BUG: small patch to _VarArray while inputting a matrix with length > 1 (#1060)

* Fix _VarArray size initialization logic

* Add test for knapsack constraint with matrix variables

---------

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

Author: Zeroto521

src/pyscipopt/scip.pxi

@@ -2509,8 +2509,8 @@ cdef class _VarArray:
         else:
             raise TypeError(f"Expected Variable or list of Variable, got {type(vars)}.")
 
-        if vars:
-            self.size = len(vars)
+        self.size = len(vars)
+        if self.size:
             self.ptr = <SCIP_VAR**> malloc(self.size * sizeof(SCIP_VAR*))
             for i, var in enumerate(vars):
                 if not isinstance(var, Variable):

tests/test_cons.py

@@ -210,6 +210,17 @@ def test_cons_indicator_with_matrix_binvar():
 
     assert m.isEQ(m.getVal(x), 1)
 
+def test_cons_knapsack_with_matrix_vars():
+    # test matrix variable vars #1043
+    m = Model()
+    vars = m.addMatrixVar(3, vtype="B")
+    m.addConsKnapsack(vars, [1, 2, 3], 5)
+
+    m.setObjective(vars.sum(), "maximize")
+    m.optimize()
+
+    assert (m.getVal(vars) == [0, 1, 1]).all()
+
 @pytest.mark.xfail(
     reason="addConsIndicator doesn't behave as expected when binary variable is False. See Issue #717."
 )