My solution to: http://www.hpc.cineca.it/content/exercise-4

ex4.c

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

int main (int argc, char** argv) {
    int i, rank, size, left, right;
    double A, B, SUM;

    MPI_Status status;

    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    A = rank;
    left = (rank == 0) ? size - 1 : rank - 1;
    right = (rank == size - 1) ? 0 : rank + 1;

    for (i = 0; i < size; i++) {
        MPI_Sendrecv(&A, 1, MPI_DOUBLE, right, 0, &B, 1, MPI_DOUBLE, left, 0, MPI_COMM_WORLD, &status);
        SUM += B;
        A = B;
    }

    printf("I am proc %d and SUM = %.2f\n", rank, SUM);

    MPI_Finalize();
    return 0;
}