You could use:
auto mask = fe.component_mask(extractor);
auto indices = DoFTools::extract_dofs(dh, mask);
for(const auti i: indices)
if (!constraints.is_constrained(i))
…
https://www.dealii.org/current/doxygen/deal.II/namespaceDoFTools.html#a45f4d01f1c4c6337e4be6f10a81fbdab
Best,
Luca.
> On 29 Apr 2025, at 10:08, Alex <[email protected]> wrote:
>
> Dear deal.II users,
> I'm working on a monolithic coupled problem using deal.II where I'm solving
> for multiple variables (e.g., displacement u, concentration c, etc.) using a
> single DoFHandler and a full matrix system (i.e., not using block
> structures). I use FEValuesExtractors to access individual components.
> I want to compute the error residual (e.g., L2 norm) for a specific component
> only, such as displacement, by extracting only the relevant DOFs from the
> global residual vector system_rhs.
>
> Here is the simplified version of what I’m doing:
> Vector<double> system_rhs; // UMFPACK
> Vector<double> error_res(dof_handler.n_dofs());
> for (unsigned int i = 0; i < dof_handler.n_dofs(); ++i)
> if (!constraints.is_constrained(i))
> error_res(i) = system_rhs(i);
> error_residual.norm = error_res.l2_norm();
>
> This computes the total residual, but how can I correctly isolate and compute
> the residual only for the displacement component (or any other specific
> field).
> Thanks in advance!
>
> Regards,
> Alex
>
>
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