Prime number calculation fun

Question

We\'re having a bit of fun here at work. It all started with one of the guys setting up a Hackintosh and we were wondering whether it was faster than a Windows Box of (nearl

Answer 1

Here's my contribution:

Machine: 2.4GHz Quad-Core i7 w/ 8GB RAM @ 1600MHz

Compiler: clang++ main.cpp -O3

Benchmarks:

Caelans-MacBook-Pro:Primer3 Caelan$ ./a.out 100

Calculated 25 prime numbers up to 100 in 2 clocks (0.000002 seconds).
Caelans-MacBook-Pro:Primer3 Caelan$ ./a.out 1000

Calculated 168 prime numbers up to 1000 in 4 clocks (0.000004 seconds).
Caelans-MacBook-Pro:Primer3 Caelan$ ./a.out 10000

Calculated 1229 prime numbers up to 10000 in 18 clocks (0.000018 seconds).
Caelans-MacBook-Pro:Primer3 Caelan$ ./a.out 100000

Calculated 9592 prime numbers up to 100000 in 237 clocks (0.000237 seconds).
Caelans-MacBook-Pro:Primer3 Caelan$ ./a.out 1000000

Calculated 78498 prime numbers up to 1000000 in 3232 clocks (0.003232 seconds).
Caelans-MacBook-Pro:Primer3 Caelan$ ./a.out 10000000

Calculated 664579 prime numbers up to 10000000 in 51620 clocks (0.051620 seconds).
Caelans-MacBook-Pro:Primer3 Caelan$ ./a.out 100000000

Calculated 5761455 prime numbers up to 100000000 in 918373 clocks (0.918373 seconds).
Caelans-MacBook-Pro:Primer3 Caelan$ ./a.out 1000000000

Calculated 50847534 prime numbers up to 1000000000 in 10978897 clocks (10.978897 seconds).
Caelans-MacBook-Pro:Primer3 Caelan$ ./a.out 4000000000

Calculated 189961812 prime numbers up to 4000000000 in 53709395 clocks (53.709396 seconds).
Caelans-MacBook-Pro:Primer3 Caelan$ 

Source:

#include  // cout
#include  // sqrt
#include  // clock/CLOCKS_PER_SEC
#include  // malloc/free

using namespace std;

int main(int argc, const char * argv[]) {
    if(argc == 1) {
        cout << "Please enter a number." << "\n";
        return 1;
    }
    long n = atol(argv[1]);
    long i;
    long j;
    long k;
    long c;
    long sr;
    bool * a = (bool*)malloc((size_t)n * sizeof(bool));

    for(i = 2; i < n; i++) {
        a[i] = true;
    }

    clock_t t = clock();

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

    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;
}

This uses the Sieve of Erastothenes approach, I've optimised it as much as I can with my knowledge. Improvements welcome.