Re: [Boost-users] multi_array: how can I efficiently have a columnselected?

29 Jan 2010

      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 <<endl;

//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() );
//..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" <rhys.ulerich@gmail.com>
To: <boost-users@lists.boost.org>
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
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Re: [Boost-users] multi_array: how can I efficiently have a columnselected?

petros mamales