Returning an Array to Python

pybind11 allows us to construct a NumPy array in C++ and have it returned to python. To do this, we need to use the pybind11 classes that create a python array.

In pybind11, the array class is pybind11::array_t<>. The constructor

Here’s an example that computes a Hilbert matrix in C++ and returns it to python.

Listing 155 array.cpp

#include <pybind11/pybind11.h>
#include <pybind11/numpy.h>

pybind11::array_t<double> hilbert(const int N) {

    // construct the numpy array we will return
    // we need to specify the strides manually

    constexpr size_t elsize = sizeof(double);
    size_t shape[2]{N, N};
    size_t strides[2]{N * elsize, elsize};
    auto H = pybind11::array_t<double>(shape, strides);
    auto H_view = H.mutable_unchecked<2>();

    for (int i = 0; i < H.shape(0); ++i) {
        for (int j = 0; j < H.shape(1); ++j) {
            H_view(i, j) = 1.0 / (static_cast<double>(i + j) + 1.0);
        }
    }

    return H;
}


PYBIND11_MODULE(array, m) {
    m.doc() = "simple example of creating a NumPy array (Hilbert matrix) in C++";
    m.def("hilbert", &hilbert);
}

The GNUmakefile is essentially the same, but we just need to give the name of the library (BASE in the GNUmakefile): GNUmakefile.