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Subject: Re: [ublas] Boundary condition application on matrices
From: Umut Tabak (u.tabak_at_[hidden])
Date: 2011-05-01 06:26:51

On 04/29/2011 01:54 PM, Karl Rupp wrote:
> Hi Umut,
> the best approach typically is to not write boundary conditions to the
> matrix at all. A little bit of extra logic during the assembly process
> can write the values directly to the right hand side vector. This is
> typically much faster than manipulating the matrix in a post-assembly
> step.
> Sorry if that's not the let-ublas-do-the-job-answer you may have been
> looking for. ;-)
> Best regards,
> Karli
Hi Karli,

I am afraid that is not possible because I am getting the matrices in
the assembled format from a commercial finite element code, and I have
to extract the information related to structural and fluid degrees of
freedom by myself. So that is the reason why I puzzled with this
approach, matrix_indirect seems to be quite fast to extract the
submatrix, however I could not find a way to efficiently assign or write
the entries to a compressed_matrix.

I guess, I will have to keep a map<int, vector<int> > for row -> col
indices, then I will have to process the key information which is rather
fast in a std::map and create the new matrix by iterating over this map
structure both reading from the original matrix and writing to the new
matrix at the same time.

BTW, for a monophysical problem, there are some workarounds, however for
a problem with more than two physics, this is the only option I see
since there are going to be different types of dofs which have to be
extracted from the matrix.