**petsc-3.8.3 2017-12-09**

# MatShift

Computes Y = Y + a I, where a is a PetscScalar and I is the identity matrix.
### Synopsis

#include "petscmat.h"
PetscErrorCode MatShift(Mat Y,PetscScalar a)

Neighbor-wise Collective on Mat

### Input Parameters

Notes: If the matrix Y is missing some diagonal entries this routine can be very slow. To make it fast one should initially
fill the matrix so that all diagonal entries have a value (with a value of zero for those locations that would not have an
entry).

Developers Note: If the local "diagonal part" of the matrix Y has no entries then the local diagonal part is
preallocated with 1 nonzero per row for the to be added values. This allows for fast shifting of an empty matrix.

### Keywords

matrix, add, shift

### See Also

MatDiagonalSet()

**Level:**intermediate

Location:src/mat/utils/axpy.c

Index of all Mat routines

Table of Contents for all manual pages

Index of all manual pages

### Examples

src/mat/examples/tutorials/ex9.c.html

src/tao/pde_constrained/examples/tutorials/parabolic.c.html

src/tao/pde_constrained/examples/tutorials/hyperbolic.c.html