[petsc-users] Better way to pre-allocate memory for matrix being read in ???

[Gaurish Telang]

Hi,

I have a text file containing the non-zero entries of a sparse matrix of
dimension 2683x1274, stored in the form (row, column, element) i.e. [i, j,
element] format.

That is ALL the information I have regarding the matrix.

However when pre-allocating memory with  MatSeqAIJSetPreallocation(Mat
B,PetscInt nz,const PetscInt nnz[]),  the parameters nz and nnz need to be
known,

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
PETSC_NULL<http://www.mcs.anl.gov/petsc/petsc-2/snapshots/petsc-dev/docs/manualpages/Sys/PETSC_NULL.html#PETSC_NULL>

which i do not know for my matrix, ( unless I resort to using MATLAB. ).

Does that mean I have to set nz= 1274 (the length of a row) and
nnz=PETSC_NULL ?  Though,  I guess this setting would consume a lot of
memory for higher order matrices.

How then, should I go about memory pre-allocation more efficiently?

Thanks,
Gaurish Telang









There is a code in the PETSc folder (/src/mat/examples/tests/ex78.c ) which
reads in a matrix of this format.
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