apr22-pi1.c

#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <omp.h>

#define CIRCLE(step, i) sqrt(4.0 - (double) (i) * (step) * (double) (i) * (step))

int main() {
    omp_set_num_threads(omp_get_num_procs());

    int N;
    FILE* datp = fopen("apr22-pi1.dat", "r");
    if (datp == NULL) {
        printf("Error. Couldn't open the data file.");
        exit(1);
    }
    if (fscanf(datp, "%d", &N) != 1) {
        printf("Error. Couldn't read the number from the data file.");
        exit(2);
    }
    fclose(datp);

    int i, ibeg, iend, size, rank;
    double step = 2.0 / (double) N;
    double f_old, f_new, S = 0.0;

#pragma omp parallel shared(step, size, N) private(i, ibeg, iend, rank, f_old, f_new) reduction(+:S)
    {
        size = omp_get_num_threads();
        rank = omp_get_thread_num();
        ibeg = (N / size) * rank;
        iend = (rank == size - 1) ? (N - 1) : ((N / size) * (rank + 1) - 1);
        f_old = CIRCLE(step, ibeg);
        S = 0.0;

        for (i = ibeg; i <= iend; ++i) {
            f_new = CIRCLE(step, i + 1);
            S += (f_old + f_new) * step / 2.0;
            f_old = f_new;
        }
    }

    printf("Pi = %f\n", S);

    return 0;
}