# Vector field within subdomains **URL:** \ **Category:** Errors\ **Created:** [April 25, 2022, 2:01pm UTC](https://fenicsproject.discourse.group/t/vector-field-within-subdomains/8142 "2022-04-25T14:01:23Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![jeannafta](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/jeannafta/32/3563_2.png) [@jeannafta](https://fenicsproject.discourse.group/u/jeannafta)\ **Post date:** [April 25, 2022, 2:01pm UTC](https://fenicsproject.discourse.group/t/vector-field-within-subdomains/8142/1 "2022-04-25T14:01:23Z")
Hello, I am working on a mesh made up of two sub-domains. I would like to define a vector field on the whole domain such as: q = [0, 0] on subdomain 1 and q = [5, 0] on subdomain 2. I have tried to implement this in the following way but I have an error, and obviously the way I am trying to do it is not appropriate. ```auto Q = FunctionSpace(mesh, ("DG", 0)) q = Function(Q) subdomain1 = (cell_tags.values == 5) q.x.array[subdomain1, :] = np.full((subdomain1.sum(), 2), [0, 0]) subdomain2 = (cell_tags.values == 6) q.x.array[subdomain2, :] = np.full((subdomain2.sum(), 2), [5, 0]) ``` The error is: ```auto --------------------------------------------------------------------------- IndexError Traceback (most recent call last) Input In [17], in () 3 q = Function(Q) 4 subdomain1 = (cell_tags.values == 5) ----> 5 q.x.array[subdomain1, :] = np.full((subdomain1.sum(), 2), [0, 0]) 6 subdomain2 = (cell_tags.values == 6) 7 q.x.array[subdomain2, :] = np.full((subdomain2.sum(), 2), [5, 0]) IndexError: too many indices for array: array is 1-dimensional, but 2 were indexed ``` Do you have any idea how to do this? Thanks a lot! ---
**Author:** ![RaphaelC](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/raphaelc/32/3394_2.png) [@RaphaelC](https://fenicsproject.discourse.group/u/RaphaelC)\ **Post date:** [April 26, 2022, 2:38pm UTC](https://fenicsproject.discourse.group/t/vector-field-within-subdomains/8142/2 "2022-04-26T14:38:49Z")
You should give a look at [Defining subdomains for different materials — FEniCSx tutorial](https://jorgensd.github.io/dolfinx-tutorial/chapter3/subdomains.html) You have replace `subdomain1.sum()` with `len(subdomain1)` ---
**Author:** ![jeannafta](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/jeannafta/32/3563_2.png) [@jeannafta](https://fenicsproject.discourse.group/u/jeannafta)\ **Post date:** [April 29, 2022, 3:30pm UTC](https://fenicsproject.discourse.group/t/vector-field-within-subdomains/8142/3 "2022-04-29T15:30:13Z")
Hello, Thank you for your reply @RaphaelC, but unfortunately I don’t think that’s where the problem lies. Indeed, even after modifying the code as suggested: ```auto Q = FunctionSpace(mesh, ("DG", 0)) q = Function(Q) subdomain1 = cell_tags.indices[cell_tags.values==5] q.x.array[subdomain1] = np.full((len(subdomain1),2), [0, 0]) subdomain2 = cell_tags.indices[cell_tags.values==6] q.x.array[subdomain2] = np.full((len(subdomain2),2), [5, 0]) ``` I still have an error: ```auto --------------------------------------------------------------------------- ValueError Traceback (most recent call last) Input In [39], in () 3 q = Function(Q) 4 subdomain1 = cell_tags.indices[cell_tags.values==5] ----> 5 q.x.array[subdomain1] = np.full((len(subdomain1),2), [0, 0]) 6 subdomain2 = cell_tags.indices[cell_tags.values==6] 7 q.x.array[subdomain2] = np.full((len(subdomain2),2), [5, 0]) ValueError: shape mismatch: value array of shape (5336,2) could not be broadcast to indexing result of shape (5336,) ``` The problem comes from the fact that q.x.array is a vector and not a two-column matrix, so it is not possible to define a vector field in this way. Do you have any other ideas for solving this problem? Thank you! ---
**Author:** ![RaphaelC](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/raphaelc/32/3394_2.png) [@RaphaelC](https://fenicsproject.discourse.group/u/RaphaelC)\ **Post date:** [April 29, 2022, 3:52pm UTC](https://fenicsproject.discourse.group/t/vector-field-within-subdomains/8142/4 "2022-04-29T15:52:44Z")
Oh, yes, you are matching a float against a vector . Either Q is a `VectorFunctionSpace` not a `FunctionSpace` or tag should be float. ---
**Author:** ![jeannafta](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/jeannafta/32/3563_2.png) [@jeannafta](https://fenicsproject.discourse.group/u/jeannafta)\ **Post date:** [April 30, 2022, 11:14am UTC](https://fenicsproject.discourse.group/t/vector-field-within-subdomains/8142/5 "2022-04-30T11:14:35Z")
Yes, that’s right, thank you! For those who would like to know, I fixed my code as follows: ```auto Q = VectorFunctionSpace(mesh, ("DG", 0)) q = Function(Q) num_cells = mesh.topology.index_map(mesh.topology.dim).size_local block_size = Q.dofmap.index_map_bs for i in range(num_cells): if cell_tags.values[i] == 5: q.x.array[[i*block_size, i*block_size+1]] = [0, 0] elif cell_tags.values[i] == 6: q.x.array[[i*block_size, i*block_size+1]] = [5, 0] ```