How to convert a numpy array to a Vector in dolfin ?

Question

element = MixedElement([P, Q, W])
V = FunctionSpace(mesh, element)
a # a bilinear form
L # a linear form
u = Function(V)**
solve(a == L, u, bc)

But now if I use some other way to solve, like below:
A = assemble(a)
b = assemble(L)
If I get the solution to the problem A*x = b, how can I get u and p,q,w = split(u), plot(u) ?

Thanks very much !!!

Comments

Please format your code by indenting it with four spaces as in my answer

Answer 1

You can create a Vector from a numpy array 'x' with:

 u = Function(V)
 u.vector().set_local(x)

This assumes that the ordering of the dofs in x is correct. Note that the suffix _local refers to 'local to this process', so you have to be careful when using this function in parallel computations, since the indices of the dofs will be, uh, local ;)

Also see the methods add() (uses global indices), add_local() and apply().

Comments

Thank you a lot.
It works.

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Glad it helped. I've added a couple of comments to the answer.

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Thank you for your excellent answer !

Another question: how could I get the more information like add_local() ? I want to learn FeniCS deeply. I have already read the tutorial book.

Thanks again !

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I don't know of any comprehensive sources. There is the FEniCS book, of course, but it's a bit outdated and often lacks many details. I read that, google, search these forums and read as much code from others as possible. Many users here provide tons of valuable examples (like user MiroK, I learn a lot from his answers).