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 Pseudorandomnumber generator uses initial values.

But i need to create a random number program, in which a number once occurred should never repeats even if the system is restarted etc. I thought that i need to store the values anywhere so that i can check if the number repeats or not, but it will be too complex when the list goes beyond limits.?

Answer 1

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.

Answer 2

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 3

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 4

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 5

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 6

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.