slicing and ranges of matrices

From: Toon Knapen (toon.knapen_at_[hidden])
Date: 2002-01-18 09:25:35

Looking at the slicing and ranging possibilities of both multi_array and 
ublas, I find the concepts as they are currently defined overlapping and 
limited in respect to e.g. Blitz++.

The most important difference with Blitz++ is that slicing in Blitz++ 
speak means also reducing the order of the array. In ublas & 
multi_array, slicing means a 'strided range'. I think it is important to 
define concepts in conformance with Blitz++ as it's still one of the 
default array libraries out there.

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 sub-array 
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.

Is there a reason this kind of subarray does not exist ? With this kind 
of functionality though, I would never have to write loops over my 
matrices again as I currently rely on ublas to do this for me. 
Currently, my only loop is for assembling my FEM matrix from all local 
element matrices which comes down basically to assigning a sub-array 
with non-strided indices.