PPT Slide
/* Compute new values (but not on boundary) */
for (i=i_first; i<=i_last; i++)
for (j=1; j<maxn-1; j++) {
xnew[i][j] = (xlocal[i][j+1] + xlocal[i][j-1] +
xlocal[i+1][j] + xlocal[i-1][j]) / 4.0;
diffnorm += (xnew[i][j] - xlocal[i][j]) *
(xnew[i][j] - xlocal[i][j]);
/* Only transfer the interior points */
for (i=i_first; i<=i_last; i++)
xlocal[i][j] = xnew[i][j];