# PCSetOperators

Sets the matrix associated with the linear system and a (possibly) different one associated with the preconditioner.

### Synopsis

    #include "petscksp.h"
    PetscErrorCode PETSCKSP_DLLEXPORT PCSetOperators(PC pc,Mat Amat,Mat Pmat,MatStructure flag)

Collective on PC and Mat

### Input Parameters

| pc | - the preconditioner context
| Amat | - the matrix associated with the linear system
| Pmat | - the matrix to be used in constructing the preconditioner, usually the same as Amat.
| flag | - flag indicating information about the preconditioner matrix structure during successive linear solves. This flag is ignored the first time a linear system is solved, and thus is irrelevant when solving just one linear system.

### Notes

The flag can be used to eliminate unnecessary work in the preconditioner during the repeated solution of linear systems of the same size. The available options are

| SAME_PRECONDITIONER | - Pmat is identical during successive linear solves. This option is intended for folks who are using different Amat and Pmat matrices and wish to reuse the same preconditioner matrix. For example, this option saves work by not recomputing incomplete factorization for ILU/ICC preconditioners.
| SAME_NONZERO_PATTERN | - Pmat has the same nonzero structure during successive linear solves.
| DIFFERENT_NONZERO_PATTERN | - Pmat does not have the same nonzero structure.

### Caution

If you specify SAME_NONZERO_PATTERN, PETSc believes your assertion and does not check the structure of the matrix. If you erroneously claim that the structure is the same when it actually is not, the new preconditioner will not function correctly. Thus, use this optimization feature carefully!

If in doubt about whether your preconditioner matrix has changed structure or not, use the flag DIFFERENT_NONZERO_PATTERN.

### More Notes about Repeated Solution of Linear Systems

PETSc does NOT reset the matrix entries of either Amat or Pmat to zero after a linear solve; the user is completely responsible for matrix assembly. See the routine MatZeroEntries() if desiring to zero all elements of a matrix.

### Keywords

PC, set, operators, matrix, linear system

### See Also

PCGetOperators(), MatZeroEntries()

Level:intermediate
Location:src/ksp/pc/interface/precon.c
Index of all PC routines
Table of Contents for all manual pages
Index of all manual pages

### Examples

src/ksp/ksp/examples/tutorials/ex21f.F.html
src/snes/examples/tutorials/ex33f.F.html