mpi_cart_shift.cpp

/* 
 * MPI_Cart_create
 */

# include <iostream>
# include <cstdlib>
# include "mpi.h"

using namespace std;

int main(int argc, char *argv[])
{
	// MPI Init and get rank
	int rank, nprocs;
	MPI_Init(&argc, &argv);
	MPI_Comm_rank(MPI_COMM_WORLD, &rank);
	MPI_Comm_size(MPI_COMM_WORLD, &nprocs);

	// Variables required by Create_cart
	int ndims, dims[2], periods[2], reorder;
	ndims = 2;		    // 2D matrix/grid 
	dims[0] = 3;		// rows 
	dims[1] = 4;		// columns 
	periods[0] = true;	// row periodic (each column forms a ring) 
	periods[1] = true;	// columns periodic (each row forms a ring)
	reorder = true;		// allows processes reordered for efficiency 

	// Create a communicator given the 2D torus topology.
	MPI_Comm new_comm;
	MPI_Cart_create(MPI_COMM_WORLD, ndims, dims, periods, reorder, &new_comm);

	// Torus rank in the new communicator
	int torus_rank;
	MPI_Comm_rank(new_comm, &torus_rank);

	// Get my coordinates in the new communicator
	int torus_coords[2];
	MPI_Cart_coords(new_comm, torus_rank, ndims, torus_coords);

	// Print my location in the 2D torus
	cout << "rank:" << rank 
		<< "> torus_rank=" << torus_rank 
		<< ", coords=(" << torus_coords[0] << "," << torus_coords[1] << ")" << endl;


	enum DIRECTIONS {DOWN, UP, LEFT, RIGHT};
	const char* neighbours_names[4] = {"DOWN", "UP", "LEFT", "RIGHT"};
	int neighbours_ranks[4];

	MPI_Cart_shift(new_comm, 0, 1, &neighbours_ranks[UP], &neighbours_ranks[DOWN]);
	MPI_Cart_shift(new_comm, 1, 1, &neighbours_ranks[LEFT], &neighbours_ranks[RIGHT]);

	int my_rank;
	MPI_Comm_rank(new_comm, &my_rank);

	for(int i=0; i<4; i++)
	{
		cout 	<< "MPI rank=" << my_rank << " I have a " << neighbours_names[i]
			<< " neighbour rank=" << neighbours_ranks[i] << endl;
	} 

	// MPI finalize and exit
	MPI_Finalize();
	return 0;
}