random number generator of Math.random() ?

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Bart Vandewoestyne - 30 May 2006 11:05 GMT

Simple question: does the Java standard specify what random
number generator algorithm should be used for Math.random() ? If
'no', then can anybody tell me what PRNG is behind Sun's version
of Math.random()?

Thanks,
Bart

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Oliver Wong - 30 May 2006 15:28 GMT

> Simple question: does the Java standard specify what random
> number generator algorithm should be used for Math.random() ?

Yes it does.

> If
> 'no', then can anybody tell me what PRNG is behind Sun's version
> of Math.random()?

See http://java.sun.com/j2se/1.5.0/docs/api/java/util/Random.html

- Oliver

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Bart Vandewoestyne - 30 May 2006 16:41 GMT

>> 'no', then can anybody tell me what PRNG is behind Sun's version
>> of Math.random()?
>
> See http://java.sun.com/j2se/1.5.0/docs/api/java/util/Random.html

Hmm... so if I understand things correctly, the pseudo-random
numbers produced by Random.nextDouble() are the same as the ones
produced by Math.random() and the underlying algorithm is the one
as specified in

http://java.sun.com/j2se/1.5.0/docs/api/java/util/Random.html#nextDouble()

namely

public double nextDouble() {
return (((long)next(26) << 27) + next(27))
/ (double)(1L << 53);
}

Looking at Random.next() then tells me that the PRNG used here is a linear
congruential pseudorandom number generator, as defined by D. H. Lehmer and
described by Donald E. Knuth in The Art of Computer Programming, Volume 2:
Seminumerical Algorithms, section 3.2.1

So am I correct if I say that the Java standard specifies the above PRNG as the PRNG to use when implementing Math.random() ?

Bart

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Oliver Wong - 30 May 2006 22:01 GMT

[...]

> public double nextDouble() {
> return (((long)next(26) << 27) + next(27))
> / (double)(1L << 53);
> }

[...]

> So am I correct if I say that the Java standard specifies the above PRNG
> as the PRNG to use when implementing Math.random() ?

Yes, I think so, or at least one which returns the exact same set of
numbers given the same initial seed.

- Oliver

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random number generator of Math.random() ?

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