On 12/4/18 6:19 AM, 'Maxi Miller' via deal.II User Group wrote:
> For the implementation of the Quasi-Newton-method according to Schubert,
> which
> is defined as
>
> C_1 = C_0 - sum(u_i u_i^T(C_0p_j - y/t)(p_i^T/(p_i^Tp_i)))
>
> with p_i the vector p with all elements set to zero where column vector i of
> matrix C_0 has zero-value entries,
> I need the position of zero- and non-zero-entries in the system matrix, in
> order to create the "sparse" vector p_i. I know that there is a method called
> "exists(i, j)", but I assume looping over that function will be rather
> time-consuming. Thus, is there another method for faster access of the
> position of the non-zero elements in a sparsity pattern, or is there a better
> approach for creating such a "sparse" vector?
Yes -- the sparse matrix (and sparsity pattern) classes have iterators that
only iterate over the nonzero entries. You can ask these iterators for
row/column indices.
Best
W.
--
------------------------------------------------------------------------
Wolfgang Bangerth email: bange...@colostate.edu
www: http://www.math.colostate.edu/~bangerth/
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