BUG: change the type of @ result from MatrixVariable to MatrixExpr (#1059)

* Add __matmul__ support to MatrixExpr

Implements the __matmul__ method for MatrixExpr, enabling matrix multiplication using the @ operator and ensuring the result is returned as a MatrixExpr instance.

* Add tests for matrix matmul return types

Adds tests to verify that matrix multiplication returns MatrixExpr instead of MatrixVariable for various input shapes.

* Use type() checks instead of isinstance in matmul tests

Replaces isinstance checks with type() comparisons for MatrixExpr in matrix matmul return type tests to ensure exact type matching.

* Fix matrix variable shapes in matmul test

Corrects the shapes of matrix variables in test_matrix_matmul_return_type to ensure proper 2D matrix multiplication and type assertion.

* Update CHANGELOG.md

---------

Co-authored-by: João Dionísio <57299939+Joao-Dionisio@users.noreply.github.com>

Author: Zeroto521

CHANGELOG.md

@@ -24,6 +24,7 @@
 ### Fixed
 - Raised an error when an expression is used when a variable is required
 - Fixed some compile warnings
+- Fixed the type of @ matrix operation result from MatrixVariable to MatrixExpr.
 ### Changed
 - MatrixExpr.sum() now supports axis arguments and can return either a scalar or MatrixExpr, depending on the result dimensions.
 - AddMatrixCons() also accepts ExprCons.

src/pyscipopt/matrix.pxi

@@ -98,7 +98,10 @@ class MatrixExpr(np.ndarray):
 
     def __rsub__(self, other):
         return super().__rsub__(other).view(MatrixExpr)
-    
+
+    def __matmul__(self, other):
+        return super().__matmul__(other).view(MatrixExpr)
+
 class MatrixGenExpr(MatrixExpr):
     pass
 

tests/test_matrix_variable.py

@@ -392,3 +392,20 @@ def test_matrix_cons_indicator():
     assert m.getVal(is_equal).sum() == 2
     assert (m.getVal(x) == m.getVal(y)).all().all()
     assert (m.getVal(x) == np.array([[5, 5, 5], [5, 5, 5]])).all().all()
+
+
+def test_matrix_matmul_return_type():
+    # test #1058, require returning type is MatrixExpr not MatrixVariable
+    m = Model()
+
+    # test 1D @ 1D → 0D
+    x = m.addMatrixVar(3)
+    assert type(x @ x) is MatrixExpr
+
+    # test 1D @ 1D → 2D
+    assert type(x[:, None] @ x[None, :]) is MatrixExpr
+
+    # test 2D @ 2D → 2D
+    y = m.addMatrixVar((2, 3))
+    z = m.addMatrixVar((3, 4))
+    assert type(y @ z) is MatrixExpr