Thread: Random number generation

rand() generates pseudo-random numbers. The numbers it generates are pretty good, but if you want better random numbers, you could use random() or urandom() if you're on Linux. I'm sure there are other functions in other libraries out there too. (random.org comes to mind.)

It depends on the implementation and your exact requirements, and the C standard does not mandate any particular implementation. The Mersenne Twister is a well known algorithm that you can use, with an implementation available from Prelude's website, but it certainly will not do for a cryptographic PRNG (which you do not seem to require).

To start from the back, yes, there are other random number generators available. The Mersenne Twister is a good candidate.

However, the question we have to clarify is how you would classify "good" in a random number generator versus a bad one. There are several qualities:

1. repeat distance - how many numbers do you need to take out before you get the same sequence of numbers back again?
2. spread/variation - if you get a high number, how likely is it that the next number is also high, or much lower - there should be no "pattern" to this.
3. Low vs. high bit variability - Do all parts of the number "vary" the equally, or are for example the
4. Predictability - If we know what the last few numbers (say 5) are, can we predict what the next number should be?

Of course, some of those variables will be more important than others for a particular system. An encryption mechanism would be more concerned with repeat distance and predictability than spread/variation. Note also the a long repeat distance doesn't necessarily mean a good randomness - just counting a 32-bit number up by 1 each time will get a 4 billion repeat distance, but very predictable.

There is a lot of research on various randum number generators...

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Mats

Additionally, you may not want the numbers to be evenly distributed, e.g. you may want to have a gaussian distribution (normal/bell-shape distribution) where 50% of samples are in the middle 30% of the range, and much less than average likelyhood to hit the outer 10% of the range.

There are other ways to distribute numbers in a range too.

rand is "even" in it's distribution, that is it's equally likely that you get a number anywhere in the range [or at least, it should be if it's a decent implementation].

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Mats

The best attempt I've ever seen at a random number generator:

Code:

int getRandomNumber() 
{
    return 4; /* randomly chosen by throw of a dice */
}

QuantumPete

Despite some of the pulicity, I don't believe rand() is BAD. It is not great either, but for most intents and purposes, and given a good seed (such as time with good spread - don't expect it to give you good variation if you run the same program 60 times in the same minute).

Obviously, as tabstop says, you need to understand what the distribution is like in reality when using random numbers for simulations - it would be rather silly to have a uniform distribution to simulate for example human height - the distribution should be "normal". On the other hand, distribution of throwing a dice should be uniform, because 1, 2, or 6 is equally likely to happen.

I found this:
http://www.dreamincode.net/code/snippet1446.htm

Not sure how useful it is.

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Mats