How to generate “random” but also “unique” numbers?

Question

How are random numbers generated.? How do languages such as java etc generate random numbers, especially how it is done for GUIDs.? i found that algorithms like Pseudorandomnumb

Answer 1

There are a lot of ways you could generate random numbers. It's usually done with a system/library call which uses a pseudo-number generator with a seed as you've already described.

But, there are other ways of getting random numbers which involve specialized hardware to get TRUE random numbers. I know of some poker sites that use this kind of hardware. It's very interesting to read how they do it.

Answer 2

I understand that you are seeking a way to generate random number using C#. If yes, RNGCryptoServiceProvider is what you are looking for.

[EDIT]

If you generate a fairly long number of bytes using RNGCryptoServiceProvider, it is likely to be unique but there is no gurantee. In theory, true random numbers doesnt mean to be unique. You roll a dice 2 times and you may get head both the times but they are still random. TRUE RANDOM!

I guess to apply the check of being unique, you just have to roll out your own mechanism of keeping history of previously generated numbers.

Answer 3

You could use this code sample: http://xkcd.com/221/ Or, you can use this book: http://www.amazon.com/Million-Random-Digits-Normal-Deviates/dp/0833030477

But seriously, don't implement it yourself, use an existing library. You can't be the first person to do this.

Answer 4

Specifically regarding Java:

• java.util.Random uses a linear congruential generator, which is not very good
• java.util.UUID#randomUUID() uses java.security.SecureRandom, an interface for a variety of cryptographically secure RNGs - the default is based on SHA-1, I believe.
• UUIDs/GUIDs are not necessarily random
• It's easy to find implementations of RNGs on the net that are much better than java.util.Random, such as the Mersenne Twister or multiply-with-carry

Answer 5

First: If the number is guaranteed to never repeat, it's not very random.

Second: There are lots of PRNG algorithms.

UPDATE:

Third: There's an IETF RFC for UUIDs (what MS calls GUIDs), but you should recognize that (U|G)UIDs are not cryptographically secure, if that is a concern for you.

UPDATE 2:

If you want to actually use something like this in production code (not just for your own edification) please use a pre-existing library. This is the sort of code that is almost guaranteed to have subtle bugs in it if you've never done it before (or even if you have).

UPDATE 3:

Here's the docs for .NET's GUID

Answer 6

Most random number generators have a way to "randomly" reïnitialize the seed value. (Sometimes called randomize).

If that's not possible, you can also use the system clock to initialize the seed.