Github user MLnick commented on a diff in the pull request:

    https://github.com/apache/spark/pull/11553#discussion_r59172573
  
    --- Diff: 
mllib/src/test/scala/org/apache/spark/ml/feature/QuantileDiscretizerSuite.scala 
---
    @@ -17,78 +17,59 @@
     
     package org.apache.spark.ml.feature
     
    -import org.apache.spark.{SparkContext, SparkFunSuite}
    -import org.apache.spark.ml.attribute.{Attribute, NominalAttribute}
    +import org.apache.spark.SparkFunSuite
     import org.apache.spark.ml.util.DefaultReadWriteTest
     import org.apache.spark.mllib.util.MLlibTestSparkContext
    -import org.apache.spark.sql.{Row, SQLContext}
    +import org.apache.spark.sql.SQLContext
     
     class QuantileDiscretizerSuite
       extends SparkFunSuite with MLlibTestSparkContext with 
DefaultReadWriteTest {
     
    -  import org.apache.spark.ml.feature.QuantileDiscretizerSuite._
    -
    -  test("Test quantile discretizer") {
    -    checkDiscretizedData(sc,
    -      Array[Double](1, 2, 3, 3, 3, 3, 3, 3, 3),
    -      10,
    -      Array[Double](1, 2, 3, 3, 3, 3, 3, 3, 3),
    -      Array("-Infinity, 1.0", "1.0, 2.0", "2.0, 3.0", "3.0, Infinity"))
    -
    -    checkDiscretizedData(sc,
    -      Array[Double](1, 2, 3, 3, 3, 3, 3, 3, 3),
    -      4,
    -      Array[Double](1, 2, 3, 3, 3, 3, 3, 3, 3),
    -      Array("-Infinity, 1.0", "1.0, 2.0", "2.0, 3.0", "3.0, Infinity"))
    -
    -    checkDiscretizedData(sc,
    -      Array[Double](1, 2, 3, 3, 3, 3, 3, 3, 3),
    -      3,
    -      Array[Double](0, 1, 2, 2, 2, 2, 2, 2, 2),
    -      Array("-Infinity, 2.0", "2.0, 3.0", "3.0, Infinity"))
    +  test("Test observed number of buckets and their sizes match expected 
values") {
    +    val sqlCtx = SQLContext.getOrCreate(sc)
    +    import sqlCtx.implicits._
     
    -    checkDiscretizedData(sc,
    -      Array[Double](1, 2, 3, 3, 3, 3, 3, 3, 3),
    -      2,
    -      Array[Double](0, 1, 1, 1, 1, 1, 1, 1, 1),
    -      Array("-Infinity, 2.0", "2.0, Infinity"))
    +    val datasetSize = 100000
    +    val numBuckets = 5
    +    val df = sc.parallelize(1.0 to datasetSize by 
1.0).map(Tuple1.apply).toDF("input")
    +    val discretizer = new QuantileDiscretizer()
    +      .setInputCol("input")
    +      .setOutputCol("result")
    +      .setNumBuckets(numBuckets)
    +    val result = discretizer.fit(df).transform(df)
     
    -  }
    +    val observedNumBuckets = result.select("result").distinct.count
    +    assert(observedNumBuckets === numBuckets,
    +      "Observed number of buckets does not equal expected number of 
buckets.")
     
    -  test("Test getting splits") {
    -    val splitTestPoints = Array(
    -      Array[Double]() -> Array(Double.NegativeInfinity, 0, 
Double.PositiveInfinity),
    -      Array(Double.NegativeInfinity) -> Array(Double.NegativeInfinity, 0, 
Double.PositiveInfinity),
    -      Array(Double.PositiveInfinity) -> Array(Double.NegativeInfinity, 0, 
Double.PositiveInfinity),
    -      Array(Double.NegativeInfinity, Double.PositiveInfinity)
    -        -> Array(Double.NegativeInfinity, 0, Double.PositiveInfinity),
    -      Array(0.0) -> Array(Double.NegativeInfinity, 0, 
Double.PositiveInfinity),
    -      Array(1.0) -> Array(Double.NegativeInfinity, 1, 
Double.PositiveInfinity),
    -      Array(0.0, 1.0) -> Array(Double.NegativeInfinity, 0, 1, 
Double.PositiveInfinity)
    -    )
    -    for ((ori, res) <- splitTestPoints) {
    -      assert(QuantileDiscretizer.getSplits(ori) === res, "Returned splits 
are invalid.")
    +    val relativeError = discretizer.getRelativeError
    +    val isGoodBucket = org.apache.spark.sql.functions.udf {
    +      (size: Int) => math.abs( size - (datasetSize / numBuckets)) <= 
(relativeError * datasetSize)
         }
    +    val numGoodBuckets = 
result.groupBy("result").count.filter(isGoodBucket($"count")).count
    +    assert(numGoodBuckets === numBuckets,
    +      "Bucket sizes are not as expected.")
    --- End diff --
    
    Could we make this a bit more clear, like `Bucket sizes are not within 
expected relative error tolerance` or something similar


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