[petsc-users] error messages while using snes

[barry]

Hi,

I want to solve a nonlinear equation tan(x) = x, and the code i wrote 
occur some error.

In the error, it say that I need to use MatAssemblyBegin/End(), but 
there aren't any Mat in my program.

How can I fix it?

This is what i write (major part):

int main(){

   .....

   VecCreate(PETSC_COMM_WORLD,&f);
   VecSetSizes(f,PETSC_DECIDE,400);
   VecSetFromOptions(f);
   VecDuplicate(f,&b);

   SNESCreate(PETSC_COMM_WORLD,&snes);
   SNESSetFunction(snes,f,FormFunction,&user);
   SNESSetComputeInitialGuess(snes,FormInitialGuess,NULL);
   SNESSetFromOptions(snes);
   SNESSolve(snes,NULL,b);

   SNESDestroy(&snes);

   .....

}

PetscErrorCode FormFunction(SNES snes,Vec b,Vec f,void *ctx)
{
   const PetscScalar   *bb;
   PetscScalar             *ff;
   PetscInt                   i;
   PetscErrorCode       ierr;

   PetscInt                   A1 = 1;
   PetscInt                   A2 = 1;

   VecGetArrayRead(b,&bb);
   VecGetArray(f,&ff);

   /* Compute function */
   for (i=0; i<400; i++) ff[i] = tan(bb[i])-(A1/A2)*bb[i];

   VecRestoreArrayRead(b,&bb);
   VecRestoreArray(f,&ff);
   return 0;
}

PetscErrorCode FormInitialGuess(SNES snes,Vec b,void *ctx)
{
   PetscScalar           *bb;
   PetscInt                  i;
   PetscErrorCode      ierr;

   VecGetArray(b,&bb);

   /* Compute initial guess */
   for (i=2; i<400; i++)
   {
     if      (i==0) bb[i] = 0;
     else if (i==1) bb[i] = 4.5;
     else           bb[i] = bb[i-1] + 3.2;
   }

   VecRestoreArray(b,&bb);
   return(0);
}


Here is the error come out:

[0]PETSC ERROR: --------------------- Error Message 
--------------------------------------------------------------
[0]PETSC ERROR: Object is in wrong state
[0]PETSC ERROR: Matrix must be assembled by calls to MatAssemblyBegin/End();
[0]PETSC ERROR: See http://www.mcs.anl.gov/petsc/documentation/faq.html 
for trouble shooting.
[0]PETSC ERROR: Petsc Release Version 3.10.2, unknown
[0]PETSC ERROR: ./test2 on a arch-linux2-c-debug named G1ngy by barry 
Thu Nov 22 21:07:18 2018
[0]PETSC ERROR: Configure options --with-cc=gcc --with-cxx=g++ 
--with-fc=gfortran --download-mpich --download-fblaslapack
[0]PETSC ERROR: #1 MatFDColoringCreate() line 469 in 
/home/barry/petsc/src/mat/matfd/fdmatrix.c
[0]PETSC ERROR: #2 SNESComputeJacobianDefaultColor() line 84 in 
/home/barry/petsc/src/snes/interface/snesj2.c
[0]PETSC ERROR: #3 SNESComputeJacobian() line 2555 in 
/home/barry/petsc/src/snes/interface/snes.c
[0]PETSC ERROR: #4 SNESSolve_NEWTONLS() line 222 in 
/home/barry/petsc/src/snes/impls/ls/ls.c
[0]PETSC ERROR: #5 SNESSolve() line 4396 in 
/home/barry/petsc/src/snes/interface/snes.c
[0]PETSC ERROR: #6 main() line 89 in /home/barry/Desktop/petsc/test2.c
[0]PETSC ERROR: No PETSc Option Table entries
[0]PETSC ERROR: ----------------End of Error Message -------send entire 
error message to petsc-maint at mcs.anl.gov----------
application called MPI_Abort(MPI_COMM_WORLD, 73) - process 0
[unset]: write_line error; fd=-1 buf=:cmd=abort exitcode=73
:
system msg for write_line failure : Bad file descriptor


Thank you,

Barry