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Subject: Re: [Boost-users] multi-array and pdes
From: Larry Evans (cppljevans_at_[hidden])
Date: 2011-05-27 15:14:17

On 05/25/11 15:44, Larry Evans wrote:
> Maybe one advantage of array_dyn is the ease of rotating the
> axes. Another may be offset calculation, because with
> array_dyn, no testing is done for the bool values of
> general_storage_order::ascending. Instead, an offset
> and negative strides achieve the same result, AFAICT.
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:


Sorry for the noise about offset calculations.

OTOH, there would be no need, in array_dyn, for incrementing
all of extents, strides, and index_bases pointers since they're
all (well, just extents and strides or really lengths and strides)
stored in the same element of an array:


; hence, only one increment would be needed. Of course that's a small
advantage, and there's probably disadvantages to this array_dyn which
I've not thought of yet.

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