[ublas] Concatenation of Matrices and Vectors in uBlas

From: James N. Knight (nate_at_[hidden])
Date: 2006-10-08 17:56:55

Hello all.

I'm trying to implement a concatenation feature as an extension to 
ublas.  I think this will be generally useful so I wanted to try and get 
some input.

A simple method to do this would look something like

concat(A,B,'v')

where

??? concat (...)
{
        // verify dimensions of A and B match given
        // 'v' or 'h' (vertical or horizontal)
        
        // declare new matrix of correct size

        // use project to assign A and B to correct parts
        // of new matrix C
        if(c = 'v'){
          project( C, range(0,A.size1()), range(0,A.size2())) = A;
          project( C, range(A.size1(),A.size1()+B.size1()), range(0,B.size2())) 
= B;
        }
        else {
          project( C, range(0,A.size1()), range(0,A.size2())) = A;
          project( C, range(0,B.size1()), 
range(A.size2(),A.size2()+B.size2())) = B;
        }
}

This has a big problem though.

If C does not need to be modified (as in D*concat(A,B,'v')) then 
creating a new matrix and copying all of the elements is a waste of time 
and space.

Anyway, what I'm really thinking is that a better way to do this would 
be to create a concat matrix expression that I can lazily evaluate when 
needed.  The expression would need to join the iterators of the two 
component matrices in some way.  When evaluated the expression's 
iterators would iterate over all of the elements of the contained 
matrices.  This would allow me to use the concatenation in a constant 
way if I want and copy it to a matrix if I want to modify it.

Does this seem feasible?

Is there any interest in something like this?

Any ideas on this are appreciated.

Thanks,
Nate

PS- I have also considered using some special type of matrix that has as 
its elements other matrices and uses special iterators to iterate over 
all elements of the submatrices.  This might be useful beyond the 
concatenation scenario.