Github user brkyvz commented on a diff in the pull request:
https://github.com/apache/spark/pull/4109#discussion_r23573354
--- Diff: mllib/src/main/scala/org/apache/spark/mllib/linalg/Matrices.scala
---
@@ -92,6 +84,16 @@ sealed trait Matrix extends Serializable {
* backing array. For example, an operation such as addition or
subtraction will only be
* performed on the non-zero values in a `SparseMatrix`. */
private[mllib] def update(f: Double => Double): Matrix
+
+ /**
+ * Applies a function `f` to all the active elements of dense and sparse
matrix.
+ *
+ * @param f the function takes three parameters where the first two
parameters are the row
+ * and column indices respectively with the type `Int`, the
third parameter is
+ * the corresponding array index, and the final parameter is
the corresponding
--- End diff --
It is used especially for sparse matrices. For example in `horzcat` and
`vertcat`, we already know how long the COO representation should be and we
allocate an `Array` instead of an `ArrayBuffer`. There, to index `Array`
correctly, we need `index`. It's not a bad thing that we can supply the `index`
of the underlying storage.
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