Re: [boost] [fusion] matrix?

From: David Abrahams (dave_at_[hidden])
Date: 2006-09-06 03:40:06

"Andy Little" <andy_at_[hidden]> writes:

> "David Abrahams" <dave_at_[hidden]> wrote in message
> news:87irk2a0rd.fsf_at_pereiro.peloton...
>> "Andy Little" <andy_at_[hidden]> writes:
>>
>>> The code below uses boost::tuple, but presumably fusion could do
>>> this stuff better?
>>
>> I should note that one of the main reasons fusion tuples (which are
>> "vector-like") are better for random access than boost::tuple (which
>> is "slist-like") is that, even taking into account the memoizing
>> nature of template instantiation, the latter generates O(N^2)
>> instantiations to access N elements of a tuple with at<>, whereas the
>> former takes only O(N).
>
> Unfortunately I couldnt check out the relative compile time performance, as I
> ran out of elements in Boost.Tuple when I tried making a 4 x 4 matrix, else I
> would have stuck with it for the moment. ( I opted just to use one
> tuple)

Especially where Boost.Tuple (a cons-list implementation) is
concerned, I don't see why one tuple would be better than 5.

> However maybe its a good move to try out Boost.Fusion. Although the docs said
> the move from tuple was as easy as changing from get to at, I found that there
> was a big change, because AFAICS Fusion uses references everywhere, 

Surely not everywhere.  fusion::tuple<int,long> contains an int and a
long, not references to int and long.

> and the compiler refused to assign anything, for reasons I am not
> clear on. Anyway after changing from result_of::at_c to
> result_of::value_at_c things seemed to go more smoothly. (That is
> using the Boost Review version of fusion). IOW I am succcessfully
> fused !  FWIW compiling a 2x2, 3x3, and 4x4 with some
> quan::quantities in, a multiply of each by itself and some output,
> takes about 22 seconds, on my AMD Atlhon 1.25Ghz system The
> trickiest part is working out an algorithm to do cofactors of the
> matrices (to get the inverse), but I may just hard code them, unless
> anyone has any suggestions ...?

Yeah, use the fusion algorithms to express what you'd ordinarily do
with looping if these were homogeneous vectors/matrices.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com