#include "mpi.h"  /* C version */
#include <stdio.h> 
#define NPROCS 8 
int main(argc,argv) 
int argc; char *argv[]; 
{ int rank, new_rank, sendbuf, recvbuf, numtasks, 
ranks1[4]={0,1,2,3}, ranks2[4]={4,5,6,7}; 
MPI_Group orig_group, new_group; 
MPI_Comm new_comm; 
MPI_Init(&argc,&argv); 
MPI_Comm_rank(MPI_COMM_WORLD, &rank); 
MPI_Comm_size(MPI_COMM_WORLD, &numtasks); 
if (numtasks != NPROCS) 
{ printf("Must specify MP_PROCS= %d. Terminating.\n",NPROCS); 
MPI_Finalize(); exit(0); } 
sendbuf = rank; /* Extract the original group handle */ 
MPI_Comm_group(MPI_COMM_WORLD, &orig_group); 
/* Divide tasks into two distinct groups based upon rank */ 
if (rank < NPROCS/2) 
{ MPI_Group_incl(orig_group, NPROCS/2, ranks1, &new_group); } 
else { MPI_Group_incl(orig_group, NPROCS/2, ranks2, &new_group); } 
/* Create new new communicator and then perform collective communications */
MPI_Comm_create(MPI_COMM_WORLD, new_group, &new_comm); 
MPI_Allreduce(&sendbuf, &recvbuf, 1, MPI_INT, MPI_SUM, new_comm); 
MPI_Group_rank (new_group, &new_rank); 
printf("rank= %d newrank= %d recvbuf= %d\n",rank,new_rank,recvbuf); 
MPI_Finalize(); }