How to extend c++ class in python with operator [], using swig

Question

I have a simple test.h file with my own array class (which uses the standard vector class):

#include <vector>
#include <string>

using namespace std;

class Array1D{
private:
   vector<double> data_; 
   int xsize_; 
public:
    Array1D(): xsize_(0) {};

    // creates vector of size nx and sets each element to t
    Array1D(const int& nx, const double& t): xsize_(nx) {
       data_.resize(xsize_, t);
    }

    double& operator()(int i) {return data_[i];}
    const double& operator[](int i) const {return data_[i];}

};

I want to be able to use the [] operator in python using swig. My current SWIG interface file looks like

 %module test

 %{ 
 #define SWIG_FILE_WITH_INIT 
 #include "test.h"
 %}

 %include "std_vector.i"

 namespace std{
%template(DoubleVector) vector<double>;
 }

 %include "test.h"

When I make the module, everything runs fine, but when I instantiate an object of Array1D, a = test.Array1D(10,2), which creates a length 10 vector with 2 in each element, and type a[1] I get TypeError: 'Array1D' object does not support indexing.

How should my SWIG interface file look in order to extend the operator method so I can output a[1] properly within python? I would also like to be able to do something like a[1] = 3.0;

You shouldn't return a const ref if you want to assign to it — Jules Gagnon-Marchand
– Jules Gagnon-Marchand, Commented Mar 29, 2014 at 20:25
Isn't this a duplicate of stackoverflow.com/questions/22736700/…? — Oliver
– Oliver, Commented Apr 1, 2014 at 19:13

Bluegreen17 · Accepted Answer · 2014-03-29 20:38:40Z

6

I figured it out. This is what I needed to add to my interface file:

%extend Array1D{
    const double& __getitem__(int i) {
        return (*self)[i];
    }
}

answered Mar 29, 2014 at 20:38

Bluegreen17

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