Qbasic RND

[PlayGGY]

It could be a scientific equation. ((If someone finds it here comes sentient A.I.)) but i asked my mom and she said 40.

Yeah, 37 is just the most common answer, not the answer. Free will by definition isn't an equation.

[whitetiger0990]

yes, by definition. But that can't be proven. So choice is just an illiusion OooOOOOOoooOOoOooo
Hey, cool, 37 is about 3/4 of 50. That partially explains why C is the most chose mulitple choice answer

[Mango]

There is no RND algorithm that can generate *truly* random numbers because a truly random number has no pattern. A pattern is something that can be replicated. An algorithm can be run twice with the same results: it generates a "pattern".

Aga's right. However, the "pattern" can be so long before it repetes that it is *very* difficult to distinguish from "truely random" data. (I put that in quotations because some people believe that, if one has access to *all* the information about *any* system, then all future states of that system are calculable. I happen to be one of those people!!!) The two important observeables in a PRNG (Pseudo Random Number Generator) are the sequence and the repete length. Consider a 64 bit generator that returns a single byte as it's random data. The 64-bit is the size of the internal state, or the "system size". There is some initial state, the return function, and the update sequence. So...you might, at first guess, suppose that a 64-bit system would have a repete length of 2^64...that is, from any given internal state, the system will return to that state after 2^64 update events. an example of such a system would be a simple counter...add 1 to the state, and the whole thing will repete after 2^64. Turns out that this makes for one ultra-crappy PRNG!! In reality, a 64-bit PRNG that had a cyle-size of 2^48 would be pretty good...iffffff the PRNG output was able to pass statistical tests for randomness. The Mersine twister was mentioned earlier as a good RRNG...this system is something like 25000 bits with a cycle-length of ~2^19000. On practical computers, such a system will never repete even on a geologic time-scale.

I have created a c++ class that spits out 32-bit "random" integers from my 1.5 GHz machine at between 300 MB/sec (whole state returned between update events) to 10 MB/sec (1/30 internal state returned between update events) if anyone is interested. It has a calculated average cycle-length of 2^(0.75 * system-size) with some weak-key states that are more like 2 ^ (0.25 * system-size). Even with weak keys, this beast's cycle length will be something like 2^16000 with 1kb returned per cycle.

If you are interested in probing the quality of "random" data, Google "Diehard random". There is a version of this test that tests a 12MB data-set, and one (diehardII) that tests a 300MB data-set). If you test the QB RND function, you will find it fails this test miserably....as does the c-function rand(). However there are many good-quality generators that will pass.

This topic is closely related to cryptography. Suppose that you have a file that consists of 100 MB of highly repetitive data...say 100MB of 'zeros'. After encryption, you would like this data to be indistinguishable from "random" data...and this is the challenge of modern cryptography...and all good cryptography schemes succeed in this endeavor...


ANyway...I (again) strayed from the topic at hand...However aga is right...computers (which follow rules by definition) can never, from a given internal state and a given input, produce variable output...However, it is fairly easy to make systems that produce chaotic output that cannot be predicted except by allowing the program to run and observing the output. Put another way...a good PRNG has the property that there are no "shortcuts" to predicting the next output bit, other than to have knowledge of the system's internal initial state, and the number of update events that have occured. The QB RND does not have this property...that is, it is easy to determine the internal state based on the RNG output, and thus, if you use RND to deal cards, if the stakes are high enough, someone will figure out your internal state, which will allow them to *know* the next card!!!


If you need good random data, make sure you understand the issues well enough to get it!!

[PlayGGY]

I don't want to quote the last post, because it is too long, but I agree with almost everything he said (except for the "know everything about a system and you can predict th enext event" thing, but that is a huge discussion by itself).

The Mersenne Twister, as he and I said, is one of the best ways of making an equation that looks random. It shifts bits around, which makes it very hard to see what is happening.

The C/C++ rand() function of course depends on which implimentation is being used, but most modern implimentations are good enough for any game (just not a casino, use an MS for that).

[Agamemnus]

You don't agree that if you know everything about a system you can predict what happens to it? Maybe you might not agree that all systems can be ever predicted, but surely the predictability of a fully known system is a fact...?

[PlayGGY]

You don't agree that if you know everything about a system you can predict what happens to it? Maybe you might not agree that all systems can be ever predicted, but surely the predictability of a fully known system is a fact...?

OK, then here we go!!

I am not the only one who dissagrees with this. And first let me clarify: If you are talking about a simple closed system, like a computer program, then of course I agree. But it isn't true for all systems. Lets take the universe.

At one time, it didn't exist, so something came from absolutely nothing. If you beleive the universe has always existed (which doesn't really make sense), then we would have traversed an infinite amount of time, which by definition is impossible.

So, the thing that acted completely on its own, without anything causing it that caused the universe to exist could not have been predicted (there was nothing to know). What ever force created itself unprovoced back then (lets call that free will) is most likely either still acting today, or could act up at any time, causing the universe to be essentially unpredictable.

[Mango]

Heading way off topic......

OK, then here we go!!

I am not the only one who dissagrees with this. And first let me clarify: If you are talking about a simple closed system, like a computer program, then of course I agree. But it isn't true for all systems. Lets take the universe.

Define "the universe", and let's see where this goes.

[PlayGGY]

Heading way off topic......

OK, then here we go!!

I am not the only one who dissagrees with this. And first let me clarify: If you are talking about a simple closed system, like a computer program, then of course I agree. But it isn't true for all systems. Lets take the universe.

Define "the universe", and let's see where this goes.

It will go completely away from my RND topic, and start a third arguement.

[Moneo]

.... Aga's right. However, the "pattern" can be so long before it repetes that it is *very* difficult to distinguish from "truely random" data......

Mango,
Thanks for an excellent and informative post.
*****