resizing boost::multi_array to match another

Question

I need to resize one multi_array to the size of another.

In Blitz++ I could just do

arr1.resize(arr2.shape());

Is there a multi_array solution of similar length? Because

arr1.resize(boost::extents[arr2.shape()[0]][arr2.shape()[1]]);

seems a little long and arduous.

Answer 1

You can use the shape() member. Sadly it cannot directly serve as an ExtentList (it doesn't model the Collection concept) but it's easy to make it into one:

using MA = multi_array<double, 2>;
MA ma(extents[12][34]);
auto& ma_shape = reinterpret_cast<boost::array<size_t, MA::dimensionality> const&>(*ma.shape());

So that

// demo
std::cout << "[" << ma_shape[0] << "][" << ma_shape[1] << "]\n";

Prints [12][34].

Now, ma_shape can directly be used to reshape/resize another array:

Live On Coliru

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

int main() {
    using namespace boost;
    using MA = multi_array<double, 2>;

    MA ma(extents[12][34]);
    auto& ma_shape = reinterpret_cast<boost::array<size_t, MA::dimensionality> const&>(*ma.shape());

    // demo
    std::cout << "[" << ma_shape[0] << "][" << ma_shape[1] << "]\n";

    // resize
    MA other;
    assert(!std::equal(ma_shape.begin(), ma_shape.end(), other.shape()));

    other.resize(ma_shape);
    assert(std::equal(ma_shape.begin(), ma_shape.end(), other.shape()));

    // reshape
    other.resize(extents[1][12*34]);
    assert(!std::equal(ma_shape.begin(), ma_shape.end(), other.shape()));

    other.reshape(ma_shape);
    assert(std::equal(ma_shape.begin(), ma_shape.end(), other.shape()));
}