Github user holdenk commented on a diff in the pull request:
https://github.com/apache/spark/pull/21942#discussion_r216022250
--- Diff:
mllib/src/main/scala/org/apache/spark/ml/feature/StandardScaler.scala ---
@@ -160,15 +160,88 @@ class StandardScalerModel private[ml] (
@Since("2.0.0")
override def transform(dataset: Dataset[_]): DataFrame = {
transformSchema(dataset.schema, logging = true)
- val scaler = new feature.StandardScalerModel(std, mean, $(withStd),
$(withMean))
-
- // TODO: Make the transformer natively in ml framework to avoid extra
conversion.
- val transformer: Vector => Vector = v =>
scaler.transform(OldVectors.fromML(v)).asML
+ val transformer: Vector => Vector = v => transform(v)
val scale = udf(transformer)
dataset.withColumn($(outputCol), scale(col($(inputCol))))
}
+ /**
+ * Since `shift` will be only used in `withMean` branch, we have it as
+ * `lazy val` so it will be evaluated in that branch. Note that we don't
+ * want to create this array multiple times in `transform` function.
+ */
+ private lazy val shift: Array[Double] = mean.toArray
+
+ /**
+ * Applies standardization transformation on a vector.
+ *
+ * @param vector Vector to be standardized.
+ * @return Standardized vector. If the std of a column is zero, it will
return default `0.0`
+ * for the column with zero std.
+ */
+ private[spark] def transform(vector: Vector): Vector = {
+ require(mean.size == vector.size)
+ if ($(withMean)) {
+ /**
+ * By default, Scala generates Java methods for member variables. So
every time
+ * member variables are accessed, `invokespecial` is called. This is
an expensive
+ * operation, and can be avoided by having a local reference of
`shift`.
+ */
+ val localShift = shift
+ /** Must have a copy of the values since they will be modified in
place. */
+ val values = vector match {
+ /** Handle DenseVector specially because its `toArray` method does
not clone values. */
+ case d: DenseVector => d.values.clone()
+ case v: Vector => v.toArray
+ }
+ val size = values.length
+ if ($(withStd)) {
+ var i = 0
+ while (i < size) {
+ values(i) = if (std(i) != 0.0) (values(i) - localShift(i)) *
(1.0 / std(i)) else 0.0
+ i += 1
+ }
+ } else {
+ var i = 0
+ while (i < size) {
+ values(i) -= localShift(i)
+ i += 1
+ }
+ }
+ Vectors.dense(values)
+ } else if ($(withStd)) {
--- End diff --
Maybe leave a comment withStd and not mean since when tracing the code by
hand the nested if/else if can get a bit confusing flow wise.
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