rand() not giving me a random number (even when srand() is used)

Question

Okay I'm starting to lose my mind. All I want to do is random a number between 0 and 410, and according to this page, my code should do that. And since I want a random number and not a pseudo-random number, I'm using srand() as well, in a way that e.g. this thread told me to do. But this isn't working. All I get is a number that is depending on how long it was since my last execution. If I e.g. execute it again as fast as I can, the number is usually 6 numbers higher than the last number, and if I wait longer, it's higher, etc. When it reaches 410 it goes back to 0 and begins all over again. What am I missing?

Edit: And oh, if I remove the srand(time(NULL)); line I just get the same number (41) every time I run the program. That's not even pseudo random, that's just a static number. Just copying the first line of code from the article I linked to above still gives me number 41 all the time. Am I the star in a sequel to "The Number 23", or have I missed something?

int main(void) {

    srand(time(NULL));
    int number = rand() % 410;

    std::cout << number << std::endl;

    system("pause");

}

Answer 1

That is what you get for using deprecated random number generation.

rand produces a fixed sequence of numbers (which by itself is fine), and does that very, very badly.

You tell rand via srand where in the sequence to start. Since your "starting point" (called seed btw) depends on the number of seconds since 1.1.1970 0:00:00 UTC, your output is obviously time depended.

The correct way to do what you want to do is using the C++11 <random> library. In your concrete example, this would look somewhat like this:

std::mt19937 rng (std::random_device{}());
std::uniform_int_distribution<> dist (0, 409);

auto random_number = dist(rng);

For more information on the evils of rand and the advantages of <random> have a look at this.

As a last remark, seeding std::mt19937 like I did above is not quite optimal because the MT's state space is much larger than the 32 bit you get out of a single call to std::random_device{}(). This is not a problem for toy programs and your standard school assignments, but for reference: Here is my take at seeding the MT's entire state space, plus some helpful suggestions in the answers.

Answer 2

From manual:

time() returns the time as the number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).

Which means that if you start your program twice both times at the same second you will initialize srand with same value and will get same state of PRNG.

And if you remove initialization via call to srand you will always get exactly same sequence of numbers from rand.

Answer 3

I'm afraid you can't get trully random numbers there. Built in functions are meant to provide just pseudo random numbers. Moreover using srand and rand, because the first uses the same approach as the second one. If you want to cook true random numbers, you must find a correct source of entrophy, working for example with atmospheric noise, as the approach of www.random.org.

The problem here consists in the seed used by the randomness algorithm: if it's a number provided by a machine, it can't be unpredictable. A normal solution for this is using external hardware.

Answer 4

Unfortunately you can't get a real random number from a computer without specific hardware (which is often too slow to be practical).

Therefore you need to make do with a pseudo generator. But you need to use them carefully.

The function rand is designed to return a number between 0 and RAND_MAX in a way that, broadly speaking, satisfies the statistical properties of a uniform distribution. At best you can expect the mean of the drawn numbers to be 0.5 * RAND_MAX and the variance to be RAND_MAX * RAND_MAX / 12.

Typically the implementation of rand is a linear congruential generator which basically means that the returned number is a function of the previous number. That can give surprisingly good results and allows you to seed the generator with a function srand.

But repeated use of srand ruins the statistical properties of the generator, which is what is happening to you: your use of srand is correlated with your system clock time. The behaviour you're observing is completely expected.

What you should do is to only make one call to srand and then draw a sequence of numbers using rand. You cannot easily do this in the way you've set things up. But there are alternatives; you could switch to a random number generator (say mersenne twister) which allows you to draw the (n)th term and you could pass the value of n as a command line argument.

As a final remark, I'd avoid using a modulus when drawing a number. This will create a statistical bias if your modulo is not a multiple of RAND_MAX.

Answer 5

Try by change the NULL in time(NULL) by time(0) (that will give you the current système time). If it doesn't work, you could try to convert time(0) into ms by doing time(0)*1000.