pratyushadk opened a new pull request, #50907:
URL: https://github.com/apache/arrow/pull/50907
### Rationale for this change
Calling `SparseCSRMatrix::Make` or `SparseCSCMatrix::Make` on a 1D dense
tensor currently returns a `NotImplemented` error ("TODO for ndim <= 1"). The
CSR/CSC representation for a 1D vector is well-defined though: it is equivalent
to a single-row sparse matrix where `indptr = [0, nnz]`, `indices` holds the
positions of non-zero values, and `values` holds the corresponding values. The
COO format already handles 1D tensors, so this fills a straightforward gap.
### What changes are included in this PR?
`cpp/src/arrow/tensor/csx_converter.cc`
The `Convert()` method in `SparseCSXMatrixConverter` now handles three cases
cleanly. `ndim == 0` returns `Status::Invalid` since scalars have no meaningful
sparse representation. `ndim == 1` takes a new path that does a single linear
scan of the vector, writes `indptr = [0, nnz]`, and records the positions and
values of non-zeros. `ndim == 2` is the existing path and is completely
unchanged.
The reverse path (`MakeTensorFromSparseCSXMatrix`) had a hardcoded
`shape[1]` access which would be out of bounds for a 1D tensor. That is fixed
to use `shape.size() > 1 ? shape[1] : shape[0]`.
`cpp/src/arrow/sparse_tensor_test.cc`
Three new tests added to the existing `TestSparseCSRMatrix` fixture covering
the 1D case.
### Are these changes tested?
Yes. Three new tests cover the main conversion path, the all-zero edge case,
and a full round-trip from dense to sparse and back. All 153 tests in the
sparse tensor test suite passed locally with no regressions.
### Are there any user-facing changes?
`SparseCSRMatrix::Make` and `SparseCSCMatrix::Make` now accept 1D tensors
instead of returning `NotImplemented`. There are no breaking changes.
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