One-line initialiser for Boost.MultiArray

问题:

I have a n-dimensional Boost.MultiArray I initialize as follows:

const int n=3, size=4; //# of dimensions and size of one dimension
boost::multi_array<char,n> arr;
boost::array<size_t,n> extents; //size of each dimension
extents.assign(size); //assign size to each dimension -> {{4, 4, 4}}
arr.resize(extents);

So I have 4 lines of code to get the MultiArray, but I'd like to do it in one line. Is there any simple way to generate an MultiArray with n dimensions each having size length (so I can write arr(samevaluearray(n,size))) or did I miss a handy constructor for MultiArray?

Edit: It should work without depending on a certain value of n, i.e. arr({{size,size}} would only work for n=2.

Since it may not be clear: boost::multi_array<char,n>(boost::extents[4][4][4]) correctly initializes a 4x4x4-array, but every time n is changed in the sourcecode, every initialization has to be updated by hand, so it's not an option.

回答1:

You can encapsulate the creation of the array into an helper function:

template <typename T, size_t N>
boost::multi_array<T, N> make_regular_matrix(const size_t m)
{
    boost::multi_array<T, N> arr;
    boost::array<size_t, N> extents;
    extents.assign(m);
    arr.resize(extents);

    return arr;
}

const int n = 3;
int size = 4; // Can be const as well, but this is not mandatory

auto arr = make_regular_matrix<char, n>(size);

If you can't use auto, you'll have to duplicate the template parameters:

boost::multi_array<char, n> arr = make_regular_matrix<char, n>(size);

The make_regular_matrix function could be shortened to use std::vector, as you did in your answer; I don't know if this implementation would be better. The aim of the helper function is to hide the creation of the array, but other versions could be written, for example to initialize the array elements with a given value:

template <size_t N, typename T> //switched order for deduction
boost::multi_array<T, N> make_regular_matrix(const size_t m, const T & value)
{
     boost::multi_array<T, N> arr(std::vector<size_t>(n, m));

     std::fill(arr.data(), arr.data() + arr.num_elements(), value);

     return arr;
}

auto arr = make_regular_matrix<4>(3, 'z'); //creates a 3x3x3x3 matrix
                                           //filled with 'z's

回答2:

Turns out, std::vector has a constructor, that constructs a vector with a constant value repeated n times, so a possible solution looks like this:

const int n=2, size=4; //# of dimensions and size of one dimension
boost::multi_array<char,n> arr(std::vector<size_t>(n,size));

This initializes a n-dimensional multi_array with each dimension's size set to size.

回答3:

From the Boost Multi-Array documentation, yes, you can initialize it one line:

typedef boost::multi_array<double, 3> array_type;
typedef array_type::index index;
array_type A(boost::extents[3][4][2]);

The typedefs are for readability, one can just as easily do for your example:

boost::multi_array<int, 2> arr(boost::extents[2][4]);