# PETSc operators **URL:** \ **Category:** General Discussion\ **Created:** [September 5, 2021, 5:41pm UTC](https://community.freefem.org/t/petsc-operators/1191 "2021-09-05T17:41:17Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Markov](https://avatars.discourse-cdn.com/v4/letter/m/f1d935/32.png) [@Markov](https://community.freefem.org/u/Markov)\ **Post date:** [September 5, 2021, 5:41pm UTC](https://community.freefem.org/t/petsc-operators/1191/1 "2021-09-05T17:41:17Z")
How to perform simple operations in PETSc like matrix addition, multiplication, constant times a matrix? I’ve tried MatMatMult and MatScale both did not work. ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [September 5, 2021, 8:26pm UTC](https://community.freefem.org/t/petsc-operators/1191/2 "2021-09-05T20:26:40Z")
`MatMatMult` should work if you have a recent enough FreeFEM installation, cf. [https://github.com/FreeFem/FreeFem-sources/blob/develop/plugin/mpi/PETSc-code.hpp#L4460](https://github.com/FreeFem/FreeFem-sources/blob/develop/plugin/mpi/PETSc-code.hpp#L4460). `MatScale` is simply `A *= alpha`. Addition is `A += alpha * B` where both `A` and `B` are `Mat`. You can also do, assuming your `Mat` follow the same distributions: ```auto matrix a = alpha * b + gamma * c; // sequential FreeFEM matrix Mat B(A, a); // B and A follow the same distribution ``` This is usually more efficient since the sequential matrix operations do not require any communication. ---
**Author:** ![Markov](https://avatars.discourse-cdn.com/v4/letter/m/f1d935/32.png) [@Markov](https://community.freefem.org/u/Markov)\ **Post date:** [September 5, 2021, 8:58pm UTC](https://community.freefem.org/t/petsc-operators/1191/3 "2021-09-05T20:58:20Z")
Is there a direct way to obtain the number of rows and columns like in sequential? ex: B.n or B.m ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [September 6, 2021, 6:57am UTC](https://community.freefem.org/t/petsc-operators/1191/4 "2021-09-06T06:57:09Z")
`.n` will give you the number of local rows of a `Mat`. `.m` does not exist, but since you need to `Mat` do define a rectangular one, i.e., `Mat C(A, B)`, instead of doing `C.m` (which does not exist), you can do `B.n`. ---
**Author:** ![Markov](https://avatars.discourse-cdn.com/v4/letter/m/f1d935/32.png) [@Markov](https://community.freefem.org/u/Markov)\ **Post date:** [September 6, 2021, 8:43am UTC](https://community.freefem.org/t/petsc-operators/1191/5 "2021-09-06T08:43:39Z")
Ah I see, thank you for your reply. I would like to add something about my first comment regarding matrix operations, whenever I try to do for instance A+=alpha I get the following error: any idea on how to proceed? [0]PETSC ERROR: ------------------------------------------------------------------------ [0]PETSC ERROR: Caught signal number 11 SEGV: Segmentation Violation, probably memory access out of range [0]PETSC ERROR: Try option -start\_in\_debugger or -on\_error\_attach\_debugger [0]PETSC ERROR: or see [https://www.mcs.anl.gov/petsc/documentation/faq.html#valgrind](https://www.mcs.anl.gov/petsc/documentation/faq.html#valgrind) [0]PETSC ERROR: or try [http://valgrind.org](http://valgrind.org) on GNU/linux and Apple Mac OS X to find memory corruption errors [0]PETSC ERROR: configure using --with-debugging=yes, recompile, link, and run [0]PETSC ERROR: to get more information on the crash. [0]PETSC ERROR: --------------------- Error Message -------------------------------------------------------------- [0]PETSC ERROR: Signal received [0]PETSC ERROR: See [https://www.mcs.anl.gov/petsc/documentation/faq.html](https://www.mcs.anl.gov/petsc/documentation/faq.html) for trouble shooting. [0]PETSC ERROR: Petsc Release Version 3.15.0, Mar 30, 2021 [0]PETSC ERROR: /softs/local/freefem/4.9/bin/FreeFem+±mpi on a named node001.cluster by mnoun Mon Sep 6 10:39:22 2021 [0]PETSC ERROR: Configure options --prefix=/softs/local/freefem/4.9/ff-petsc/r MAKEFLAGS= --with-debugging=0 COPTFLAGS="-O3 -mtune=generic" CXXOPTFLAGS="-O3 -mtune=generic" FOPTFLAGS="-O3 -mtune=generic" --with-cxx-dialect=C++11 --with-ssl=0 --with-x=0 --with-fortran-bindings=0 --with-cudac=0 --with-scalar-type=real --with-blaslapack-include= --with-blaslapack-lib="-llapack -lblas" --download-metis --download-ptscotch --download-hypre --download-parmetis --download-mmg --download-parmmg --download-superlu --download-suitesparse --download-tetgen --download-slepc --download-hpddm --with-cc=gcc --with-cxx=g++ --download-mpich --with-fc=gfortran --download-scalapack --download-mumps --download-slepc-configure-arguments=–download-arpack=https://github.com/prj-/arpack-ng/archive/b64dccb.tar.gz PETSC\_ARCH=fr [0]PETSC ERROR: #1 User provided function() at unknown file:0 [0]PETSC ERROR: Run with -malloc\_debug to check if memory corruption is causing the crash. ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [September 6, 2021, 5:56pm UTC](https://community.freefem.org/t/petsc-operators/1191/6 "2021-09-06T17:56:06Z")
That’s a very generic error. It’s nearly impossible to help without a [MWE](https://en.wikipedia.org/wiki/Minimal_working_example) to reproduce the error. ---
**Author:** ![Markov](https://avatars.discourse-cdn.com/v4/letter/m/f1d935/32.png) [@Markov](https://community.freefem.org/u/Markov)\ **Post date:** [September 6, 2021, 6:21pm UTC](https://community.freefem.org/t/petsc-operators/1191/7 "2021-09-06T18:21:35Z")
This is a part of the code : [testpetsc.edp](https://community.freefem.org/uploads/short-url/ylZrylCqAxEXLpwiqpCa0QEswEK.edp) (3.0 KB) I defined three matrices K, C and M and I am trying to combine them in the matrix a1. ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [September 6, 2021, 6:46pm UTC](https://community.freefem.org/t/petsc-operators/1191/8 "2021-09-06T18:46:14Z")
Neither `a1` nor `a2` are defined, so it doesn’t make sense to do `a1 += something` with an undefined `a1`. Just put everything in a `matrix linearComb`, and then do `Mat a1(K, linearComb)`. ---
**Author:** ![Markov](https://avatars.discourse-cdn.com/v4/letter/m/f1d935/32.png) [@Markov](https://community.freefem.org/u/Markov)\ **Post date:** [September 6, 2021, 6:50pm UTC](https://community.freefem.org/t/petsc-operators/1191/9 "2021-09-06T18:50:09Z")
I defined a1 and a2 on line 81 as Mat a1, a2. Could you elaborate a little bit more your method please? Edit: K, C and M are Mat type also ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [September 6, 2021, 7:16pm UTC](https://community.freefem.org/t/petsc-operators/1191/10 "2021-09-06T19:16:57Z")
Sorry, I meant that none of the matrices are initialized, so you cannot do `unintializedMat += something`, it’s undefined behavior (and results in a segfault). Instead of ```auto K = stiffness(Vh, Vh); C = damping(Vh,Vh); M = mass(Vh,Vh); ``` Do ```auto matrix locK = stiffness(Vh, Vh); K = locK; matrix locC = damping(Vh,Vh); C = locC; matrix locM = damping(Vh,Vh); M = locM; matrix locLinearComb = alpha * locK + beta * locC + gamma * locM; Mat linearComb(M, locLinearComb); ```