#include <mpi.h>
#include <stdio.h>

int main(int argc, char** argv) {
    /* our rank and size */
    int rank, size;

    /* we need to create groups of processes */
    MPI_Group orig_group, new_group;

    /* find our rank and size in MPI_COMM_WORLD */
    MPI_Init(&argc,&argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    /* make arrays storing the ranks of the processes in group1 and group2 */
    int ranks1[size / 2], ranks2[size / 2], i;
    for (i = 0; i < size/2; i++) {
        ranks1[i] = i;
    }
    for(i = size/2; i < size; i++) {
        ranks2[i - size/2] = i;
    }

    /* find the original group */
    MPI_Comm_group(MPI_COMM_WORLD, &orig_group);

    /* Divide tasks into two distinct groups based upon rank */
    if (rank < size/2) {
        MPI_Group_incl(orig_group, size/2, ranks1, &new_group);
    } else {
        MPI_Group_incl(orig_group, size/2, ranks2, &new_group);
    }

    /* Create new communicator for our group */
    MPI_Comm new_comm;
    MPI_Comm_create(MPI_COMM_WORLD, new_group, &new_comm);

    /* have the processes sum the ranks of each group */
    int send = rank, recv;
    MPI_Allreduce(&send, &recv, 1, MPI_INT, MPI_SUM, new_comm);

    /* get our rank within the new group */
    int grank;
    MPI_Comm_rank(new_comm, &grank);

    /* print the results */
    printf("Process %d (%d in sub-group) has %d!\n", rank, grank, recv);

    /* quit */
    MPI_Finalize();
    return 0;
}