[petsc-users] From CSR format to MatSetValues

[John Fettig]

On Fri, Jun 8, 2012 at 2:31 AM, w_ang_temp <w_ang_temp at 163.com> wrote:
> Hello,
>     When solving a linear system Ax=b, the first step is assigning values to
> matrix A. In my program, the subroutine PETSCSOLVE, which is used to slove
> Ax=b
> with PETSc, gets the CSR format matrix(values, columns, rowIndex) to set
> values
> to PETSc Mat A.
>     The variables 'values'、'columns'、'rowIndex' belong to the main function.
> They are used to represent a matrix in CSR format. The following table
> describes
> the arrays in terms of the values, row, and column positions of the non-zero
> elements in a sparse matrix.
>     values: A real or complex array that contains the non-zero elements of a
>             sparse matrix. The non-zero elements are mapped into the values
>             array using the row-major upper triangular storage mapping
> described
>             above.
>     columns: Element i of the integer array columns is the number of the
> column
>              that contains the i-th element in the values array.
>     rowIndex: Element j of the integer array rowIndex gives the index of the
>               element in the values array that is first non-zero element in
> a row j.
>
> codes:
> ---------Set Values to A From CSR
> Format(values,rowIndex,columns)-------------------
>       !values:Non-Symmetric Matrix
>       ione = 1
>       do 10,II=Istart+1,Iend
>         !non-zero numbers of this row
>         rowNum=rowIndex(II+1)-rowIndex(II)
>         do 11,JJ=1,rowNum
>
>             !elemnt index of the values/columns
>             kValNn=rowIndex(II)+JJ-1
>             !column index of this element
>             nCol=columns(kValNn)-1
>
>             !Setdata:II-RowIndex,nCol-ColIndex
>             nRow=II-1
>             call
> MatSetValues(A,ione,nRow,ione,nCol,values(kValNn),INSERT_VALUES,ierr)
>   11    continue
>   10  continue
> --------------------------------------------------------------------------------------
>
>     As we can see, it has to loop a number of times because it can only set
> one data per
> time. And this leads to low efficiency.

Why not call MatSetValues once per row?

John