# MatCreateSeqAIJViennaCL
Creates a sparse matrix in `MATSEQAIJVIENNACL` (compressed row) format (the default parallel PETSc format).  This matrix will ultimately be pushed down to GPUs and use the ViennaCL library for calculations. For good matrix assembly performance the user should preallocate the matrix storage by setting the parameter nz (or the array nnz).  By setting these parameters accurately, performance during matrix assembly can be increased substantially. 
## Synopsis
```
#include "petscmat.h" 
PetscErrorCode MatCreateSeqAIJViennaCL(MPI_Comm comm, PetscInt m, PetscInt n, PetscInt nz, const PetscInt nnz[], Mat *A)
```
Collective


## Input Parameters

- ***comm -*** MPI communicator, set to `PETSC_COMM_SELF`
- ***m -*** number of rows
- ***n -*** number of columns
- ***nz -*** number of nonzeros per row (same for all rows)
- ***nnz -*** array containing the number of nonzeros in the various rows
(possibly different for each row) or NULL



## Output Parameter

- ***A -*** the matrix


It is recommended that one use the `MatCreate()`, `MatSetType()` and/or `MatSetFromOptions()`,
MatXXXXSetPreallocation() paradigm instead of this routine directly.
[MatXXXXSetPreallocation() is, for example, `MatSeqAIJSetPreallocation()`]


## Notes
If nnz is given then nz is ignored

The AIJ format, also called
compressed row storage, is fully compatible with standard Fortran 77
storage.  That is, the stored row and column indices can begin at
either one (as in Fortran) or zero.  See the users' manual for details.

Specify the preallocated storage with either nz or nnz (not both).
Set nz = `PETSC_DEFAULT` and nnz = NULL for PETSc to control dynamic memory
allocation.  For large problems you MUST preallocate memory or you
will get TERRIBLE performance, see the users' manual chapter on matrices.




## See Also
 `MATSEQAIJVIENNACL`, `MatCreate()`, `MatCreateAIJ()`, `MatCreateAIJCUSPARSE()`, `MatSetValues()`, `MatSeqAIJSetColumnIndices()`, `MatCreateSeqAIJWithArrays()`, `MatCreateAIJ()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/aij/seq/seqviennacl/aijviennacl.cxx.html#MatCreateSeqAIJViennaCL">src/mat/impls/aij/seq/seqviennacl/aijviennacl.cxx</A>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/mat/impls/aij/seq/seqviennacl/aijviennacl.cxx)


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