` and `categoryIndex`:
+
+~~~~
+ id | categoryIndex
+----|---------------
+ 0 | 0.0
+ 1 | 2.0
+ 2 | 1.0
+ 3 | 0.0
+ 4 | 0.0
+ 5 | 1.0
+~~~~
+
+Applying `IndexToString` with `categoryIndex` as the input column,
+`originalCategory` as the output column and the previous `StringIndexer`'s
+labels as labels, we are able to retrieve our original labels:
+
+~~~~
+ id | categoryIndex | originalCategory
+----|---------------|-----------------
+ 0 | 0.0 | a
+ 1 | 2.0 | b
+ 2 | 1.0 | c
+ 3 | 0.0 | a
+ 4 | 0.0 | a
+ 5 | 1.0 | c
+~~~~
+
+<div class="codetabs">
+<div data-lang="scala" markdown="1">
+
+Refer to the [IndexToString Scala
docs](api/scala/index.html#org.apache.spark.ml.feature.IndexToString)
+for more details on the API.
+
+{% highlight scala %}
+import org.apache.spark.ml.feature.{IndexToString, StringIndexer}
+
+val df = sqlContext.createDataFrame(Seq(
+ (0, "a"),
+ (1, "b"),
+ (2, "c"),
+ (3, "a"),
+ (4, "a"),
+ (5, "c")
+)).toDF("id", "category")
+
+val indexer = new StringIndexer()
+ .setInputCol("category")
+ .setOutputCol("categoryIndex")
+ .fit(df)
+val indexed = indexer.transform(df)
+
+val converter = new IndexToString()
+ .setInputCol("categoryIndex")
+ .setOutputCol("originalCategory")
+ .setLabels(indexer.labels)
--- End diff --
You probably don't need to specify labels; they should be pulled from
column metadata.
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