converts a sparse matrix to a list of (row, column) vectors indicating which positions are occupado.

torcmtx.cpp

#include <octave/oct.h>

// SPECIAL FUNCTION THAT CONVERTS A SPARSE MATRIX 
// TO A MATRIX THAT LISTS (ROW COLUMN) OF THE FILLED VALUES

DEFUN_DLD (torcmtx, args,  nargout, "extracts edges of a sparse matrix to a list matrix")
{
  int nargin = args.length();
  if (nargin != 1)
    print_usage ();  //make sure we actually sent one value in.
  else
  {
  	//pull the sparse matrix from the argument.
  	SparseMatrix sm = args(0).sparse_matrix_value ();

    //temporarily cache the ridx and cidx arrays.
  	int* ri = sm.ridx();
  	int* ci = sm.cidx();

  	//count how many elements we have, this should also be
  	//the length of ri array
    int count = sm.nelem();

    //cursor will be used to iterate over ci array.
    int cursor = 0;

    //allocate a new matrix to store our answer.
    Matrix res(count,2);

    //iterating over the results:
    for (int index = 0; index < count; index++)
    {
       //the ri array has the row, just put that in the result matrix.
       //remember that internally octave indexes the array using 0-indexing
       //but octave wants 1s-indexing.

       res(index, 0) = ri[index] + 1;

       //find the appropriate row in the ci array
       while (ci[cursor] <= index)
       	 cursor++;

       //since the cursor *passed* the corresponding index that says
       //what the column is, no need to subtract one to get 1s-indexing.
       res(index, 1) = cursor;
    }
   
    //return it.
    return octave_value (res);
  }
  return octave_value_list ();
}