[SPARK-7446] [MLLIB] Add inverse transform for string indexer

It is useful to convert the encoded indices back to their string representation for result inspection. We can add a function which creates an inverse transformation.

Author: Holden Karau <holden@pigscanfly.ca>

Closes #6339 from holdenk/SPARK-7446-inverse-transform-for-string-indexer and squashes the following commits:

7cdf915 [Holden Karau] scala style comment fix
b9cffb6 [Holden Karau] Update the labels param to have the metadata note
6a38edb [Holden Karau] Setting the default needs to come after the value gets defined
9e241d8 [Holden Karau] use Array.empty
21c8cfa [Holden Karau] Merge branch 'master' into SPARK-7446-inverse-transform-for-string-indexer
64dd3a3 [Holden Karau] Merge branch 'master' into SPARK-7446-inverse-transform-for-string-indexer
4f06c59 [Holden Karau] Fix comment styles, use empty array as the default, etc.
a60c0e3 [Holden Karau] CR feedback (remove old constructor, add a note about use of setLabels)
1987b95 [Holden Karau] Use default copy
71e8d66 [Holden Karau] Make labels a local param for StringIndexerInverse
8450d0b [Holden Karau] Use the labels param in StringIndexerInverse
7464019 [Holden Karau] Add a labels param
868b1a9 [Holden Karau] Update scaladoc since we don't have labelsCol anymore
5aa38bf [Holden Karau] Add an inverse test using only meta data, pass labels when calling inverse method
f3e0c64 [Holden Karau] CR feedback
ebed932 [Holden Karau] Add Experimental tag and some scaladocs. Also don't require that the inputCol has the metadata on it, instead have the labelsCol specified when creating the inverse.
03ebf95 [Holden Karau] Add explicit type for invert function
ecc65e0 [Holden Karau] Read the metadata correctly, use the array, pass the test
a42d773 [Holden Karau] Fix test to supply cols as per new invert method
16cc3c3 [Holden Karau] Add an invert method
d4bcb20 [Holden Karau] Make the inverse string indexer into a transformer (still needs test updates but compiles)
e8bf3ad [Holden Karau] Merge branch 'master' into SPARK-7446-inverse-transform-for-string-indexer
c3fdee1 [Holden Karau] Some WIP refactoring based on jkbradley's CR feedback. Definite work-in-progress
557bef8 [Holden Karau] Instead of using a private inverse transform, add an invert function so we can use it in a pipeline
88779c1 [Holden Karau] fix long line
78b28c1 [Holden Karau] Finish reverse part and add a test :)
bb16a6a [Holden Karau] Some progress

1 files changed, 13 insertions, 0 deletions

diff --git a/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala b/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala
index 99f82bea42..d0295a0fe2 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala
@@ -47,6 +47,19 @@ class StringIndexerSuite extends SparkFunSuite with MLlibTestSparkContext {
     // a -> 0, b -> 2, c -> 1
     val expected = Set((0, 0.0), (1, 2.0), (2, 1.0), (3, 0.0), (4, 0.0), (5, 1.0))
     assert(output === expected)
+    // convert reverse our transform
+    val reversed = indexer.invert("labelIndex", "label2")
+      .transform(transformed)
+      .select("id", "label2")
+    assert(df.collect().map(r => (r.getInt(0), r.getString(1))).toSet ===
+      reversed.collect().map(r => (r.getInt(0), r.getString(1))).toSet)
+    // Check invert using only metadata
+    val inverse2 = new StringIndexerInverse()
+      .setInputCol("labelIndex")
+      .setOutputCol("label2")
+    val reversed2 = inverse2.transform(transformed).select("id", "label2")
+    assert(df.collect().map(r => (r.getInt(0), r.getString(1))).toSet ===
+      reversed2.collect().map(r => (r.getInt(0), r.getString(1))).toSet)
   }
 
   test("StringIndexer with a numeric input column") {