Subject: Re: [Boost-users] multi-array and pdes
From: Larry Evans (cppljevans_at_[hidden])
Date: 2011-06-27 11:54:02
On 05/27/11 14:14, Larry Evans wrote:
[snip]
> After finding:
>
> Reference access(boost::type<Reference>,index idx,TPtr base,
> const size_type* extents,
> const index* strides,
> const index* index_bases) const {
>
> BOOST_ASSERT(idx - index_bases[0] >= 0);
> BOOST_ASSERT(size_type(idx - index_bases[0]) < extents[0]);
> // return a sub_array<T,NDims-1> proxy object
> TPtr newbase = base + idx * strides[0];
> return Reference(newbase,extents+1,strides+1,index_bases+1);
>
> in boost/multi_array/base.hpp, I'm guessing that array_dyn has
> no advantage in offset calculation since there's no test in
> that base.hpp code on general_storage_order<>::ascending.
> However, I'm still not sure how multi_array does it, unless
> it allows strides to be negative to account for a false
> ascending value and increments the base value to account for
> the negative strides, much like array_dyn's:
>
> box_domain::init_iter_strides
>
> Sorry for the noise about offset calculations.
After reading:
http://www.boost.org/doc/libs/1_46_1/libs/multi_array
/doc/reference.html#memory_layout
which provides an example:
Finally, both dimensions could be stored in descending order:
int data[] = {11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0};
int *a = data + 11;
int s[] = { -4, -1 };
which strongly suggest that multi_array *does* use negative
strides:
int s[] = { -4, -1 };
*and* somehow performs an offset calculation to account for
those:
int *a = data + 11;
I'm guessing that there's really no difference in the way array_dyn
implements this reversal and offset calculation. The only difference
is in the interface which uses a bool to specify whether an axis
is reversed rather than a negative length.
I just didn't see where this was done from looking at code :(
-regards
Larry