Re: [Boost-users] Help with a generic algorithm using boost::MultiArray/Multiple nested range traversal

AMDG Jesse Perla wrote:

Thanks for the response. I also think this is a pretty important use case for this great library. Let me justify why I think an Iterator based approach is the best:

* For maximum efficiency (I will be using this in numerical simulations), we will want have an incrementing pointer within the data structure pointing to the actual object, and not just the iterator.

* Also, if we have a single forward iterator, then we should be able to reuse a bunch of standard algorithms in the cases where we do not need the actual index values or the order of execution doesn't matter.

* In the iterator algorithm, I would love to check the ordering of the data from the template parameters. If it is fortran ordering then we would do the loops in the opposite order for example, and this should be based on querying the data structure in the iterator code.

* As for your range based use case, I don't know enough about how iterators and ranges interact to see how that would work.

* As to implementation of this, thanks for the pointer to Fusion. This indeed seems to be the best approach since the loops can be generated at compile time, and compilers could then do some very fancy optimization on them. Sadly, I am not a template metaprogrammer, or a very good C++ programmer in general, and am terrified to learn it and lose sight of my non-CS research.

Now for the semantics of this, it seems to me that the way the iterator concepts are designed, the distance() function is intended to pass back the related index information. Now my question is whether it is OK for the semantics of distance to pass back a boost::array<NUMDIMS> instead? Seems reasonable to me that it would not be based on passing back a scalar value in this case because that is arbitrarily dependent on the column vs. row ordering. And we can easily generate a boost::array from the distance function by looking at the distance between the .begin() for all of our current positions in the sub-iterators?

If you just want to iterate over the elements you can use data to get a pointer: multi_array m; m.data(), m.data() + m.num_elements() Given a pointer into a multi_array, you can deduce its index: int index_of(const multi_array& m, int* iter, int dimension) { int offset = iter - m.origin(); return(offset / m.strides()[dimension] % m.shape()[dimension] + m.index_bases()[dimension]); } In Christ, Steven Watanabve