Re: [ViennaCL-support] compressed_matrix operator() with preallocation

[Karl R. <ru...@iu...>]

Hi!



On 04/06/2017 06:44 PM, Chris Marsh wrote:
> Hi,
>
> I know the number of non-zero entries for a sparse matrix so I am trying
> to pre-allocate it with
>
>  viennacl::compressed_matrix<vcl_scalar_type>  vl_C(row, col, nnz);

At this point your matrix is still empty (i.e. no nonzeros). It only 
preallocated an array to hold up to 'nnz' entries.


> and access it with vl_C.operator().

Unless you are changing only a few entries, this is likely to be too slow.


> I am using host only memory context, with ViennaCL 1.7.1 from homebrew.
>
> How should I proceed with this?

To fill the CSR format efficiencly, have a look here:
https://sourceforge.net/p/viennacl/discussion/1143678/thread/325a937c/?limit=25#d6f0

For host-based memory, an example of how to get pointers to the three 
CSR arrays is here:
https://github.com/viennacl/viennacl-dev/blob/master/viennacl/linalg/host_based/sparse_matrix_operations.hpp#L115

Best regards,
Karli