C++ Array to Numpy with SWIG Problem with TypeError

Question

I am studying the SWIG for calling C++ libraries in Python. One problem is that when I use 1-dimension array in C++ and want to call it in Python as Numpy arrary, I got the error.

Header file: example.h

#include <iostream>
using namespace std;
class Example {
  public:
  void say_hello();
  void add(int x, int y, int *result);
  int sub(int *x, int *y);
  void array_add(int *a, int *b, int *c);
};

C++ file: example.cpp

#include "example.h"

void Example::say_hello() {
    cout<<"Hello Example."<<endl;
}

void Example::add(int x, int y, int *result) {
    *result = x + y;
}

int Example::sub(int *x, int *y) {
    return *x-*y;
}


void Example::array_add(int *a, int *b, int *c) { 
    c[0] = a[0] + b[0]; 
    c[1] = a[1] + b[1]; 
}

SWIG interface file: example.i

%module example
%{
    #define SWIG_FILE_WITH_INIT
    #include "example.h"
%}

%include "typemaps.i"
%include "numpy.i"
%init %{
   import_array();
%}

%apply int *OUTPUT { int *result };
%apply int *INPUT { int *x, int *y};

%apply int *INPLACE_ARRAY1 {int *a, int *b, int *c};

%include "example.h"

Setup file: setup.py

#!/usr/bin/env python

from distutils.core import setup, Extension
import numpy
import os

example_module = Extension('_example',
    sources=['example.cpp', 'example_wrap.cxx',],
)
setup (
    name = 'example',
    version = '0.1',
    author = "Frank Tang",
    description = """Simple swig C\+\+/Python example""",
    ext_modules = [example_module],
    py_modules = ["example"],
)

file: test_example.py test_example.py

After I ran "python test_example.py" I got the error message as follows. I use macOS.

(virtualenv) bogon:source tangsg$ python test_example.py 
Hello Example.
7
3
Traceback (most recent call last):
  File "test_example.py", line 18, in <module>
    example.Example().array_add(a, b, c)
  File "/Users/tangsg/Projects/test/source/example.py", line 115, in 
array_add
    return _example.Example_array_add(self, a, b, c)
TypeError: in method 'Example_array_add', argument 2 of type 'int *'
(virtualenv) bogon:source tangsg$ ›

Error message

Please edit your question and add the error message in text form, not as an image — Nico Haase
– Nico Haase, Commented Sep 6, 2018 at 7:51
How is array_add going to know the length of these arrays? — Henri Menke
– Henri Menke, Commented Sep 6, 2018 at 10:31

Henri Menke · Accepted Answer · 2020-09-21 16:30:32Z

Your typemaps and your declaration of the array_add function are not valid. NumPy arrays always have a size and this has to be communicated to C++. There is quite extensive documentation for the usage of NumPy with SWIG.

Two other things:

The function signatures of add and sub are inefficient. Rather than using clumsy pointer arguments, just call by value and return a value, i.e.
```
  int add(int x, int y) { return x + y; }
  int sub(int x, int y) { return x - y; }
```
Then you can also remove %apply int *OUTPUT { int *result }; and %apply int *INPUT { int *x, int *y}; from the interface file.
Never ever do using namespace std; in a header file! (Why is "using namespace std;" considered bad practice?)

example.h

#include <algorithm>
#include <iostream>

class Example {
public:
    void array_add(int *a, int len_a, int *b, int len_b, int *c, int len_c) {
        int const end = std::min(len_a, std::min(len_b, len_c));
        for (int i = 0; i < end; ++i) {
            c[i] = a[i] + b[i];
        }
    }
};

example.i

%module example
%{
    #define SWIG_FILE_WITH_INIT
    #include "example.h"
%}

%include "numpy.i"
%init %{
   import_array();
%}

%apply (int *IN_ARRAY1, int DIM1) { (int *a, int len_a), (int *b, int len_b) };
%apply (int *INPLACE_ARRAY1, int DIM1) { (int *c, int len_c) };

%include "example.h"

test.py

import example
import numpy as np
E = example.Example()
a = np.array([1,1], dtype=np.int32)
b = np.array([1,1], dtype=np.int32)
c = np.array([1,1], dtype=np.int32)
E.array_add(a,b,c)
print(c)

Example invocation:

$ swig -python -c++ example.i
$ clang++ -Wall -Wextra -Wpedantic -I/usr/include/python2.7 -fPIC -shared example_wrap.cxx -o _example.so -lpython2.7
$ python test.py
[2 2]

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