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# ISLocalToGlobalMappingCreate

Creates a mapping between a local (0 to n) ordering and a global parallel ordering.

## Synopsis

    #include "petscis.h"
    PetscErrorCode ISLocalToGlobalMappingCreate(MPI_Comm comm, PetscInt bs, PetscInt n, const PetscInt indices[], PetscCopyMode mode, ISLocalToGlobalMapping *mapping)

Not Collective, but communicator may have more than one process

## Input Parameters

- comm - MPI communicator

- bs - the block size

- n - the number of local elements divided by the block size, or equivalently the number of block indices

- indices - the global index for each local element, these do not need to be in increasing order (sorted), these values should not be scaled (i.e. multiplied) by the blocksize bs

- mode - see PetscCopyMode

## Output Parameter

- mapping - new mapping data structure

## Notes

There is one integer value in indices per block and it represents the actual indices bs*idx + j, where j=0,..,bs-1

For “small” problems when using `ISGlobalToLocalMappingApply()` and `ISGlobalToLocalMappingApplyBlock()`, the `ISLocalToGlobalMappingType` of `ISLOCALTOGLOBALMAPPINGBASIC` will be used; this uses more memory but is faster; this approach is not scalable for extremely large mappings.

For large problems `ISLOCALTOGLOBALMAPPINGHASH` is used, this is scalable. Use `ISLocalToGlobalMappingSetType()` or call `ISLocalToGlobalMappingSetFromOptions()` with the option `-islocaltoglobalmapping_type` <`basic`,`hash`> to control which is used.

## See Also

Low-level Vector Communication, `ISLocalToGlobalMapping`, `ISLocalToGlobalMappingDestroy()`, `ISLocalToGlobalMappingCreateIS()`, `ISLocalToGlobalMappingSetFromOptions()`, `ISLOCALTOGLOBALMAPPINGBASIC`, `ISLOCALTOGLOBALMAPPINGHASH`, `ISLocalToGlobalMappingSetType()`, `ISLocalToGlobalMappingType`

## Level

advanced

## Location

src/vec/is/utils/isltog.c

## Examples

src/vec/is/is/tutorials/ex4.c
src/ksp/ksp/tutorials/ex59.c
src/vec/is/is/tutorials/ex5.c
src/vec/vec/tutorials/ex8f.F90
src/vec/vec/tutorials/ex8.c
src/mat/tutorials/ex3.c
src/ksp/ksp/tutorials/ex71.c
src/ksp/ksp/tutorials/ex85.c

Index of all IS routines
Table of Contents for all manual pages
Index of all manual pages