How to constraint only one component of the vector field using boundary conditions?

Question

Is there a way to constraint only say the 3rd component of a vector field at the boundary?
For example, I can fix all the components of a 3-dimensional vector field by doing :

bc1 = DirichletBC(V, (0, 0 ,0), boundary)

But is there a way to say constraint only the 3rd component and not the other components ?

Thanks!!

Comments

Regarding this, the V1.5 interface has changed slightly so this would be

std::vector< std::size_t > sub{2};
bc1 = DirichletBC(*(V.extract_sub_space(sub)), Constant(0), boundary);

Answer 1

Try

bc1 = DirichletBC(V.sub(2), Constant(0), boundary)