Re: slicing and ranges of matrices

From: jhrwalter (walter_at_[hidden])
Date: 2002-01-24 05:32:16

--- In boost_at_y..., Kresimir Fresl <fresl_at_g...> wrote:
> 
> jhrwalter wrote:
> 
> > Toon Knapen <toon.knapen_at_s...> wrote:
> 
> >> But another concept I'm missing, and which is in neither of the 
> >> three libraries, is creating a sub-array (of the same order as 
the 
> 
> >> array) that contains a specific selection of indices for every 
> 
> >> dimension of the array. e.g. in the 2D case I would like to do
> 
> 
> >>   matrix< double > A(8,8);
> >>   boost::array<3> i1 = { 0,1,6 };
> >>   boost::array<5> i2 = { 5,6,7 };
> >>   sub_matrix sub_A = submatrix(A, i1,i2 );
> 
> >> where sub_A contains all element that are in columns 5,6 and 7 
and 
> >> in rows 0,1 and 6. The difference with a range is thus that this 
> >> subarray is not a continuous block in the original array. The 
diff with the 
> >> current definition of slicing is that the selection does not 
need to be 
> >> strided.
> 
> In 1D case this is very similar to `indirect_array' view of 
`std::valarray'.  
> 
> >> Is there a reason this kind of subarray does not exist ? 
> 
> > Interesting question. Should this sub matrix support dense 
matrices 
> > only or sparse matrices also (the old question of genericity ;-)?
> 
> Both. FEM matrices, for example, are usually sparse. 

I believe, that it's difficult to estimate the complexity of 
operations on such a view for sparse matrices (in other words: they 
could be rather inefficient). May be we currently should be more 
interested in some gather and scatter functionality for sparse 
matrices.
 
Regards
 
Joerg