Rand() % 14 only generates the values 6 or 13

Question

Whenever I run the following program the returned values are always 6 or 13.

#include <iostream>
#include <fstream>
#include <ctime>
#include <cstdlib>
using namespace std;

//void randomLegs();
//void randomPush();
//void randomPull();
//void randomMisc();


int main(int argc, const char * argv[])
{
    srand(time(NULL));
    //randomLegs();
    cout << rand() % 14;
    return 0;
}

I have run the program close to a hundred times during today and yesterday.

Can anyone tell me what I'm doing wrong?

Thank you.

EDIT: By the way, if I change the range of rand() to say 13 or 15 it works just fine.

Answer 1

I can reproduce the problem on Mac OS X 10.9 with Xcode 5 - it looks like it might actually be a bug, or at least a limitation with rand()/srand() on OS X 10.9.

I recommend you use arc4random() instead, which works a lot better than rand(), and which doesn't require that you randomize the seed:

#include <iostream>
#include <cstdlib>

using namespace std;

int main(int argc, const char * argv[])
{
    cout << (arc4random() % 14) << endl;
    return 0;
}

Test:

$ g++ -Wall -O3 srand.cpp && ./a.out
5
$ ./a.out
8
$ ./a.out
0
$ ./a.out
8
$ ./a.out
11
$ ./a.out
8
$ ./a.out
3
$ ./a.out
13
$ ./a.out
9
$