Jacobi Iteration: update and error

for (j = 1; j <= xpoints; j++)

{

for (i = 1; i <= ypoints; i++)

{

if (mask[i][j]) {

phi[i][j] = 0.25 * (oldphi[i-1][j] +

oldphi[i+1][j]

+ oldphi[i][j-1] +

oldphi[i][j+1]);

diff = max(diff, abs(phi[i][j] - oldphi[i][j]));

} }}

/* maximum difference over all processors */

MPI_Allreduce(&diff, &err, 1, MPI_DOUBLE, MPI_MAX, comm2d);

if (err < ((double)TOLERANCE)) done = 1;

}