pi.c

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

int main(int argc, char* argv[]) {
  int n, myid, numprocs, i;
  char me[255];
  double PI25DT = 3.141592653589793238462643;
  double mypi, pi, h,sum, x;
  double starttime, endtime;
  gethostname(me, 254);
  printf("%s before call to MPI_Init\n", me);
  MPI_Init(&argc, &argv);
  MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
  MPI_Comm_rank(MPI_COMM_WORLD, &myid);
  printf("Starting pi on %s myid=%d\n", me, myid);
  printf("numprocs=%d\n", numprocs);
  while (1) {
    if (myid == 0) {
      printf("Enter the number of intervals: (0 quits) \n");
      scanf("%d", &n);
    }
    starttime = MPI_Wtime();
    MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);
    if (n == 0) break;
    else {
      h = 1.0 / (double) n;
      sum = 0.0;
      for (i = myid + 1; i <= n; i += numprocs) {
	x = h * ((double) i - 0.5);
	sum += (4.0 / (1.0 + x*x));
      }
      mypi = h * sum;
      endtime = MPI_Wtime();
      MPI_Reduce(&mypi, &pi, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
      if (myid == 0) {
	printf("pi is approximately %.16f, Error is %.16f\n",
	       pi, fabs(pi - PI25DT));
        printf("time is %f seconds\n", endtime - starttime);
      }
    }
  }
  MPI_Finalize();
  return 0;
}