关于 c ：遍历任何 boost::multi_array 的除第 d 维之外的所有维度

#include"boost/multi_array.hpp"
#include <ostream>

using namespace boost;

typedef multi_array_types::index index_t;
typedef multi_array_types::index_range range;

template <template <typename, std::size_t, typename…> class Array,
          typename T, std::size_t NDims, typename index_t, std::size_t NDimsNew>
multi_array_ref<T, NDimsNew>
reshape(Array<T, NDims>& A, const array<index_t, NDimsNew>& dims) {
    multi_array_ref<T, NDimsNew> a(A.origin(), dims);
    return a;
}

template <template <typename, std::size_t, typename…> class Array, typename T>
void f(Array<T, 1>& A) {
    for (auto it : A) {
        // do something with it
        std::cout << it <<"";
    }
    std::cout << std::endl;
}

template <template <typename, std::size_t, typename…> class Array,
          typename T, std::size_t NDims>
void f(Array<T, NDims>& A, long d) {
    auto dims = A.shape();
    typedef typename std::decay<decltype(*dims)>::type type;

    // collapse dimensions [0,d) and (d,Ndims)
    array<type, 3> dims3 = {
        std::accumulate(dims, dims + d, type(1), std::multiplies<type>()),
        dims[d],
        std::accumulate(dims + d + 1, dims + NDims, type(1), std::multiplies<type>())
    };

    // reshape to collapsed dimensions
    auto A3 = reshape(A, dims3);

    // call f for each slice [i,:,k]
    for (auto Ai : A3) {
        for (index_t k = 0; k < dims3[2]; ++k) {
            auto S = Ai[indices[range()][k]];
            f(S);
        }
    }
}

template <template <typename, std::size_t, typename…> class Array,
          typename T, std::size_t NDims>
void f(Array<T, NDims>& A) {
    for (long d = NDims; d—; ) {
        f(A, d);
    }
}

#include"f.hpp"

int main() {
    boost::multi_array<double, 3> A(boost::extents[2][2][3]);
    boost::multi_array_ref<double, 1> a(A.data(), boost::extents[A.num_elements()]);
    auto Ajk = A[1];
    auto Aik = A[boost::indices[range()][1][range()]];

    int i = 0;
    for (auto& ai : a) ai = i++;

    std::cout <<"work on boost::multi_array_ref" << std::endl;
    f(a);

    std::cout <<"work on boost::multi_array" << std::endl;
    f(A);

    std::cout <<"work on boost::detail::multi_array:sub_array" << std::endl;
    f(Ajk);

    std::cout <<"work on boost::detail::multi_array:sub_array" << std::endl;
    f(Aik);   // wrong result, since reshape() ignores strides!

    //f(A[1]);   // fails: rvalue A[1] is boost::multi_array_ref<double, 3ul>::reference
}

def idx_iterator(s, d, idx):
    if len(s) == 0:
        yield idx
    else:
        ii = (slice(None),) if d == 0 else xrange(s[0])
        for i in ii:
            for new_idx in idx_iterator(s[1:], d – 1, idx + [i]):
                yield new_idx

def iterator(A, d=0):
    for idx in idx_iterator(A.shape, d, []):
        yield A[idx]

def f(A):
    for d in reversed(xrange(A.ndim)):
        for it in iterator(A, d):
            print it
        print

import numpy as np
A = np.arange(12).reshape((2, 2, 3))

print"Work on flattened array"
f(A.ravel())

print"Work on array"
f(A)

print"Work on contiguous slice"
f(A[1])

print"Work on discontiguous slice"
f(A[:,1,:])

#include"boost/multi_array.hpp"

template <template <typename, std::size_t, typename…> class Array,
          typename T, std::size_t NDims>
struct SliceIterator {
    typedef Array<T, NDims> array_type;
    typedef typename array_type::size_type size_type;
    typedef boost::multi_array_types::index_range range;
    typedef boost::detail::multi_array::multi_array_view<T, 1> slice_type;
    typedef boost::detail::multi_array::index_gen<NDims, 1> index_gen;

    array_type& A;
    const size_type* shape;
    const long d;
    index_gen indices;
    bool is_end = false;

    SliceIterator(array_type& A, long d) : A(A), shape(A.shape()), d(d) {
        int i = 0;
        for (; i != d; ++i) indices.ranges_[i] = range(0);
        indices.ranges_[i++] = range();
        for (; i != NDims; ++i) indices.ranges_[i] = range(0);
    }

    SliceIterator& operator++() {
        // addition with carry, excluding dimension d
        int i = NDims – 1;
        while (1) {
            if (i == d) —i;
            if (i < 0) {
                is_end = true;
                return *this;
            }
            ++indices.ranges_[i].start_;
            ++indices.ranges_[i].finish_;
            if (indices.ranges_[i].start_ < shape[i]) {
                break;
            } else {
                indices.ranges_[i].start_ = 0;
                indices.ranges_[i].finish_ = 1;
                —i;
            }
        }
        return *this;
    }

    slice_type operator*() {
        return A[indices];
    }

    // fakes for iterator protocol (actual implementations would be expensive)
    bool operator!=(const SliceIterator& r) {
        return !is_end;
    }

    SliceIterator begin() {return *this;}
    SliceIterator end()   {return *this;}
};

template <template <typename, std::size_t, typename…> class Array,
          typename T, std::size_t NDims>
SliceIterator<Array, T, NDims> make_slice_iterator(Array<T, NDims>& A, long d) {
    return SliceIterator<Array, T, NDims>(A, d);
}

// overload for rvalue references
template <template <typename, std::size_t, typename…> class Array,
          typename T, std::size_t NDims>
SliceIterator<Array, T, NDims> make_slice_iterator(Array<T, NDims>&& A, long d) {
    return SliceIterator<Array, T, NDims>(A, d);
}

for (auto S : make_slice_iterator(A, d)) {
    f(S);
}