petsc-3.4.5 2014-06-29

PCGetOperators

Gets the matrix associated with the linear system and possibly a different one associated with the preconditioner.

Synopsis

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

Not collective, though parallel Mats are returned if the PC is parallel

Input Parameter

pc -the preconditioner context

Output Parameters

	Amat	- the matrix defining the linear system
	Pmat	- the matrix from which the preconditioner is constructed, usually the same as Amat.
	flag	- flag indicating information about the preconditioner matrix structure. See PCSetOperators() for details.

Notes: Does not increase the reference count of the matrices, so you should not destroy them

Alternative usage: If the operators have NOT been set with KSP/PCSetOperators() then the operators are created in PC and returned to the user. In this case, if both operators mat and pmat are requested, two DIFFERENT operators will be returned. If only one is requested both operators in the PC will be the same (i.e. as if one had called KSP/PCSetOperators() with the same argument for both Mats). The user must set the sizes of the returned matrices and their type etc just as if the user created them with MatCreate(). For example,

        KSP/PCGetOperators(ksp/pc,&Amat,NULL,NULL); is equivalent to

          set size, type, etc of Amat

        MatCreate(comm,&mat);

        KSP/PCSetOperators(ksp/pc,Amat,Amat,SAME_NONZERO_PATTERN);

        PetscObjectDereference((PetscObject)mat);

          set size, type, etc of Amat

and

        KSP/PCGetOperators(ksp/pc,&Amat,&Pmat,NULL); is equivalent to

          set size, type, etc of Amat and Pmat

        MatCreate(comm,&Amat);

        MatCreate(comm,&Pmat);

        KSP/PCSetOperators(ksp/pc,Amat,Pmat,SAME_NONZERO_PATTERN);

        PetscObjectDereference((PetscObject)Amat);

        PetscObjectDereference((PetscObject)Pmat);

          set size, type, etc of Amat and Pmat

The rational for this support is so that when creating a TS, SNES, or KSP the hierarchy of underlying objects (i.e. SNES, KSP, PC, Mat) and their livespans can be completely managed by the top most level object (i.e. the TS, SNES, or KSP). Another way to look at this is when you create a SNES you do not NEED to create a KSP and attach it to the SNES object (the SNES object manages it for you). Similarly when you create a KSP you do not need to attach a PC to it (the KSP object manages the PC object for you). Thus, why should YOU have to create the Mat and attach it to the SNES/KSP/PC, when it can be created for you?

Keywords

PC, get, operators, matrix, linear system

Examples