program int_pi1
c
c For API
c
c This simple program approximates pi by computing pi = integral
c from 0 to 1 of 4/(1+x*x)dx which is approximated by sum from
c k=1 to N of 4 / ((1 + (k-1/2)**2 ).  The only input data required is N.
c
c Parallel version #1: All instances are started at load time, but no 
c messages or division of work
c RLF 4/4/93 16:02
c revised: 6/4/93 riordan
c Converted to MPI: 11/12/94 Xianneng Shen
c
       include "mpif.h"
       integer ierr, status(MPI_STATUS_SIZE)
       parameter (maxproc=100)
       real err, f, pi, sum, w
       integer i, N, nprocs, mynum
       f(x) = 4.0/(1.0+x*x)
       pi = 4.0*atan(1.0)
c
c All instances call the startup routine to get their instance number (mynum)
       call MPI_INIT(ierr)
       call MPI_COMM_RANK(MPI_COMM_WORLD, mynum, ierr)
       call MPI_COMM_SIZE(MPI_COMM_WORLD, nprocs, ierr)

c -------  Each new approximation to pi begins here. -------------------
5      continue

c Step (1): get a value for N, the number of intervals in the approximation
c (Parallel versions: the initial instance, or master, reads this in.)
c This would be a good place to add message-passing, so that the master
c can send N to the other nodes, and the other nodes can get N from the
c master.  
       if (mynum .eq. 0) then
         print *,'Enter number of approximation intervals:(0 to exit)'
         read *, N
       else
         N=0
       endif

c Step (2): check for exit condition. 
       if (N .le. 0) then
	  print *,'node (',mynum, ') left'
	  call exit
       endif

c Step (3): do the computation in N steps
c (Ultimately, this work should be divided up among the processes)
       w = 1.0/N
       sum = 0.0
       do i = 1,N
	  sum = sum + f((i-0.5)*w)
       enddo
       sum = sum * w

c Step (4): print the results  
c (Ultimately, partial results will have to be sent to the master,
c  who will then print the answer)
       err = sum - pi
       print *, 'sum, err =', sum, err
       go to 5
       call MPI_FINALIZE(ierr)
       end