What would be a good way to access a 2dim numpy array in c++? I've already checked out the numpy/c api and some other posts but that doesn't brought me further. Here's the situation:
I defined in a python file called Testfile.py the following numpy array:
import numpy as np
A = np.array([[1, 2, 3], [1, 2, 3], [1, 2, 3]])
Now, I would like to access this array in c++ to use it for further calculations. Here is what i did so far. Note: For simplicity, i left out error handling and reference counting code snippets.
#define NPY_NO_DEPRECATED_APINPY_1_7_API_VERSION
#define PY_ARRAY_UNIQUE_SYMBOL cool_ARRAY_API
#include <Python.h>
#include <arrayobject.h> // numpy!
#include <iostream>
using namespace std;
#include <stdlib.h>
#include <string>
int main(){
// Name of input-file
char pyfilename[] = "Testfile";
// initilaize python interpreter
Py_Initialize();
import_array();
// load input-file
PyObject *pyName = PyUnicode_FromString(pyfilename);
PyObject *pyModule = PyImport_Import(pyName);
// import my numpy array object
char pyarrayname[] = "A";
PyObject *obj = PyObject_GetAttrString(pyModule, pyarrayname);
//------------------------------------------------------------
// The Problem starts here..
// Array Dimensions
npy_intp Dims[] = { PyArray_NDIM(obj) }; // array dimension
Dims[0] = PyArray_DIM(obj, 0); // number of rows
Dims[1] = PyArray_DIM(obj, 1); // number of columns
// PyArray_SimpleNew allocates the memory needed for the array.
PyObject *ArgsArray = PyArray_SimpleNew(2, Dims, NPY_DOUBLE);
// The pointer to the array data is accessed using PyArray_DATA()
double *p = (double *)PyArray_DATA(ArgsArray);
for (int i = 0; i<Dims[0]; i++)
{
for (int j = 0; j<Dims[1]; j++)
{
p[i * Dims[1] + j] = *((int *)PyArray_GETPTR2(obj, i, j));
}
}
// -----------------------------------------------------------
Py_Finalize();
return 0;
}
I use python 3.6 and MSVC 2015.
EDIT: I added the headers i use and changed the problem formulation a bit.
EDIT: I added the proposed solution strategies provided by Swift and Alan Stokes
After you access array by
double *p = (double*)PyArray_DATA(ArgsArray);
int* pA = (int*)PyArray_DATA(obj);
you can work with it like with array. What are dimensions?
int height = PyArray_DIM(obj, 0);
int width = PyArray_DIM(obj, 1);
Now you can use that pointer to access data in array
for ( int i = 0; y<height; y++)
{
for (int j = 0; x<width; x++)
{
p[i * width + j] = pA[i * width + j];
}
}
Actually, if you just need to copy array, use memcpy or std::copy at this point.
Tbh, I'd considered to use boost.python instead, but it's my own preference.
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