Re: [Rd] [R] RNG Cycle and Duplication (PR#12540)

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Fri, 15 Aug 2008 01:58:29 -0400

shli_at_stat.wvu.edu wrote:
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> I didn't describe the problem clearly. It's about the number of distinct=20
> values. So just ignore cycle issue.
>
> My tests were:
>
> RNGkind(kind=3D"Knuth-TAOCP");
> sum(duplicated(runif(1e7))); #return 46552
>
> RNGkind(kind=3D"Knuth-TAOCP-2002");
> sum(duplicated(runif(1e7))); #return 46415
>
> #These collision frequency suggested there were 2^30 distinct values by=20
> birthday problem.
>

The birthday problem distribution applies to independent draws, but they are only pseudo-independent. I think the only ways to know for sure if there are 2^30 values are to look at the code, or run through a complete cycle. And you need to determine the cycle by looking at .Random.seed, not at the returned value.
>
> RNGkind(kind=3D"Marsaglia-Multicarry");
> sum(duplicated(runif(1e7))); #return 11682
>
> RNGkind(kind=3D"Super-Duper");
> sum(duplicated(runif(1e7))); #return 11542
>
> RNGkind(kind=3D"Mersenne-Twister");
> sum(duplicated(runif(1e7))); #return 11656
>
> #These indicated there were 2^32 distinct values, which agrees with the=20
> help info.
>
>

If there are 2^30 distinct values for the two generators above, that also agrees with the documentation.

> RNGkind(kind=3D"Wichmann-Hill");
> sum(duplicated(runif(1e7))); #return 0
>
> #So for this method, there should be more than 2^32 distinct values.
>
> You may not get the exact numbers, but they should be close. So how to=20
> explain above problem?
>

You haven't demonstrated what you claim, but if you look at the source, you'll see that in fact the man page is wrong: Wichmann-Hill is based on 3 integer values, which each take on approximately 15 bits of different values. So Wichmann-Hill could take nearly 2^45 different values (actually 30269*30307*30323).

The source is in https://svn.r-project.org/R/trunk/src/main/RNG.c if you want to check the others.
> I need generate a large sample without any ties, it seems to me=20
> "Wichmann-Hill" is only choice right now.
>

An alternative would be to construct a new value from two (or more) runif() values, but be careful that you don't mess up the distribution when you do that.

Duncan Murdoch
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> Shengqiao Li
>
> The Department of Statistics
> PO Box 6330
> West Virginia University
> Morgantown, WV 26506-6330
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>
> On Thu, 14 Aug 2008, Peter Dalgaard wrote:
>
>
>> Shengqiao Li wrote:
>>
>>> Hello all,
>>> =20
>>> I am generating large samples of random numbers. The RNG help page says:=
>>>
> =20
>
>>> "All the supplied uniform generators return 32-bit integer values that a=
>>>
> re=20
>
>>> converted to doubles, so they take at most 2^32 distinct values and long=
>>>
> =20
>
>>> runs will return duplicated values." But I find that the cycles are not =
>>>
> the=20
>
>>> same as the 32-bit integer.
>>> =20
>>> My test indicated that the cycles for Knuth's methods were 2^30 while=20
>>> Wichmann-Hill's cycle was larger than 2^32! No numbers were duplicated i=
>>>
> n=20
>
>>> 10M numbers generated by runif using Wichmann-Hill. The other three meth=
>>>
> ods=20
>
>>> had cycle length of 2^32.
>>> =20
>>> So, anybody can explain this? And any improvement to the implementation =
>>>
> can=20
>
>>> be made to increase the cycle length like the Wichmann-Hill method?
>>> =20
>>>
>> What test? These are not simple linear congruential generators. Just beca=
>>
> use=20
>
>> you get the same value twice, it doesn't mean that the sequence is repeat=
>>
> ing.=20
>
>> Perhaps you should read the entire help page rather than just the note.
>>
>> --=20
>> O__ ---- Peter Dalgaard =D8ster Farimagsgade 5, Entr.B
>> c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
>> (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
>> ~~~~~~~~~~ - (p.dalgaard_at_biostat.ku.dk) FAX: (+45) 35327907
>>
>>
>>
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>
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