/* Update the temperature based on the four point stencil. */

    for ( i = 1; i <= n; i++ )
    {
      h_new[i] = h[i] 
      + ( time_delta * k / x_delta / x_delta ) * ( h[i-1] - 2.0 * h[i] + h[i+1] ) 
      + time_delta * rhs ( x[i], time );
    }
/*  Correct settings of first H in first interval, last H in last interval. */

    if ( 0 == id ) h_new[1] = boundary_condition ( x[1], time_new );

    if ( id == p - 1 ) h_new[n] = boundary_condition ( x[n], time_new );

/* Update time and temperature. */

    time = time_new;

    for ( i = 1; i <= n; i++ ) h[i] = h_new[i];

/* End of time loop. */
 }