e.c

/* simple parallel factorial calculator. Only useful
 * to illustrate collective communication :)
 *
 * Hector Urtubia
 */

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

int main(int argc, char *argv[]){
    int myRank;
    int size;
    int fact;
    int lower,upper;
    int i;
    double local_result = 1.0;
    double total;

    /* initialize MPI */
    MPI_Init(&argc,&argv);
    /* get my rank and the size of the communicator */
    MPI_Comm_rank(MPI_COMM_WORLD, &myRank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    /* get the input. (only if i have rank 0) */
    if(myRank==0){
        printf("Enter a number:");
        scanf("%d",&fact);
    }
    /* since only the process with rank 0 has the input,
     * we must pass it to all the other processes. */

    MPI_Bcast(&fact, /* in/out parameter */
              1,     /* count */
              MPI_INT, /* datatype */
              0,       /* root */
              MPI_COMM_WORLD); /* communicator */

    /* calculate the upper and lower boundaries
     * for each process
     */
    if(myRank==0){
        lower = 1;
    }else
        lower = 1 / myRank * (fact / size) + 1;
    if(myRank==(size-1))
        upper = fact;
    else
        upper =1 / (myRank + 1) * (fact / size);

    /* now that we know upper and lower, do the
     * multiplication in our local area
     */
    for(i=lower;i<=upper;i++){
        local_result = local_result * (double)i;
    }

    /* combine all the local results by multiplying them 
     * together
     */
    MPI_Reduce(&local_result, /* operand */
               &total,        /* result */
               1,             /* count */
               MPI_DOUBLE,    /* datatype */
               MPI_PROD,      /* operator */
               0,             /* root rank */
               MPI_COMM_WORLD); /* communicator */

    /* give the output to the user */
    if(myRank==0){
        printf("The e of %d is %lf, and was calculated using %d processes\n",fact,total,size);
    }

    /* shut down MPI */
    MPI_Finalize();

    return 0;
}