Boost multi_array Views

Question

I have a 3 dimensioned boost::multi_array representing layers of 2d tilemaps. I would like to be able to clear a layer - aka iterate through all tiles on one layer and set its value to 0, but I can't figure out how to do this - I believe I have to use views but the documentation doesn't make much sense to me. For example, when looking at this example code:

// myarray = 2 x 3 x 4

//
// array_view dims: [base,bound) (dimension striding default = 1)
// dim 0: [0,2) 
// dim 1: [1,3) 
// dim 2: [0,4) (strided by 2), 
// 

typedef boost::multi_array_types::index_range range;
// OR typedef array_type::index_range range;
array_type::array_view<3>::type myview = myarray[ boost::indices[range(0,2)][range(1,3)][range(0,4,2)] ];

for (array_type::index i = 0; i != 2; ++i)
    for (array_type::index j = 0; j != 2; ++j)
        for (array_type::index k = 0; k != 2; ++k) 
            assert(myview[i][j][k] == myarray[i][j+1][k*2]);

How does boost::indices work? what do those ranges even represent? Sorry if this is obvious to some, I just can't get the hang of it.

Thanks in advance, ell.

Cubbi · Accepted Answer · 2012-12-07 03:57:14Z

That example creates a 3D view which accesses a part of a larger 3D array, and it's a little bit complicated because they are showing off strided views at the same time as bounded.

It'd be easier to understand if they used [range(0,2)][range(1,3)][range(0,4)], which would cut out a solid 3D block (0..2 along the first dimension, 1..3 along the second, 0..4 along the third)

Your goal is a 2D view (slice), so let's look at that, I'll make it simple:

you have a 3x3x3 array, and you want to zero out a plane that goes through its center. There are three such planes, so let's create a 2D view for each of the three:

#include <iostream>
#include <iomanip>
#include <boost/multi_array.hpp>

int main()
{
    typedef boost::multi_array<int, 3> array_t;

    array_t a(boost::extents[3][3][3]); // 3D array of 3x3x3

    int values = 0;
    for(array_t::index i = 0; i != 3; ++i)
        for(array_t::index j = 0; j != 3; ++j)
            for(array_t::index k = 0; k != 3; ++k)
                a[i][j][k] = values++;

    for(array_t::index i = 0; i != 3; ++i) {
        for(array_t::index j = 0; j != 3; ++j) {
            for(array_t::index k = 0; k != 3; ++k)
                std::cout << std::setw(2) << a[i][j][k] << ' ';
            std::cout << '\n';
        }
        std::cout << '\n';
    }

so far so good.. now let's cut some 2D planes out:

typedef boost::multi_array_types::index_range range_t;
array_t::index_gen indices;

// 2D plane that's constant in the 1st dimension
array_t::array_view<2>::type plane =
    a[ indices[1][range_t(0,3)][range_t(0,3)] ];

for (array_t::index i = 0; i != 3; ++i)
    for (array_t::index j = 0; j != 3; ++j)
        plane[i][j] = 0;

try printing this cube now, you'll see that one of the three planes is zeroed.

Then replace the initializer for plane with

a[ indices[range_t(0,3)][1][range_t(0,3)] ];

and with

a[ indices[range_t(0,3)][range_t(0,3)][1] ];

and print the cubes now.. You'll get it!

Is there a built in way to extract a copy of a sub-array? That is, instead of using a View of the same data, actually copy the data without manually iterating over corresponding indices of the view and a new multi_array?

Collectives™ on Stack Overflow

Boost multi_array Views

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related