Hi
I'm having a problem with the boost/multi_array and I'm hoping someone here
can help, or at least explain why the following doesn't work.
The problem: I have a multi_array of ints that I want to be able to view as
being either row major or column major to simplify a number algorithms. I
don't want to change the internal representation of the array (the default
c++ row major storage layout is fine), I just want to be able to interchange
the indexing order from array[row][col] to array[col][row] for the same
multi_array.
To do so, I attempted to define 2 array_views of the same multi_array with
different stride values, like this:
#include <iostream>
using namespace std;
#include
using namespace boost;
int main(void)
{
typedef multi_array intarray2;
typedef intarray2::array_view<2>::type intarray2view2;
typedef intarray2::array_view<1>::type intarray2view1;
typedef intarray2::index_range intarray2range;
intarray2 arr(extents[3][3]);
// test data
int a = 0;
for (int r = 0; r < 3; r++)
{
for (int c = 0; c < 3; c++)
{
arr[r][c] = a++;
}
}
intarray2view2 rows =
arr[indices[intarray2range().stride(3)][intarray2range().stride(1)]];
intarray2view2 cols =
arr[indices[intarray2range().stride(1)][intarray2range().stride(3)]];
/*
0 1 2
3 4 5
6 7 8
*/
cout << rows[2][1]; // i expect this to print 7
cout << cols[2][1]; // i expect this to print 5
}
Unfortunately, this doesn't work at all (an assert fails when indexing the
views). I debugged (using Visual C 2005), and found that the 'rows'
array_view has a stride collection of [9, 1], while the 'cols' array_view
stride collection is [3, 3]. Both are different than what I specified
during construction. It appears that they're being multiplied by the
extents of the first dimension of the multi_view array (in this example: 3).
I can produce a column major view of a single column of the array using a 1
dimensional view constructed using a degenerate range, like this:
intarray2view1 col= arr[indices[1][intarray2range()]]; // column 1 in arr
but that's not what I'm hoping to achieve.
Can someone explain the behavior above and/or suggest a way of creating a 2d
column major array_view of a 2d multi_array?
Thanks
- Chris Dion
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