calc_pi.c

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

int main(int argc, char *argv[])
{
    int cpu, num;
    unsigned int i, n;
    double sub, pi, x, dx, sum;
    double PI25DT = 3.141592653589793238462643;

    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &num);
    MPI_Comm_rank(MPI_COMM_WORLD, &cpu);

    n = 4000000000;

    dx  = 1.0 / (double) n;
    sum = 0.0;
    for (i = cpu; i < n; i += num) {
        x = dx * ((double)i - 0.5);
        sum += (4.0 / (1.0 + x*x));
    }
    sub = dx * sum;

    MPI_Reduce(&sub, &pi, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);

    if (cpu == 0)
        printf("pi is approximately %.15f, Error is %.12f\n",
                  pi, fabs(pi - PI25DT));

    MPI_Finalize();
    return 0;
}