Initializing the values of b (and u)

145: PetscOptionsHasName(PETSC_NULL,"–random_exact_sol",&flg);

146: if ( ! flg) {

h = 1.0 / (m+1);

for (I=Istart; I<Iend; I++) {

v = 0; i = I/n; j = I – i*n; h = 1/(m+1);

if (i==0) v = v + /* u(-1,j): */ h * 0;

if (i==m-1) v = v + /* u(m,j): */ h * (m+1);

if (j==0) v = v + /* u(i,-1): */ h * (i+1);

if (j==n-1) v = v + /* u(i, n): */ h * (i+1);

if (v != 0) ; VecSetValues(b,1,&I,&v,INSERT_VALUES);

v = /* u(i, j): */ h * (i+1); VecSetValues(u,1,&I,&v,INSERT_VALUES);

}

VecAssemblyBegin(b); VecAssemblyEnd(b);

VecAssemblyBegin(u); VecAssemblyEnd(u);

160: } else {

PetscRandomCreate(PETSC_COMM_WORLD,RANDOM_DEFAULT,&rctx);

VecSetRandom(rctx,u); PetscRandomDestroy(rctx);

MatMult(A,u,b);

164: }

167: /* View the exact solution vector if desired */

169: PetscOptionsHasName(PETSC_NULL,"–view_exact_sol",&flg);

170: if (flg) {VecView(u,PETSC_VIEWER_STDOUT_WORLD);}