[GRASS-dev] raster row alloc?

[Hamish]

Joel Pitt wrote:
> Glynn wrote:
> > Hamish wrote:
> > > I'd like to use  G_set_null_value(array, nrows*ncols, map_type);
> > > to fill some nrows with NULL in one go instead of a loop over the
> > > number of rows, and perhaps G_raster_cpy() to copy a few lines
> > > over at a time.
> > >
> > > nrows*(ncols+1) ?
> >
> > Not necessary.
> 
> Glynn, as lovely as your succinct answers are, would you be able to
> clarify whether you meant that Hamish's wish is not necessary or if
> the the "+1"  in "nrows*(ncols+1)" is not necessary to perform this
> action?

+1 isn't necessary; nrows*ncols is enough. i.e. you can copy chunks of
memory without worrying about stopping on each individual line.

I think it might be a common mistake to do (at least it is one I've
done in the past):

row_array = G_allocate_raster_buf(map_type);
ptr = row_array;
loop {
 ptr = G_incr_void_ptr(ptr, G_raster_size(map_type));
 G_set_raster_value_d(ptr, value, map_type);
}

when the order should be:

row_array = G_allocate_raster_buf(map_type);
ptr = row_array;
loop {
 G_set_raster_value_d(ptr, value, map_type);
 ptr = G_incr_void_ptr(ptr, G_raster_size(map_type));
}

By allocating an extra byte for row_array you mitigate the effect
of this sort of dumb mistake. It doesn't cost much for a single row, 
and the bug doesn't have an effect as long as you are consistently +1.



Hamish