Possible fix for taking scalar derivatives of vector coefficients usi…

…ng coefficient derivative maps (#124)

* A possible fix for taking the derivative of vector coefficients w.r.t. a scalar.

Also adding a test.

* A slightly neater version that handles scalar derivatives of vectors.

---------

Co-authored-by: Matthew Scroggs <[email protected]>

test/test_derivative.py

@@ -483,6 +483,30 @@ def test_coefficient_derivatives(self):
     self.assertEqual(replace(actual, fd.function_replace_map), expected)
 
 
+def test_vector_coefficient_scalar_derivatives(self):
+    V = FiniteElement("Lagrange", triangle, 1)
+    VV = VectorElement("Lagrange", triangle, 1)
+
+    dv = TestFunction(V)
+
+    df = Coefficient(VV, count=0)
+    g = Coefficient(VV, count=1)
+    f = Coefficient(VV, count=2)
+    u = Coefficient(V, count=3)
+    cd = {f: df}
+
+    integrand = inner(f, g)
+
+    i0, i1, i2, i3, i4 = [Index(count=c) for c in range(5)]
+    expected = as_tensor(df[i1]*dv, (i1,))[i0]*g[i0]
+
+    F = integrand*dx
+    J = derivative(F, u, dv, cd)
+    fd = compute_form_data(J)
+    actual = fd.preprocessed_form.integrals()[0].integrand()
+    assert (actual*dx).signature() == (expected*dx).signature()
+
+
 def test_vector_coefficient_derivatives(self):
     V = VectorElement("Lagrange", triangle, 1)
     VV = TensorElement("Lagrange", triangle, 1)

ufl/algorithms/apply_derivatives.py

@@ -835,9 +835,9 @@ def coefficient(self, o):
             dosum = Zero(o.ufl_shape)
             for do, v in zip(dos, self._v):
                 so, oi = as_scalar(do)
-                rv = len(v.ufl_shape)
-                oi1 = oi[:-rv]
-                oi2 = oi[-rv:]
+                rv = len(oi) - len(v.ufl_shape)
+                oi1 = oi[:rv]
+                oi2 = oi[rv:]
                 prod = so * v[oi2]
                 if oi1:
                     dosum += as_tensor(prod, oi1)