Re: [ublas] beginners question

Subject: Re: [ublas] beginners question
From: Ralf Denzer (Ralf.Denzer_at_[hidden])
Date: 2010-01-06 06:17:22

Hi

> > OK, I'm kidding but the real question is similar: does the new traits
> > system allow to manipulate, it's my Holy Grail, matrices like I do in
> > matlab ? Being able to address a matrix with vectors, matrices, the
> > famous ':' operator, etc...
> > Any pointer to a documentation would be most appreciated.

Maybe, ublas::indirect_array<> is what you like to do?
It works like ublas::range but you use an 'index vector' (indirect_array)
to extract arbitrary submatrices or subvectors.
Unfortunately, this feature is not documented, but works well
to my experience. You should check the source code 
in the include files:

#include <boost/numeric/ublas/storage.hpp>
#include <boost/numeric/ublas/vector_proxy.hpp>
#include <boost/numeric/ublas/matrix_proxy.hpp>

Check this example:

// ----- snip -----
#include <boost/numeric/ublas/vector.hpp>
#include <boost/numeric/ublas/vector_proxy.hpp>

#include <boost/numeric/ublas/matrix.hpp>
#include <boost/numeric/ublas/matrix_proxy.hpp>

namespace ublas = boost::numeric::ublas;

int main()
{
    const unsigned N = 7;
    ublas::matrix<int> M(N,N);
    ublas::vector<int> V(N);

    // initialize M and V
    std::cout << "M=" << std::endl;
    for (unsigned i = 0; i < M.size1(); ++i)
    {
        V(i) = 111*(i+1);
        for (unsigned j = 0; j < M.size2(); ++j)
        {
            M(i,j) = 10*i+j;
            std::cout.width(2);
            std::cout.fill('0');
            std::cout << M(i,j) << "  ";
        }
        std::cout << std::endl;
    }
    std::cout << "V=\n" << V << std::endl;
    //************************************************
    // define an indirect_array
    const unsigned n = 3;
    ublas::indirect_array<> ia(n);
    ia[0] = 1;
    ia[1] = 2;
    ia[2] = 4;
    //*************************************************

    // ia(n) to extract some indices
    ublas::matrix_indirect<ublas::matrix<int> > Mindirect(M, ia,ia);
    std::cout << "Mindirect=\n" << Mindirect << std::endl;

    // or use ublas::project
    std::cout << "Mprojected=\n" << ublas::project(M, ia, ia) << std::endl;

    // similar for vectors
    ublas::vector_indirect<ublas::vector<int> > Vindirect(V,ia);
    std::cout << "Vindirect=\n" << Vindirect << std::endl;

    // and
    std::cout << "Vprojected=\n" << ublas::project(V, ia) << std::endl;
}
// ---- snip -----

Output:

 M=
00  01  02  03  04  05  06
10  11  12  13  14  15  16
20  21  22  23  24  25  26
30  31  32  33  34  35  36
40  41  42  43  44  45  46
50  51  52  53  54  55  56
60  61  62  63  64  65  66
V=
[7](111,222,333,444,555,666,777)
Mindirect=
[3,3]((11,12,14),(21,22,24),(41,42,44))
Mprojected=
[3,3]((11,12,14),(21,22,24),(41,42,44))
Vindirect=
[3](222,333,555)
Vprojected=
[3](222,333,555)

With ublas::indirect_array<>  you can simulate Matlab possibilities to extract 
submatrices and subvectors quite well.

Ciao

Ralf

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