python,c++,sorting , C++ lib in Python: custom sorting method


C++ lib in Python: custom sorting method

Question:

Tag: python,c++,sorting

I want to make a custom sorting method in C++ and import it in Python. I am not an expert in C++, here are implementation of "sort_counting"

#include <iostream>
#include <time.h>

using namespace std;

const int MAX = 30;

class cSort
{
public:
    void sort( int* arr, int len )
    {
        int mi, mx, z = 0; findMinMax( arr, len, mi, mx );
        int nlen = ( mx - mi ) + 1; int* temp = new int[nlen];
        memset( temp, 0, nlen * sizeof( int ) );

        for( int i = 0; i < len; i++ ) temp[arr[i] - mi]++;

        for( int i = mi; i <= mx; i++ )
        {
            while( temp[i - mi] )
            {
                arr[z++] = i;
                temp[i - mi]--;
            }
        }

        delete [] temp;
    }

private:
    void findMinMax( int* arr, int len, int& mi, int& mx )
    {
        mi = INT_MAX; mx = 0;
        for( int i = 0; i < len; i++ )
        {
            if( arr[i] > mx ) mx = arr[i];
            if( arr[i] < mi ) mi = arr[i];
        }
    }
};

int main( int* arr )
{
    cSort s;
    s.sort( arr, 100 );

    return *arr;
}

and then using it in python

from ctypes import cdll
lib = cdll.LoadLibrary('sort_counting.so')

result = lib.main([3,4,7,5,10,1])

compilation goes nice

How to rewrite a C++ method to receive an array and then return a sorted array?


Answer:

The error is quite clear: ctypes doesn't know how to convert a python list into a int * to be passed to your function. In fact a python integer is not a simple int and a list is not just an array.

There are limitations on what ctypes can do. Converting a generic python list to an array of ints is not something that can be done automatically.

This is explained here:

None, integers, bytes objects and (unicode) strings are the only native Python objects that can directly be used as parameters in these function calls. None is passed as a C NULL pointer, bytes objects and strings are passed as pointer to the memory block that contains their data (char * or wchar_t *). Python integers are passed as the platforms default C int type, their value is masked to fit into the C type.

If you want to pass an integer array you should read about arrays. Instead of creating a list you have to create an array of ints using the ctypes data types and pass that in instead.

Note that you must do the conversion from python. It doesn't matter what C++ code you write. The alternative way is to use the Python C/API instead of ctypes to only write C code.


A simple example would be:

from ctypes import *
lib = cdll.LoadLibrary('sort_counting.so')

data = [3,4,7,5,10,1]

arr_type = c_int * len(data)
array = arr_type(*data)

result = lib.main(array)

data_sorted = list(result)

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