:orphan:
# MatMatMult
Performs matrix-matrix multiplication C=A*B.
## Synopsis
```
#include "petscmat.h"
PetscErrorCode MatMatMult(Mat A, Mat B, MatReuse scall, PetscReal fill, Mat *C)
```
Neighbor-wise Collective
## Input Parameters
- ***A -*** the left matrix
- ***B -*** the right matrix
- ***scall -*** either `MAT_INITIAL_MATRIX` or `MAT_REUSE_MATRIX`
- ***fill -*** expected fill as ratio of nnz(C)/(nnz(A) + nnz(B)), use `PETSC_DEFAULT` if you do not have a good estimate
if the result is a dense matrix this is irrelevant
## Output Parameter
- ***C -*** the product matrix
## Notes
Unless scall is `MAT_REUSE_MATRIX` C will be created.
`MAT_REUSE_MATRIX` can only be used if the matrices A and B have the same nonzero pattern as in the previous call and C was obtained from a previous
call to this function with `MAT_INITIAL_MATRIX`.
To determine the correct fill value, run with -info and search for the string "Fill ratio" to see the value actually needed.
In the special case where matrix B (and hence C) are dense you can create the correctly sized matrix C yourself and then call this routine with `MAT_REUSE_MATRIX`,
rather than first having `MatMatMult()` create it for you. You can NEVER do this if the matrix C is sparse.
## Example of Usage
```none
MatProductCreate(A,B,NULL,&C);
MatProductSetType(C,MATPRODUCT_AB);
MatProductSymbolic(C);
MatProductNumeric(C); // compute C=A * B
MatProductReplaceMats(A1,B1,NULL,C); // compute C=A1 * B1
MatProductNumeric(C);
MatProductReplaceMats(A2,NULL,NULL,C); // compute C=A2 * B1
MatProductNumeric(C);
```
## See Also
[](ch_matrices), `Mat`, `MatProductType`, `MATPRODUCT_AB`, `MatTransposeMatMult()`, `MatMatTransposeMult()`, `MatPtAP()`, `MatProductCreate()`, `MatProductSymbolic()`, `MatProductReplaceMats()`, `MatProductNumeric()`
## Level
intermediate
## Location
src/mat/interface/matrix.c
## Examples
src/ksp/ksp/tutorials/ex27.c
src/snes/tutorials/ex70.c
src/tao/pde_constrained/tutorials/elliptic.c
src/tao/pde_constrained/tutorials/parabolic.c
---
[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/mat/interface/matrix.c)
[Index of all Mat routines](index.md)
[Table of Contents for all manual pages](/manualpages/index.md)
[Index of all manual pages](/manualpages/singleindex.md)