#858 discretisation of VariableDot results in StateVectorDot

Author: martinjrobins

pybamm/discretisations/discretisation.py

@@ -855,6 +855,13 @@ def _process_symbol(self, symbol):
             disc_children = [self.process_symbol(child) for child in symbol.children]
             return symbol._function_new_copy(disc_children)
 
+        elif isinstance(symbol, pybamm.VariableDot):
+            return pybamm.StateVectorDot(
+                *self.y_slices[symbol.get_variable().id],
+                domain=symbol.domain,
+                auxiliary_domains=symbol.auxiliary_domains
+            )
+
         elif isinstance(symbol, pybamm.Variable):
             # Check if variable is a standard variable or an external variable
             if any(symbol.id == var.id for var in self.external_variables.values()):

pybamm/expression_tree/variable.py

@@ -87,6 +87,18 @@ class VariableDot(Variable):
     def __init__(self, name, domain=None, auxiliary_domains=None):
         super().__init__(name, domain=domain, auxiliary_domains=auxiliary_domains)
 
+    def get_variable(self):
+        """
+        return a :class:`.Variable` corresponding to this VariableDot
+
+        Note: Variable._jac adds a dash to the name of the corresponding VariableDot, so
+        we remove this here
+
+        """
+        return Variable(self.name[:-1],
+                        domain=self._domain,
+                        auxiliary_domains=self._auxiliary_domains)
+
 
 class ExternalVariable(Variable):
     """A node in the expression tree representing an external variable variable

tests/unit/test_discretisations/test_discretisation.py

@@ -329,6 +329,14 @@ def test_process_symbol_base(self):
         var_disc = disc.process_symbol(var)
         self.assertIsInstance(var_disc, pybamm.StateVector)
         self.assertEqual(var_disc.y_slices[0], disc.y_slices[var.id][0])
+
+        # variable dot
+        var_dot = pybamm.VariableDot("var'")
+        var_vec_dot = pybamm.VariableDot("var vec'", domain=["negative electrode"])
+        var_dot_disc = disc.process_symbol(var_dot)
+        self.assertIsInstance(var_dot_disc, pybamm.StateVectorDot)
+        self.assertEqual(var_dot_disc.y_slices[0], disc.y_slices[var.id][0])
+
         # scalar
         scal = pybamm.Scalar(5)
         scal_disc = disc.process_symbol(scal)
@@ -1086,7 +1094,6 @@ def test_mass_matirx_inverse(self):
             model.mass_matrix_inv.entries.toarray(), mass_inv.toarray()
         )
 
-
 if __name__ == "__main__":
     print("Add -v for more debug output")
     import sys