BUG: sparse: 'diagonal' of sparse matrices fixed to match numpy's dia…

…gonal behaviour (scipy#11970)

* BUG: sparse: Added check for size in method 'diagonal' of sparse matrices to deal with matrices of size 0

'diagonal' raise ValueError if a sparse matrix size is 0,
but equivalent function 'numpy.diagonal' return an empty array, as mentioned in issue scipy#11949.
'diagonal' check if k is less than size, but in case of an empty matrix,
k is equal to the size and ValueError rises.
Added check so if any of sizes is 0, 'diagonal' return an empty array, like 'numpy.diagonal'.

* BUG: sparse: Removed raising a ValueError from `diagonal` method and fixed type of returned array

To match the behaviour of `numpy.diagonal` replaced raising ValueError
when passed a value exceeds the number of rows or columns with returning an empty array.
Also changed dtype of a returned empty array to match dtype of a sparse matrix.

* TST: sparse: added tests to cover offset in `diagonal` greater than the size of a matrix

New test cover that and few more shapes of
an empty sparse matrix(when one of the sizes is 0 and other not)

* MAINT: Fixed typo in comments

Co-authored-by: Warren Weckesser <[email protected]>

Co-authored-by: Warren Weckesser <[email protected]>

scipy/sparse/bsr.py

@@ -296,7 +296,7 @@ def __repr__(self):
     def diagonal(self, k=0):
         rows, cols = self.shape
         if k <= -rows or k >= cols:
-            raise ValueError("k exceeds matrix dimensions")
+            return np.empty(0, dtype=self.data.dtype)
         R, C = self.blocksize
         y = np.zeros(min(rows + min(k, 0), cols - max(k, 0)),
                      dtype=upcast(self.dtype))

scipy/sparse/compressed.py

@@ -530,7 +530,7 @@ def _mul_sparse_matrix(self, other):
     def diagonal(self, k=0):
         rows, cols = self.shape
         if k <= -rows or k >= cols:
-            raise ValueError("k exceeds matrix dimensions")
+            return np.empty(0, dtype=self.data.dtype)
         fn = getattr(_sparsetools, self.format + "_diagonal")
         y = np.empty(min(rows + min(k, 0), cols - max(k, 0)),
                      dtype=upcast(self.dtype))

scipy/sparse/coo.py

@@ -456,7 +456,7 @@ def todok(self, copy=False):
     def diagonal(self, k=0):
         rows, cols = self.shape
         if k <= -rows or k >= cols:
-            raise ValueError("k exceeds matrix dimensions")
+            return np.empty(0, dtype=self.data.dtype)
         diag = np.zeros(min(rows + min(k, 0), cols - max(k, 0)),
                         dtype=self.dtype)
         diag_mask = (self.row + k) == self.col

scipy/sparse/dia.py

@@ -321,7 +321,7 @@ def transpose(self, axes=None, copy=False):
     def diagonal(self, k=0):
         rows, cols = self.shape
         if k <= -rows or k >= cols:
-            raise ValueError("k exceeds matrix dimensions")
+            return np.empty(0, dtype=self.data.dtype)
         idx, = np.nonzero(self.offsets == k)
         first_col, last_col = max(0, k), min(rows + k, cols)
         if idx.size == 0:

scipy/sparse/tests/test_base.py

@@ -724,13 +724,19 @@ def test_diagonal(self):
         for m in mats:
             rows, cols = array(m).shape
             sparse_mat = self.spmatrix(m)
-            for k in range(-rows + 1, cols):
+            for k in range(-rows-1, cols+2):
                 assert_equal(sparse_mat.diagonal(k=k), diag(m, k=k))
-            assert_raises(ValueError, sparse_mat.diagonal, -rows)
-            assert_raises(ValueError, sparse_mat.diagonal, cols)
+            # Test for k beyond boundaries(issue #11949)
+            assert_equal(sparse_mat.diagonal(k=10), diag(m, k=10))
+            assert_equal(sparse_mat.diagonal(k=-99), diag(m, k=-99))
 
         # Test all-zero matrix.
         assert_equal(self.spmatrix((40, 16130)).diagonal(), np.zeros(40))
+        # Test empty matrix
+        # https://github.com/scipy/scipy/issues/11949
+        assert_equal(self.spmatrix((0, 0)).diagonal(), np.empty(0))
+        assert_equal(self.spmatrix((15, 0)).diagonal(), np.empty(0))
+        assert_equal(self.spmatrix((0, 5)).diagonal(10), np.empty(0))
 
     def test_reshape(self):
         # This first example is taken from the lil_matrix reshaping test.