Re: [boost] Re: Boost Mathematicians

From: Walter Landry (wlandry_at_[hidden])
Date: 2004-06-05 10:02:13

David Abrahams <dave_at_[hidden]> wrote:
> Walter Landry <wlandry_at_[hidden]> writes:
> 
> > Have you looked at
> >
> >   http://www.oonumerics.org/FTensor/
> >
> > It implements a fair number of different tensor types (e.g. symmetric,
> > antisymmetic), the element type is templated, and it has natural ways
> > of specifying contractions.  It doesn't have separate covariant and
> > contravariant tensors, but that wouldn't be that hard to implement.
> > It would just be a fair amount of work for quesionable gain.  I used
> > it for my General Relativity code, and I never missed the distinction
> > between covariant and contravariant indices.
> >
> > I bill it as a high performance tensor library, though now I think it
> > might actually be better to use ordinary loops instead of the
> > compile-time loops.
> 
> Really?  Please say more.
> 
> I clearly remember your talk on that work; weren't you saying that
> ordinary loops were so hard to write correctly that they compromised
> the chances of success in your projects?

Sorry, I was not clear.  Yes, if I had written out all of those loops
by hand, I would never have finished.  Rather, my comment above is
about the internal mechanism to auto-generate the loops.  In FTensor,
I used compile time loops.  Now I think it might be better to just use
run-time loops.  To use a factorial as an example, instead of
something like

template<int N>
class Factorial {
public:
    enum { value = N * Factorial<N-1>::value };
};

class Factorial<1> {
public:
    enum { value = 1 };
};

I think it might be better to ditch a lot of the template
metaprogramming stuff and just use

int factorial(int F)
  {
    int result(1);
    for(int i=F; i>1; --i)
      result*=i;
    return result;
  }

I described some benchmarks here:

  http://groups.google.com/groups?q=g:thl3194341146d&dq=&hl=en&lr=&ie=UTF-8&selm=87d6rdlirf.fsf%40grue.ucsd.edu&rnum=7

Of course, it is possible that I just screwed up the implementation of
FTensor ;)

Cheers,
Walter Landry
wlandry_at_[hidden]