[petsc-users] IS Invert Not Permutation

[Florian Lindner]

Hello,

I have a range of local input data indices that I want to use for row indexing, say { 3, 4, 6}.

For that, I create a matrix with a local number of rows of 3 and map the indices {3, 4, 5} to these rows.

I create an index set:

ISCreateGeneral(comm, myIndizes.size(), myIndizes.data(), PETSC_COPY_VALUES, &ISlocal);
ISSetPermutation(ISlocal);
ISInvertPermutation(ISlocal, myIndizes.size(), &ISlocalInv);
ISAllGather(ISlocalInv, &ISglobal); // Gather the IS from all processors
ISLocalToGlobalMappingCreateIS(ISglobal, &ISmapping); // Make it a mapping

MatSetLocalToGlobalMapping(matrix, ISmapping, ISmapping); // Set mapping for rows and cols

The InvertPermutation is required because, well, it seems that the direction the mapping stuff works in PETSc.

Now I can use MatSetValues local to set rows {3, 4, 5} and actually set the local rows {1, 2, 3}.

The problem is that the range of data vertices is not always contiguous, i.e. could be {3, 4, 6}. This seems to be not a permutation of PETSc, therefore ISSetPermutation fails and subsequently ISInvertPermutation.

How can I still create such a mapping, so that:

3 -> 1
4 -> 2
6 -> 6

row 5 is unassigned and will never be addressed.

Thanks!
Florian