pi.reducer.c

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/* pi.reducer.c (2014-04-04) */

/** \file pi.reducer.c
    \brief Example for KMRRUN.  It is a reducer for PI calculation.

    How to run.
    1. create input files in a directory.
         work/
            000
            001
            002
            ...

       Each file have one line which represents number of points to plot.
         $ cat work/000
         100000

    2. run by kmrrun
         $ mpirun -np 2 ./kmrrun -m "./pi.mapper" \
         -k "./pi.kvgen.sh" -r "./pi.reducer" ./work
*/

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define LINELEN 80

/** \brief Main function.
    Read a file which has key-values separated by lines.
    One line is like this.

       0 7932/10000

    '0' is key and '7932/10000' is value.
    7932 is number of points plotted in a circle and 10000 is
    total number of points plotted.
    By reading these numbers, it calculates pi and writes result
    to a file. */
int
main(int argc, char *argv[])
{
    char line[LINELEN];
    FILE *ifp, *ofp;

    if (argc != 2) {
        fprintf(stderr, "specify an input file\n");
        return 1;
    }

    int sum_count = 0;
    int sum_point = 0;
    ifp = fopen(argv[1], "r");
    while (fgets(line, sizeof(line), ifp) != NULL) {
        char *count_s, *point_s;
        char *cp = line;
        int len = (int)strlen(line);

        // chomp
        if (cp[len-1] == '\n') {
            cp[len-1] = '\0';
        }

        // find In count position
        cp = strchr(line, ' ');
        count_s = cp + 1;

        // find Total point position
        cp = strchr(line, '/');
        point_s = cp + 1;
        cp[0] = '\0';

        sum_count += atoi(count_s);
        sum_point += atoi(point_s);
    }
    fclose(ifp);

    double pi = 4.0 * sum_count / sum_point;

    // write result to stdout
    printf("%f\n", pi);
    // write result to a file
    ofp = fopen("pi.out", "w");
    fprintf(ofp, "%f\n", pi);
    fclose(ofp);

    return 0;
}