// Argo Beowulf Cluster: MPI Commands and Examples
// http://www.uic.edu/depts/accc/hardware/argo/mpi_routines.html
// See MPI_Gather for more information
// gather.c

#include "mpi.h"
#include <stdio.h>
#define elements 4

int main( int argc, char *argv[])
{
  int numprocs, myrank, loop;
  int   y[elements] = {2, 4,6,8};
  int   *products;   // this is used to collect the answers from each process
  int   sum = 0;
  int   i; //loop counter

  MPI_Init(&argc,&argv);
  MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
  MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
  products = (int *)malloc(sizeof(int)*numprocs);  // Allocate space for the
                                                  // array of answers.

   if (myrank == 0) printf("\nExample program to demonstrate MPI_Gather\n\n");

   MPI_Bcast(y, elements, MPI_INT, 0, MPI_COMM_WORLD);

   if (myrank == 0) {
        for(loop=0; loop < elements; loop++) {
            sum = sum + y[loop];
        }
        printf("In process %d, product=%d\n", myrank, sum);
   }
   else {
        printf("In process %d:\n");
        printf("Array y:\n");
        for(loop=0; loop < elements; loop++) {
              printf("\t%d", y[loop]);
        }
        printf("\n");
        for(loop=0; loop < elements; loop++) {
             sum = sum + (myrank+1)*y[loop];
        }
            printf("In process %d, dot product=%d\n", myrank, sum);
    }
    MPI_Gather(&sum, 1, MPI_INT, products, 1, MPI_INT, 0, MPI_COMM_WORLD);
    if (myrank==0) {
          printf("Process 0, products = {");
          for(i=0; i<numprocs; i++) {
             printf("%5d", products[i]);
           }
          printf("}\n");
        }

    MPI_Finalize();
    return 0;
}

/*
 cc gather.c -o /tmp/yourusername/a.out
 mpirun -np 6 /tmp/yourusername/a.out 

Example program to demonstrate MPI_Gather

In process 0, product=20
In process 4:
Array y:
        2       4       6       8
In process 1, dot product=40
In process 4:
Array y:
        2       4       6       8
In process 2, dot product=60
In process 4:
Array y:
        2       4       6       8
In process 4, dot product=100
In process 4:
Array y:
        2       4       6       8
In process 5, dot product=120
In process 4:
Array y:
        2       4       6       8
In process 3, dot product=80
Process 0, products = {   20   40   60   80  100  120}

*/