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7c673cae FG |
1 | How to access data using raw pointers |
2 | ===================================== | |
3 | ||
4 | One of the advantages of the ndarray wrapper is that the same data can be used in both Python and C++ and changes can be made to reflect at both ends. | |
5 | The from_data method makes this possible. | |
6 | ||
7 | Like before, first get the necessary headers, setup the namespaces and initialize the Python runtime and numpy module:: | |
8 | ||
9 | #include <boost/python/numpy.hpp> | |
10 | #include <iostream> | |
11 | ||
12 | namespace p = boost::python; | |
13 | namespace np = boost::python::numpy; | |
14 | ||
15 | int main(int argc, char **argv) | |
16 | { | |
17 | Py_Initialize(); | |
18 | np::initialize(); | |
19 | ||
20 | Create an array in C++ , and pass the pointer to it to the from_data method to create an ndarray:: | |
21 | ||
22 | int arr[] = {1,2,3,4,5}; | |
23 | np::ndarray py_array = np::from_data(arr, np::dtype::get_builtin<int>(), | |
24 | p::make_tuple(5), | |
25 | p::make_tuple(sizeof(int)), | |
26 | p::object()); | |
27 | ||
28 | Print the source C++ array, as well as the ndarray, to check if they are the same:: | |
29 | ||
30 | std::cout << "C++ array :" << std::endl; | |
31 | for (int j=0;j<4;j++) | |
32 | { | |
33 | std::cout << arr[j] << ' '; | |
34 | } | |
35 | std::cout << std::endl | |
36 | << "Python ndarray :" << p::extract<char const *>(p::str(py_array)) << std::endl; | |
37 | ||
38 | Now, change an element in the Python ndarray, and check if the value changed correspondingly in the source C++ array:: | |
39 | ||
40 | py_array[1] = 5 ; | |
41 | std::cout << "Is the change reflected in the C++ array used to create the ndarray ? " << std::endl; | |
42 | for (int j = 0; j < 5; j++) | |
43 | { | |
44 | std::cout << arr[j] << ' '; | |
45 | } | |
46 | ||
47 | Next, change an element of the source C++ array and see if it is reflected in the Python ndarray:: | |
48 | ||
49 | arr[2] = 8; | |
50 | std::cout << std::endl | |
51 | << "Is the change reflected in the Python ndarray ?" << std::endl | |
52 | << p::extract<char const *>(p::str(py_array)) << std::endl; | |
53 | } | |
54 | ||
55 | As we can see, the changes are reflected across the ends. This happens because the from_data method passes the C++ array by reference to create the ndarray, and thus uses the same locations for storing data. | |
56 |