Why does my mexed eigen code print out the wrong entries of the matrix?

Question

The following is a C++ code that takes a sparse matrix from Matlab.

#include "mex.h"
#include <Eigen/Sparse>
#include<iostream>
using namespace Eigen;
using namespace std;


void mexFunction( int nlhs, mxArray *plhs[],
        int nrhs, const mxArray *prhs[]) {

    mwSize     m = mxGetM (prhs[0]);
    mwSize     n = mxGetN (prhs[0]);
    mwSize    nz = mxGetNzmax (prhs[0]);
    double  * pr = mxGetPr (prhs[0]);
    int * ir = (int*) mxGetIr (prhs[0]);
    int * jc = (int*) mxGetJc (prhs[0]);
    Map<SparseMatrix<double> > gmap (m, n, nz, jc, ir, pr);

    cout << gmap.coeffRef(0,0);
    cout << gmap.coeffRef(0,1)<< "\n";

    cout << gmap.coeffRef(1,0);
    cout << gmap.coeffRef(1,1)<< "\n";

}

I simply pass it a small sparse format 2x2 matrix and print out the entries. Why are the entries wrong? Here's the output from the Matlab command window:

>> a=magic(2)

a =

     1     3
     4     2

>> example(sparse(a))
11
13

Update: solved thanks to suggestions in comments. I'll post an answer.

Answer 1

I've updated the code with suggestions from the comments and it works now.

1. I've changed int* to mwIndex*.

2. I've copied the mwIndex* arrays ir and jc arrays re-casting each entry to int so it will be compatible with Eigen. This compiles successfully. Here is the revised working code:

   #include "mex.h"
   #include <Eigen/Sparse>
   #include<iostream>
   using namespace Eigen;
   using namespace std;
   
   
   void mexFunction( int nlhs, mxArray *plhs[],
           int nrhs, const mxArray *prhs[]) {
   
       mwSize     m = mxGetM (prhs[0]);
       mwSize     n = mxGetN (prhs[0]);
       mwSize    nz = mxGetNzmax (prhs[0]);
       double  * pr = mxGetPr (prhs[0]);
       mwIndex * ir = mxGetIr (prhs[0]);
       mwIndex * jc = mxGetJc (prhs[0]);
   
       //copy and cast mwIndex
       int* ir_int=(int*)mxMalloc(nz*sizeof(int));
       int* jc_int=(int*)mxMalloc(nz*sizeof(int));
       for (int ii=0;ii<nz;ii++){
          ir_int[ii]=(int) ir[ii];   
          jc_int[ii]=(int) jc[ii]; 
       }
   
       Map<SparseMatrix<double> > gmap (m, n, nz, jc_int, ir_int, pr);
   
       cout << gmap.coeffRef(0,0);
       cout << gmap.coeffRef(0,1)<< "\n";
   
       cout << gmap.coeffRef(1,0);
       cout << gmap.coeffRef(1,1)<< "\n";
   
   }
   

   magic(2)

   ans =

    1     3
    4     2
   

   example(sparse(magic(2))) 13 42