Example

int dim_sizes[2];

int wrap_around[2];

int reorder = 1;

dim_sizes[0] = dim_sizes[1] = q;

wrap_around[0] = wrap_around[1] = 1;

MPI_Cart_create( MPI_COMM_WORLD,2, dim_sizes, wrap_around,

reorder, &grid_comm);

int coords[2];

int my_grid_rank;

MPI_Comm_rank( grid_comm, &my_grid_rank);

MPI_Cart_coords( grid_comm, my_grid_rank, 2, coords);

(need to call MPI_Comm_rank because we’ve reordered)

MPI_Cart_rank( grid_comm, coords, &grid_rank);