Re: [Boost-Users] matrix inside a matrix?

From: Kresimir Fresl (fresl_at_[hidden])
Date: 2002-11-26 02:36:47

Stephen Crowley wrote:

> Is it possible to define a matrix that is part of a larger matrix, without
> having to create temporaries and copy part of the larger?
> 
> /----------\
> |    |     |
> | SM |     |
> |----| LM  |
> |          |
> \----------/
> 
> I need to extract a sub-section of a matrix, multiply it, and then copy it back. 
> I am currently using a temporary, but I have a feeling there must be a better way.

I suppose you are using ublas ;o)

`template <typename Matrix> class matrix_range' seems to be
what you are looking for:
==============================================
   using namespace boost::numeric::ublas;
   matrix<double> LM (10, 10);
   // init LM
   matrix<double> M1 (4, 4);
   // init M1
   matrix_range<matrix<double> > SM (LM, range (1, 5), range (2, 6));
   LM = prod (M1, LM);
===============================================

matrix_range<> gives you a view of a part (a block) of a
matrix. Note that class `range' follows `convention' of iterator
ranges: `range (1, 5)'  is `[1, 5)', i.e. `1, 2, 3, 4'.

There is also function `project()', so there's no need to declare
matrix_range<>:
===============================================
   project (M, range (1, 5), range (2, 6))
       = prod (M1, project (M, range (1, 5), range (2, 6)));
===============================================

More details on:

http://64.226.201.52/libs/numeric/ublas/doc/matrix_proxy.htm#matrix_range

Hope this help,

fres