NickName:El Sampsa Ask DateTime:2016-09-14T14:33:28

Accessing numpy array data in C (for numpy 1.7+)

Following examples and the numpy C-API (http://docs.scipy.org/doc/numpy/reference/c-api.html), I'm trying to access numpy array data in cpp, like this:

#include <Python.h>
#include <frameobject.h>
#define NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION // TOGGLE OR NOT
#include "numpy/ndarraytypes.h"
#include "numpy/arrayobject.h"
...
// here I have passed "some_python_object" to the C code
// .. and "some_python_object" has member "infobuf" that is a numpy array
//
unsigned long* fInfoBuffer;
PyObject* infobuffer = PyObject_GetAttrString(some_python_object, "infobuf");
PyObject* x_array    = PyArray_FROM_OT(infobuffer, NPY_UINT32);
fInfoBuffer          = (unsigned long*)PyArray_DATA(x_array); // DOES NOT WORK WHEN API DEPRECATION IS TOGGLED

When the API deprecation is toggled, I get, when compiling:

error: cannot convert ‘PyObject* {aka _object*}’ to ‘PyArrayObject* {aka tagPyArrayObject*}’ for argument ‘1’ to ‘void* PyArray_DATA(PyArrayObject*)’

What would be the legitimate way of doing this in numpy 1.7+ ?

Copyright Notice：Content Author:「El Sampsa」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/39483862/accessing-numpy-array-data-in-c-for-numpy-1-7

Answers

Quant 2017-04-12T00:30:44

You could try using a higher-level library that wraps numpy arrays in C++ containers with proper container semantics.\n\nTry out xtensor and the xtensor-python bindings.\n\n\nNumpy to xtensor cheat sheet http://xtensor.readthedocs.io/en/latest/numpy.html\nThe xtensor-python project\nhttp://xtensor-python.readthedocs.io/en/latest/\n\n\nThere is also a cookiecutter to generate a minimal C++ extension project with all the boilerplate for testing, html documentation and setup.p...\n\nExample: C++ code\n\n#include <numeric> // Standard library import for std::accumulate\n#include \"pybind11/pybind11.h\" // Pybind11 import to define Python bindings\n#include \"xtensor/xmath.hpp\" // xtensor import for the C++ universal functions\n#define FORCE_IMPORT_ARRAY // numpy C api loading\n#include \"xtensor-python/pyarray.hpp\" // Numpy bindings\n\ndouble sum_of_sines(xt::pyarray<double> &m)\n{\n auto sines = xt::sin(m);\n // sines does not actually hold any value, which are only computed upon access\n return std::accumulate(sines.begin(), sines.end(), 0.0);\n}\n\nPYBIND11_PLUGIN(xtensor_python_test)\n{\n xt::import_numpy();\n pybind11::module m(\"xtensor_python_test\", \"Test module for xtensor python bindings\");\n\n m.def(\"sum_of_sines\", sum_of_sines,\n \"Computes the sum of the sines of the values of the input array\");\n\n return m.ptr();\n}\n\n\nPython Code:\n\nimport numpy as np\nimport xtensor_python_test as xt\n\na = np.arange(15).reshape(3, 5)\ns = xt.sum_of_sines(v)\ns\n",