! reduceF.f90
! This program uses MPI_Reduce to collect data from all the processors -
! including the root process - and apply some operation on all of this data.
! In this program the operation is to sum all of the data collected from all
! processes.  The first parameter in MPI_Reduce is the value being sent from
! each process.  The second parameter is the variable storing the sum of
! the data collected from all processes (in this case, the sum of all the
! PIs).  The fifth parameter, MPI_SUM, is the operation to be done on all
! of the data collected from all processors.
! The total sum, piSum, has a valid value only in the root process.
! The root process is defined in the sixth parameter of MPI_Reduce ('source').

        program main
        include "mpif.h"

        double precision PI25DT
        parameter (PI25DT = 3.141592653589793238462643d0)
        double precision pi
        integer rank, size, count, source
        integer ierr, status(MPI_STATUS_SIZE)

        call MPI_INIT(ierr)
        call MPI_COMM_RANK(MPI_COMM_WORLD, rank, ierr)
        call MPI_COMM_SIZE(MPI_COMM_WORLD, size, ierr)

        count = 1
        source = 0

        if (rank == source) then
                pi = PI25DT
                print *, 'Rank ', rank, ' is broadcasting ', pi
        end if
        call MPI_BCAST(pi, 1, MPI_DOUBLE, source, MPI_COMM_WORLD, ierr);
        if (rank /= source) then
                print *, 'Rank ', rank, ' receives ', pi, &
                        ' from process ', source
        end if

        call MPI_REDUCE(pi, piSum, 1, MPI_DOUBLE, MPI_SUM, source, &
                                MPI_COMM_WORLD, ierr)
        if (rank == source) then
                print *, 'Inside process ', source, &
                                " sum of pi's = ", piSum
        end if
        call  MPI_FINALIZE(ierr)

        end program

! CRAY SV1:

! cc reduce.c
! mpirun -np 8 a.out

! Rank  0  is broadcasting  3.14159265358979323846264300001E+0
! Rank  2  receives  3.1415926535897824578569270833E+0  from process  0
! Rank  3  receives  3.1415926535897824578569270833E+0  from process  0
! Rank  4  receives  3.1415926535897824578569270833E+0  from process  0
! Rank  5  receives  3.1415926535897824578569270833E+0  from process  0
! Inside process  0  sum of pi's =  25.13274122871826
! Rank  1  receives  3.1415926535897824578569270833E+0  from process  0
! Rank  6  receives  3.1415926535897824578569270833E+0  from process  0
! Rank  7  receives  3.1415926535897824578569270833E+0  from process  0

! mpirun -np 4 a.out

! Rank  0  is broadcasting  3.14159265358979323846264300001E+0
! Rank  1  receives  3.1415926535897824578569270833E+0  from process  0
! Rank  2  receives  3.1415926535897824578569270833E+0  from process  0
! Rank  3  receives  3.1415926535897824578569270833E+0  from process  0
! Inside process  0  sum of pi's =  12.56637061435913