[petsc-users] Question about MATMPIAIJ and column indexes ordering

[Barry Smith]

   Added clarification to the man pages in https://gitlab.com/petsc/petsc/-/merge_requests/7170


> On Jan 8, 2024, at 4:31 AM, Deuse, Mathieu via petsc-users <petsc-users at mcs.anl.gov> wrote:
> 
> Hello,
>  
> I have a piece of code which generates a matrix in CSR format, but the without sorting the column indexes in increasing order within each row. This seems not to be 100% compatible with the MATMPIAIJ format: the documentation of MatCreateMPIAIJWithArrays indeed mentions 'row-major ordering'.
>  
> For example, consider the 2x2 matrix (1 2; 3 4), which in my code could be stored as i=[0, 2, 4], j=[1, 0, 0, 1], v=[2, 1, 3, 4]. I can generate the matrix as follows (on 1 proc): MatCreateMPIAIJWithArrays(PETSC_COMM_SELF, 2, 2, 2, 2, i, j, v, &matrix). This appears to work fine, and I can then use the matrix in a KSP for example. However, if I try to update the entry values (same order and values v=[2, 1, 3, 4]) with MatUpdateMPIAIJWithArray(matrix, v), it seems that PETSc does not memorize the order of the column indexes and the matrix that I get now is (2 1; 3 4). I get the same result with MatUpdateMPIAIJWithArrays(matrix, 2, 2, 2, 2, i, j, v). On the other hand, if the column indexes are sorted within each row (i=[0, 2, 4], j=[0, 1, 0, 1], v=[1, 2, 3, 4]), then it works fine. I have attached a minimal working example (C++).
>  
> Can I safely rely on MatCreateMPIAIJWithArrays working fine with unsorted column indexes as long as I do not use MatUpdateMPIAIJWithArray(s)? Or should I do the sorting myself before calling MatCreateMPIAIJWithArrays? (or alternatively use another matrix format).
>  
> Thanks in advance for the help.
>  
> Kind regards,
>  
> Mathieu Deuse
> <main.cpp>

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