Cython example for sending array/vector to a c++ script

Question

Say we have a 1 dimensional numpy array or list that we use as an input to a function, i.e., func(np.array[1.5, 2.5, 3.5]), where this function comes from a wrapped c++ class and our goal is to print the values of this array or vector inside the c++ script.

However, I don't know how to define the c++ function for this matter and specify it in the pyx file. And wanted to ask if you can share a snippet as simple as having the above functionality, where low latency is of most importance.

Basically, on the c++ side I want to do:

void func (/* where I don't what to write */) {

std::cout <<  array_[0] << std::endl; // which prints 1.5 (since np.array[1.5, 2.5, 3.5][0] was 1.5)
std::cout <<  array_[1] << std::endl; // which prints 2.5 (since np.array[1.5, 2.5, 3.5][1] was 2.5)
std::cout <<  array_[2] << std::endl; // which prints 3.5 (since np.array[1.5, 2.5, 3.5][2] was 3.5)
}

It seems memoryviews offer a possible solution that also has low latency, however looking at Cython's documentation about memoryviews, I'm quite confused.

The example from here mentions (int* a, int n) as the argument of the above function, but I didn't understant what a and n are about and how do I sill index the array on teh c++ side. And finally writing the pyx mirror is also unclear to me.

I would appreciate if anyone could give a very simple snippet example for such c++ function and it's pyx counterpart. Any suggestion other than memoryviews that is not slow is also greatly welcome:)

DavidW · Accepted Answer · 2020-06-20 20:27:30Z

2

a is a variable that contains a pointer to zero or more int.
n is a the number of values that a contains.

You can index a using the standard C++ array access syntax a[i]. Make sure i < n.

The C++ header would then look like:

void func(int *a, int n);

The .pyx file would be something like

cdef extern from "myheader.hpp":
    void func(int *a, int n)

def func_wrapper(int[::1] mview):
    func(&mview[0], mview.shape[0])

You will need to compile it in the usual way being sure to list both your Cython source file and your C source file (the .cpp file, not the header) in the sources list.

answered Jun 20, 2020 at 20:27

DavidW

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2 Comments

Alejandro Over a year ago

Thanks. since the array I wanted to access on c++ side has float numbers I changed it to (float *a, int n) and (float[::1] mview), which worked. I hope it was ok to do this.

DavidW Over a year ago

Yes - that's the right way to handle floats instead of ints

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