Python to c++ interface example files. Used to demonstrate how to pass multidimensional numpy arrays to c++ vectors and back (see http://www.birving.com/blog/2014/05/13/passing-numpy-arrays-between-python-and/)

README.md

Python <-> c++ interface example for multidimensional arrays

python_test.py

# Python running example

from __future__ import print_function

import numpy as np
from rect import PyRectangle

# Initialising the wrapped c++ function
R1 = PyRectangle(0, 0, 9, 9);

#Test1: Doing the area example calculation
print("Test 1, Area: ", R1.getArea())

#Test2: Passing a list from python to c++
list1 = [1,2,3,4,5]
print("Test 2, Sum list: ", R1.sum_vec(list1))

#Test3: Passing a numpy array from python to c++
list2 = np.array([1,2,3,4,5])
print(" Test 3, Sum list: ", R1.sum_vec(list2))

#Test4: Passing a 2D list from python to c++
list3 = [[1, 2, 3], [4, 5, 6]]
print("Test 4, Sum list: ", R1.sum_mat(list3))

#Test5: Passing a 2D numpy array from python to c++
# Passing by value makes a second copy so best to pass by reference as demonstrated in the next example
list4 = np.array([[1, 2, 3], [4, 5, 6]])
print("Test 5, Sum list: ", R1.sum_mat(list4))

#Test6: Passing a 2D numpy array from python to c++ as a constant reference
list5 = np.array([[1, 2, 3], [4, 5, 6]])
print("Test 6, Sum list: ", R1.sum_mat_ref(list5))

#Test7: Returning a 2D numpy array from c++
list6 = np.array([[1, 2, 3], [4, 5, 6]])
print("Test 7, Sum list: ", R1.ret_mat(list6))

# This should work with any n-dimensional array

rect.pyx

# distutils: language = c++
# distutils: sources = Rectangle.cpp

# Cython interface file for wrapping the object
#
#

from libcpp.vector cimport vector

# c++ interface to cython
cdef extern from "Rectangle.h" namespace "shapes":
  cdef cppclass Rectangle:
        Rectangle(int, int, int, int) except +
        int x0, y0, x1, y1
        int getLength()
        int getHeight()
        int getArea()
        void move(int, int)
        double sum_vec(vector[double])
        double sum_mat(vector[vector[double]])
        double sum_mat_ref(vector[vector[double]] &)
        vector[vector[double]] ret_mat(vector[vector[double]])

# creating a cython wrapper class
cdef class PyRectangle:
    cdef Rectangle *thisptr      # hold a C++ instance which we're wrapping
    def __cinit__(self, int x0, int y0, int x1, int y1):
        self.thisptr = new Rectangle(x0, y0, x1, y1)
    def __dealloc__(self):
        del self.thisptr
    def getLength(self):
        return self.thisptr.getLength()
    def getHeight(self):
        return self.thisptr.getHeight()
    def getArea(self):
        return self.thisptr.getArea()
    def move(self, dx, dy):
        self.thisptr.move(dx, dy)
    def sum_vec(self, sv):
        return self.thisptr.sum_vec(sv)
    def sum_mat(self, sv):
        return self.thisptr.sum_mat(sv)
    def sum_mat_ref(self, sv):
        return self.thisptr.sum_mat_ref(sv)
    def ret_mat(self, sv):
        return self.thisptr.ret_mat(sv)

Rectangle.cpp

/*
Passing variables / arrays between cython and cpp
Example from 
http://docs.cython.org/src/userguide/wrapping_CPlusPlus.html

Adapted to include passing of multidimensional arrays

*/

#include "Rectangle.h"

using namespace shapes;

Rectangle::Rectangle(int X0, int Y0, int X1, int Y1)
{
    x0 = X0;
    y0 = Y0;
    x1 = X1;
    y1 = Y1;
}

Rectangle::~Rectangle()
{
}

int Rectangle::getLength()
{
    return (x1 - x0);
}

int Rectangle::getHeight()
{
    return (y1 - y0);
}

int Rectangle::getArea()
{
    return (x1 - x0) * (y1 - y0);
}

void Rectangle::move(int dx, int dy)
{
    x0 += dx;
    y0 += dy;
    x1 += dx;
    y1 += dy;
}

/*
Inputting a 1D vectoror list and returning its sum
*/

double Rectangle::sum_vec(std::vector<double> sv)
{

double tot=0;

int svs = sv.size();
std::cout << "vector length " << svs << std::endl;

for (int ii=0; ii<svs; ii++)
{
        tot = tot + sv.at(ii);
}
        return tot;

}

/*
Inputting a 2D vector or list and returning its sum
*/

double Rectangle::sum_mat(std::vector< std::vector<double> > sv)
{

double tot=0;

int svrows = sv.size();
int svcols = sv[0].size();
std::cout << "vector length " << svrows << " , " << svcols << std::endl;

for (int ii=0; ii<svrows; ii++)
{
        for (int jj=0; jj<svcols; jj++)
        {
                tot = tot + sv.at(ii).at(jj);
        }
}        
return tot;

}

/*
Passing a 2D vector by reference or list and returning its sum
*/

double Rectangle::sum_mat_ref(const std::vector< std::vector<double> > & sv)
{

double tot=0;

int svrows = sv.size();
int svcols = sv[0].size();
std::cout << "vector length " << svrows << " , " << svcols << std::endl;

for (int ii=0; ii<svrows; ii++)
{
        for (int jj=0; jj<svcols; jj++)
        {
                tot = tot + sv.at(ii).at(jj);
        }
}        
return tot;

}


/*
Inputting a 2D vector, performing a simple operation and returning a new 2D vector
*/
std::vector< std::vector<double> > Rectangle::ret_mat(std::vector< std::vector<double> > sv)
{

int svrows = sv.size();
int svcols = sv[0].size();

std::vector< std::vector<double> > tot;
tot.resize(svrows, std::vector<double> (svcols, -1));


std::cout << "vector length " << svrows << " , " << svcols << std::endl;

for (int ii=0; ii<svrows; ii++)
{
        for (int jj=0; jj<svcols; jj++)
        {
                tot.at(ii).at(jj) = (2*sv.at(ii).at(jj));
        }
}        
return tot;

}

Rectangle.h

/*
Passing variables / arrays between cython and cpp
Example from 
http://docs.cython.org/src/userguide/wrapping_CPlusPlus.html

Adapted to include passing of multidimensional arrays

*/

#include <vector>
#include <iostream>

namespace shapes {
    class Rectangle {
    public:
        int x0, y0, x1, y1;
        Rectangle(int x0, int y0, int x1, int y1);
        ~Rectangle();
        int getLength();
        int getHeight();
        int getArea();
        void move(int dx, int dy);
        double sum_vec(std::vector<double> sv);
        double sum_mat(std::vector< std::vector<double> > sv);
        double sum_mat_ref(const std::vector< std::vector<double> > & sv);
        std::vector< std::vector<double> > ret_mat(std::vector< std::vector<double> > sv);
    };
}

setup.py

# Cython compile instructions

from distutils.core import setup
from Cython.Build import cythonize

# Use python setup.py build --inplace
# to compile

setup(
  name = "rectangleapp",
  ext_modules = cythonize('*.pyx'),
)

Is Test7 supposed to return a 2D numpy array? Because I'm getting a list. Thanks.

Thanks WillieMaddox. For my test I was just interested in passing the data in and out, and I think I must have converted to an array later with np.array() . I guess this could also be put in the rect.pyx file if more convenient.