Code Explanation (MPICH) [closed]

Question

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


double f(double a)
{
    return (4.0 / (1.0 + a*a));
}

void main(int argc, char *argv[])
{
    int done = 0, n, myid, numprocs,i;
    double PI25DT = 3.141592653589793238462643;
    double mypi, pi, h, sum, x;
    double startwtime, endwtime;
    int  namelen;
    char processor_name[MPI_MAX_PROCESSOR_NAME];    
    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&myid);
    MPI_Get_processor_name(processor_name,&namelen);    
    fprintf(stderr,"Process %d on %s\n",
        myid, processor_name);
    fflush(stderr); 
    n = 0;
    while (!done)
    {
        if (myid == 0)
        {
        printf("Enter the number of intervals: (0 quits) ");fflush(stdout);
        scanf("%d",&n);

        startwtime = MPI_Wtime();
        }
        MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);
        if (n == 0)
            done = 1;
        else
        {
            h   = 1.0 / (double) n;
            sum = 0.0;
            for (i = myid + 1; i <= n; i += numprocs)
            {
                x = h * ((double)i - 0.5);
                sum += f(x);
            }
            mypi = h * sum;

            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));
        endwtime = MPI_Wtime();
        printf("wall clock time = %f\n", endwtime-startwtime);         
        }
        }
    }
    MPI_Finalize();
}

Can anyone explain me the above code what it does? I am in lab and my miss has asked me to explain and I don't understand.

Answer 1

It's the example of using parrallel processing with the MPICH library to calculate pi. This example is included in the mpich installation. It essentially divides up the job of calculating pi by adding together fractions amongst a number of processors or hosts.