[SPARK-5886][ML] Add StringIndexer as a feature transformer

This PR adds string indexer, which takes a column of string labels and outputs a double column with labels indexed by their frequency.

TODOs:
- [x] store feature to index map in output metadata

Author: Xiangrui Meng <meng@databricks.com>

Closes #4735 from mengxr/SPARK-5886 and squashes the following commits:

d82575f [Xiangrui Meng] fix test
700e70f [Xiangrui Meng] rename LabelIndexer to StringIndexer
16a6f8c [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-5886
457166e [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-5886
f8b30f4 [Xiangrui Meng] update label indexer to output metadata
e81ec28 [Xiangrui Meng] Merge branch 'openhashmap-contains' into SPARK-5886-2
d6e6f1f [Xiangrui Meng] add contains to primitivekeyopenhashmap
748a69b [Xiangrui Meng] add contains to OpenHashMap
def3c5c [Xiangrui Meng] add LabelIndexer

1 files changed, 52 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
new file mode 100644
index 0000000000..00b5d094d8
--- /dev/null
+++ b/mllib/src/test/scala/org/apache/spark/ml/feature/StringIndexerSuite.scala
@@ -0,0 +1,52 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.ml.feature
+
+import org.scalatest.FunSuite
+
+import org.apache.spark.ml.attribute.{Attribute, NominalAttribute}
+import org.apache.spark.mllib.util.MLlibTestSparkContext
+import org.apache.spark.sql.SQLContext
+
+class StringIndexerSuite extends FunSuite with MLlibTestSparkContext {
+  private var sqlContext: SQLContext = _
+
+  override def beforeAll(): Unit = {
+    super.beforeAll()
+    sqlContext = new SQLContext(sc)
+  }
+
+  test("StringIndexer") {
+    val data = sc.parallelize(Seq((0, "a"), (1, "b"), (2, "c"), (3, "a"), (4, "a"), (5, "c")), 2)
+    val df = sqlContext.createDataFrame(data).toDF("id", "label")
+    val indexer = new StringIndexer()
+      .setInputCol("label")
+      .setOutputCol("labelIndex")
+      .fit(df)
+    val transformed = indexer.transform(df)
+    val attr = Attribute.fromStructField(transformed.schema("labelIndex"))
+      .asInstanceOf[NominalAttribute]
+    assert(attr.values.get === Array("a", "c", "b"))
+    val output = transformed.select("id", "labelIndex").map { r =>
+      (r.getInt(0), r.getDouble(1))
+    }.collect().toSet
+    // 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)
+  }
+}