Prime Calculator (C++)

#include <iostream> // cout
#include <math.h> // sqrt
#include <time.h> // clock/CLOCKS_PER_SEC
#include <stdlib.h> // malloc/free

using namespace std;

int main(int argc, const char * argv[]) {
    int n = 1000000000;
    int i;
    int j;
    int k;
    int s;
    int c;
    int sr;
    int * a = (int*)malloc((size_t)n * sizeof(int));

    for(i = 0; i < n; i++) {
        a[i] = 1;
    }

    clock_t t = clock();

    sr = sqrt(n);
    for(i = 2; i <= sr; i++) {
        if(a[i]) {
            s = i * i;
            for(k = 0, j = 0; j <= n; j = s + (k * i), k++) {
                a[j] = 0;
            }
        }
    }

    t = clock() - t;

    c = 0;
    for(i = 2; i < n; i++) {
        if(a[i]) {
            //cout << i << " ";
            c++;
        }
    }

    cout << fixed << "\nCalculated " << c << " prime numbers up to " << n << " in " << t << " clocks (" << ((float)t) / CLOCKS_PER_SEC << " seconds).\n";

    free(a);

    return 0;
}