# [petsc-users] Only one column is in the matrix after solving the inverse matrix

Yaoyu Hu huyaoyu1986 at gmail.com
Tue Nov 22 09:23:19 CST 2011

```Hi, everyone,

I am new to PETSc, and I have just begun to use it together with slepc. It
is really fantastic and I like it!

It was not from me but one of my colleges who wanted to solve an inverse of
a matrix, which had 400 rows and columns.

I know that it is not good to design a algorithm that has a process for
solving the inverse of a matrix. I just wanted to give it a try. However,
things turned out wired. I followed the instructions on the FAQ web page of
PETSc, the one using MatLUFractor() and MatMatSolve(). After I finished the
coding, I tried the program. The result I got was a matrix which only has
its first column but nothing else. I did not know what's happened. The
following is the codes I used. It is kind of badly organized and the gmail
web page ignores my 'Tab's, forgive me for that.

============Code Begins=============

/*
* MI.cpp
*
*  Created on: Nov 22, 2011
*      Author: huyaoyu
*/

static char help[] = "Give the inverse of a matrix.\n\n";

#include <slepceps.h>

#include <iostream>
#include <fstream>

#undef __FUNCT__
#define __FUNCT__ "main"

#define DIMENSION 2

int main(int argc,char **args)
{
PetscErrorCode ierr;
PetscMPIInt    size;
Mat A,CA;
Mat BB,XX;
IS is_row;
IS is_col;
MatFactorInfo mfinfo;
PetscMPIInt n;
PetscScalar* array_scalar = new PetscScalar[DIMENSION*DIMENSION];

for(int i=0;i<DIMENSION*DIMENSION;i++)
{
array_scalar[i] = 0.0;
}
for(int i=0;i<DIMENSION;i++)
{
array_scalar[i*DIMENSION + i] = 1.0;
}

int idxm[DIMENSION];
for(int i=0;i<DIMENSION;i++)
{
idxm[i] = i;
}

PetscInitialize(&argc,&args,(char *)0,help);
ierr = MPI_Comm_size(PETSC_COMM_WORLD,&size);CHKERRQ(ierr);
if (size != 1) SETERRQ(PETSC_COMM_WORLD,1,"This is a uniprocessor example
only!");
ierr = PetscOptionsGetInt(PETSC_NULL,"-n",&n,PETSC_NULL);CHKERRQ(ierr);

// B & X
ierr =
MatCreateSeqDense(PETSC_COMM_WORLD,DIMENSION,DIMENSION,PETSC_NULL,&BB);
CHKERRQ(ierr);
ierr =
MatCreateSeqDense(PETSC_COMM_WORLD,DIMENSION,DIMENSION,PETSC_NULL,&XX);
CHKERRQ(ierr);

ierr =
MatSetValues(BB,DIMENSION,idxm,DIMENSION,idxm,array_scalar,INSERT_VALUES);
CHKERRQ(ierr);
ierr = MatAssemblyBegin(BB,MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
ierr = MatAssemblyEnd(BB,MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);

ierr = ISCreateStride(MPI_COMM_WORLD,DIMENSION,0,1,&is_row); CHKERRQ(ierr);
ierr = ISCreateStride(MPI_COMM_WORLD,DIMENSION,0,1,&is_col); CHKERRQ(ierr);
ierr = MatFactorInfoInitialize(&mfinfo); CHKERRQ(ierr);

// matrix A
std::fstream in_file;
double temp_scalar;
in_file.open("./A.txt",std::ifstream::in);
if(!in_file.good())
{
ierr = PetscPrintf(PETSC_COMM_SELF,"File open failed!\n"); CHKERRQ(ierr);
return 1;
}

for(int i=0;i<DIMENSION;i++)
{
for(int j=0;j<DIMENSION;j++)
{
in_file>>temp_scalar;

array_scalar[i*DIMENSION + j] = temp_scalar;
}
}
in_file.close();

ierr =
MatCreateSeqDense(PETSC_COMM_WORLD,DIMENSION,DIMENSION,PETSC_NULL,&A);
CHKERRQ(ierr);
ierr =
MatCreateSeqDense(PETSC_COMM_WORLD,DIMENSION,DIMENSION,PETSC_NULL,&CA);
CHKERRQ(ierr);
ierr =
MatSetValues(A,DIMENSION,idxm,DIMENSION,idxm,array_scalar,INSERT_VALUES);
CHKERRQ(ierr);

ierr = MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
ierr = MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);

ierr = MatDuplicate(A,MAT_COPY_VALUES,&CA); CHKERRQ(ierr);

ierr = MatLUFactor(A,is_row,is_col,&mfinfo); CHKERRQ(ierr);
ierr = MatMatSolve(A,BB,XX); CHKERRQ(ierr);
ierr = PetscPrintf(PETSC_COMM_SELF,"The inverse of A matrix is:\n");
CHKERRQ(ierr);
ierr = MatView(XX,PETSC_VIEWER_STDOUT_SELF); CHKERRQ(ierr);

ierr = MatMatMult(CA,XX,MAT_REUSE_MATRIX,PETSC_DEFAULT,&BB);
ierr = MatView(BB,PETSC_VIEWER_STDOUT_SELF); CHKERRQ(ierr);

// destroy
ierr = MatDestroy(&A); CHKERRQ(ierr);
ierr = MatDestroy(&BB); CHKERRQ(ierr);
ierr = MatDestroy(&XX); CHKERRQ(ierr);
ierr = MatDestroy(&CA); CHKERRQ(ierr);

ierr = PetscFinalize();

delete[] array_scalar;

return 0;
}

============Code Ends==============
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