sum.c

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

#define SIZE 1000000
#define NUM_TIMES 10

void init(int n, double tab[]) {
    for (int i = 0; i < n; ++i) {
        tab[i] = (double)rand() / (double)RAND_MAX;
    }
}

double sum(int n, double tab[]) {
    double s = tab[0];
    for (int i = 1; i < n; ++i) {
        s += tab[i] * tab[i] * tab[i] * tab[i];
    }
    return s;
}

int main() {
    double tab[SIZE];
    init(SIZE, tab);

    struct timespec tstart = {0, 0}, tend = {0, 0};
    clock_gettime(CLOCK_MONOTONIC, &tstart);
    double s = 0;
    for (int i = 0; i < NUM_TIMES; ++i) {
        s += sum(SIZE, tab);
    }
    clock_gettime(CLOCK_MONOTONIC, &tend);
    printf("the computation of %f took about %.5f seconds\n", s,
        (((double)tend.tv_sec + 1e-9 * tend.tv_nsec) -
            ((double)tstart.tv_sec + 1e-9 * tstart.tv_nsec)) /
            NUM_TIMES);

    return 0;
}