Github user sethah commented on a diff in the pull request: https://github.com/apache/spark/pull/15628#discussion_r107689306 --- Diff: mllib-local/src/main/scala/org/apache/spark/ml/linalg/Matrices.scala --- @@ -291,31 +396,60 @@ class DenseMatrix @Since("2.0.0") ( override def numActives: Int = values.length /** - * Generate a `SparseMatrix` from the given `DenseMatrix`. The new matrix will have isTransposed - * set to false. + * Generate a `SparseMatrix` from the given `DenseMatrix`. + * + * @param colMajor Whether the resulting `SparseMatrix` values will be in column major order. */ - @Since("2.0.0") - def toSparse: SparseMatrix = { - val spVals: MArrayBuilder[Double] = new MArrayBuilder.ofDouble - val colPtrs: Array[Int] = new Array[Int](numCols + 1) - val rowIndices: MArrayBuilder[Int] = new MArrayBuilder.ofInt - var nnz = 0 - var j = 0 - while (j < numCols) { - var i = 0 - while (i < numRows) { - val v = values(index(i, j)) - if (v != 0.0) { - rowIndices += i - spVals += v - nnz += 1 + private[ml] override def toSparseMatrix(colMajor: Boolean): SparseMatrix = { + if (!colMajor) this.transpose.toSparseMatrix(colMajor = true).transpose + else { + val spVals: MArrayBuilder[Double] = new MArrayBuilder.ofDouble + val colPtrs: Array[Int] = new Array[Int](numCols + 1) + val rowIndices: MArrayBuilder[Int] = new MArrayBuilder.ofInt + var nnz = 0 + var j = 0 + while (j < numCols) { + var i = 0 + while (i < numRows) { + val v = values(index(i, j)) + if (v != 0.0) { + rowIndices += i + spVals += v + nnz += 1 + } + i += 1 } - i += 1 + j += 1 + colPtrs(j) = nnz } - j += 1 - colPtrs(j) = nnz + new SparseMatrix(numRows, numCols, colPtrs, rowIndices.result(), spVals.result()) + } + } + + /** + * Generate a `DenseMatrix` from this `DenseMatrix`. + * + * @param colMajor Whether the resulting `DenseMatrix` values will be in column major order. + */ + private[ml] override def toDenseMatrix(colMajor: Boolean): DenseMatrix = { + if (!(isTransposed ^ colMajor)) { + val newValues = new Array[Double](numCols * numRows) + var j = 0 --- End diff -- I don't see how there is an unnecessary copy. `toArray` copies the elements of the current matrix to a new Array, then uses that as the backing array of a new `DenseMatrix`. We cannot modify the original matrix values.
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