Problem 3

/*
int MPI_Graph_create(MPI_Comm oldcomm, int nnodes, int *index, int *edges, int reorder, MPI_Comm *graphcomm),
where:
oldcomm - the initial communicator,
nnodes - the number of the graph vertices,
index - An index into the list of edges, giving the arcs proceeding from each vertex,
edges - the sequential list of the graph arcs,
reorder - the flag for pointing out if the process ranks can be reodered,
cartcomm - the created communicator with the graph type topology.
*/
#include<mpi.h>
#include<stdio.h>
int main(int argc, char *argv[]) {
    MPI_Status status;
    int rank,i;
    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    /***************************************************/
    int reorder=1;   /* Can the processes be reordered, may result in improved performance.*/
    int index[] = { 4,7,10,13,16};
    int edges[] = { 1,2,3,4,0,2,3,0,1,4,0,1,4,0,2,3};
    /***************************************************/
    int nneighbors;
    int * intptr=&nneighbors;
    int the_neighbors[10];
    MPI_Comm MPI_Tree_World;
    MPI_Graph_create(MPI_COMM_WORLD,5, index, edges, reorder,&MPI_Tree_World);
    /*int MPI_Graph_neighbors_count(MPI_Comm comm,int rank, int *nneighbors); */
    MPI_Graph_neighbors_count(MPI_Tree_World,rank, intptr);
    /*int MPI_Graph_neighbors (MPI_Comm comm,int rank,int mneighbors, int *neighbors); */
    MPI_Graph_neighbors (MPI_Tree_World,rank,nneighbors, the_neighbors);
    printf(" %i has %i neighbors.\n",rank,nneighbors);
   for (i=0;i<nneighbors;i++) {
    if (the_neighbors[i]!=rank) printf("     A neighbor of %i is %i .\n",rank,the_neighbors[i]);
   }
    MPI_Finalize();
}

A Run



$ mpirun -n 5 mytopology
 0 has 4 neighbors.
     A neighbor of 0 is 1 .
     A neighbor of 0 is 2 .
     A neighbor of 0 is 3 .
     A neighbor of 0 is 4 .
 1 has 3 neighbors.
     A neighbor of 1 is 0 .
     A neighbor of 1 is 2 .
     A neighbor of 1 is 3 .
 2 has 3 neighbors.
     A neighbor of 2 is 0 .
     A neighbor of 2 is 1 .
     A neighbor of 2 is 4 .
 3 has 3 neighbors.
     A neighbor of 3 is 0 .
     A neighbor of 3 is 1 .
     A neighbor of 3 is 4 .
 4 has 3 neighbors.
     A neighbor of 4 is 0 .
     A neighbor of 4 is 2 .
     A neighbor of 4 is 3 .