show sparse matrix in mex file

mex_show_sparse.cpp

#include "mex.h"
// undef needed for LCC compiler
#undef EXTERN_C

// mex mex_show_sparse.cpp -O -largeArrayDims
// a = sparse(rand(10)<0.1)
// mex_show_sparse(a);
// b = a .* rand(10)
// mex_show_sparse(b);
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
  if(nrhs < 1) { mexErrMsgTxt("no input"); }
  if(!mxIsSparse(prhs[0])) { mexErrMsgTxt("input array is not sparse"); }
  const mxArray* mxSparse = prhs[0];
  size_t m = mxGetM(mxSparse);
  size_t n = mxGetN(mxSparse);
  double* sr = NULL;
  bool* bsr = NULL;
  if(mxIsLogical(mxSparse)) { bsr = mxGetLogicals(mxSparse); }
  else { sr = mxGetPr(mxSparse); }
  mwIndex* irs = mxGetIr(mxSparse);
  mwIndex* jcs = mxGetJc(mxSparse);
  mwSize nzmax = mxGetNzmax(mxSparse);
  mexPrintf("%d x %d sparse matrix has %d values\n", m, n, nzmax);
  for (int i = 0; i < n; i++) {
    int c = jcs[i+1] - jcs[i];
    if (c > 0) {
      for (int j = 0; j < c; j++) {
        int k = jcs[i] + j;
        if (mxIsLogical(mxSparse)){ mexPrintf("(%d,%d): %d\n", irs[k]+1, i+1, bsr[k]); }
        else { mexPrintf("(%d,%d): %f\n", irs[k]+1, i+1, sr[k]); }
      }
    }
  }
}