Thank you very much for your quick response. To be more concrete: #include "boost/multi_array.hpp" //stl #include <algorithm> #include <iostream> using namespace std; int main(int argc, char **argv){ const int M = 3; const int N = 4; typedef boost::multi_array< int, 2> array_t; typedef array_t::index index; typedef array_t::index_range range_t; typedef array_t::array_view<1>::type array_1d_view_t; array_t A( boost::extents[M][N], boost::fortran_storage_order() ); //initialize: int num = 0; for ( index i = 0; i < M ; ++i ) { for ( index j = 0; j < N ; ++j ) { A[i][j] = ++num; } } //show initialization cout << "A: 3x4 array" << endl; for ( index i = 0; i < M ; ++i ) { for ( index j = 0; j < N ; ++j ) { cout << A[i][j] << "\t"; } cout << endl; } cout < col( &(col_view[0]), boost::extents[M], boost::fortran_storage_order() ); //..and show it cout << endl << "It's third column is:" << endl; for ( boost::multi_array_ref< int, 1>::iterator rowIt = col.begin(); rowIt != col.end(); ++rowIt ) { cout << *(rowIt) << endl; } //and the same for row: array_1d_view_t row_view = A[ boost::indices[2][range_t(0,N)] ]; boost::multi_array_ref< int, 1> row( &(row_view[0]), boost::extents[N], boost::fortran_storage_order() ); cout << endl << "It's third row is:" << endl; for ( boost::multi_array_ref< int, 1>::iterator colIt = row.begin(); colIt != row.end(); ++colIt ) { cout << *(colIt) << "\t"; } cout << endl; return 1; } Using Boost 1.40 on msvc 2010 win ista (32 bit) Because I store things in fortran storage style, I derive the column quite naturally. However the row is a disaster: A: 3x4 array 1 2 3 4 5 6 7 8 9 10 11 12 It's third column is: 3 7 11 It's third row is: 9 2 6 10 I understand this to be because Two questions: a) The only way I could think of to "promote" the view to a m_a_r (multi_array_ref) was: array_1d_view_t col_view = A[ boost::indices[ range_t(0,M) ][2] ]; boost::multi_array_ref< int, 1> col( &(col_view[0]), boost::extents[M], boost::fortran_storage_order() ); (o/wise there would be no way I could use the iterators) Is this an overkill? Is there a simpler way? b) about the real issue of the failure of the stepping through the row elements: is there a workaround that would still allow me to use iterators? there is no way for the lower-dimension-array to know/remember the stepping sequence? Thank you very much in advance, Best Regards, Petros ----- Original Message ----- From: "Rhys Ulerich" To: Sent: Friday, January 29, 2010 2:41 PM Subject: Re: [Boost-users] multi_array: how can I efficiently have a columnselected?

I, say, have a 3d array (fortran storage), and want to create a view to it so that I select say: grid[ indices[ [range(9,10)] [3], [range(9,10)] ];

I am not clear on exactly what this creates: from the documentation, I see this creating a 3d view.

I think this would be a 2d slice since you provided the single value 3 in the second position. The underling type would be accessible through either the templated array_view typedefs or else the array_view_gen mechanism.

Can I use stl-like iterators on this, to use stl type algorithms?

You can use stl-algorithms on the 2d view. Be aware that iterators retrieved from the 2D view (via begin(), end()) walk over 1D views in your example.

Is there any other repository of examples that would use high-level constructs like this so that I could look into a few examples?

Other than the tutorial, the reference, and an article [1], I'm not aware of any. - Rhys [1] http://www3.interscience.wiley.com/journal/109793810/abstract _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users

Thank you very much. ----- Original Message ----- From: "Rhys Ulerich" To: Sent: Wednesday, February 03, 2010 6:46 PM Subject: Re: [Boost-users] multi_array: how can I efficiently have acolumnselected? Re: Petros' question about getting row and column views of a 2D, Fortran order multi_array...

//choose the third column: array_1d_view_t col_view = A[ boost::indices[ range_t(0,M) ][2] ]; boost::multi_array_ref< int, 1> col( &(col_view[0]), boost::extents[M], boost::fortran_storage_order() );

multi_array_ref wraps a contiguous block of storage [1]. Since matrix A has Fortran ordering, the column is contiguous and your code works.

// and the same for row array_1d_view_t row_view = A[ boost::indices[2][range_t(0,N)] ]; boost::multi_array_ref< int, 1> row( &(row_view[0]), boost::extents[N], boost::fortran_storage_order() );

Rows in Fortran ordering are not contiguous, row_view is not contiguous, and "wrapping" row_view with multi_array_ref isn't what you want. Had you worked with C ordering in your sample code, the columns would fail but the rows would work. You should be using the views directly. Each view models the MultiArray concept and has (nearly) all the iterator goodness that multi_array and multi_array_ref do: //choose the third column: array_1d_view_t col_view = A[ boost::indices[ range_t(0,M) ][2] ]; //..and show it cout << endl << "It's third column is:" << endl; for ( array_1d_view_t::iterator it = col_view.begin(); it != col_view.end(); ++it ) { cout << *(it) << endl; } //and the same for row: array_1d_view_t row_view = A[ boost::indices[2][range_t(0,N)] ]; cout << endl << "It's third row is:" << endl; for ( array_1d_view_t::iterator it = row_view.begin(); it != row_view.end(); ++it ) { cout << *(it) << endl; } - Rhys [1] http://www.boost.org/doc/libs/1_42_0/libs/multi_array/doc/reference.html#mul... _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users