Re: [boost] Boost.Numeric: multi_array spec/docs

From: Serge Barral (sbarral_at_[hidden])
Date: 2001-04-02 05:25:58

On Sun, 01 Apr 2001 23:20:46 Larry Evans wrote:
> Jeremy Siek wrote:
> 
> > On Sun, 1 Apr 2001, Toon Knapen wrote:
> > toon> Isn't that confusing. And mathematically there's a hugh
> difference
> > toon> between operations where the result has the same dimension or
> > toon> a lower dimension. Why would we make they be identical then
> > toon> in C++ syntax ?
> >
> > Well, its not identical syntax, but similar:
> >
> > assume A is 3 dimensional
> >
> > A[3 <= stride(1) < 7][2][0 <= stride(1) < 10]
> >   gives you a 2 dimensional array
> >
> > A[3 <= stride(2) < 7][all][0 <= stride(1) < 10]
> >   gives you a 3 dimensional array
> >
> > At least to me, the above syntax is quite clear.
> > Now, according to Blitz terminology, the 1st thing is a slice
> > and the 2nd thing is a subarray. However, in the docs for
> > operator[] I'd rather just say that the thing returned is
> > a subarray.
> 
> So, just to be sure I understand, the following expression:
> 
> A[3 <= stride(2) < 7][all][0 <= stride(1) < 10]
> [all][all][all]
> 
> would be a 3 dimensional array.  This means that you could
> have an infinite number of [sliceExpr] operators strung together,
> where sliceExpr is either all or something like [3<=stride(1)<7].

Actually, the result of A[3 <= stride(2) < 7][all][0 <= stride(1) < 10]
would be a 3 dimensional subarray.
The current proposal suggests that subarrays will support indexing, but how
about slicing? Is it really often necessary in practice?

I guess some confusion comes from the name "subarray" being different from
"array". As I understand it, the distinction is not based on mathematical
considerations, but is a distinction in the types that makes the
implementation more efficient (a subarray is only a "view" of a multi_array
and thus avoids the creation of a temporary multi_array).

But if my assumption above is correct, I don't understand how the result of
indirect slicing and the result of slicing with Range objects can be of the
same type, since these are two different kind of views.

     Serge