convert sparse matrix form matlab format into ijv coordinate format

I have sparse matrices in matlab format and I want to convert them into ijv coordinate format sorted by row(the first column) . Below is my attempt but it is not so efficient because the matrices are very large(sometimes nnz > 1M) and this code requires a lot of memory. Any better idea is highly appreciated.

function printIJV(file_name,S)
  [i,j,v] = find(S);
 temp2 = [i,j,v];
 temp = sortrows(temp2 ,temp2(1) );
  file_id = fopen(file_name,'wt');
  % header: n-rows n-columns n-values
  fprintf(file_id,'%d %d %d\n', size(S,1) ,size(S,2),size(v,1) );
  n = size(temp , 1);
  for i=1:n
  fprintf(file_id,'%d %d %g\n',temp(i,1)-1 , temp(i,2)-1 , temp(i,3));
  end
  fclose(file_id);
end