Re: [Rd] Randomness not due to seed

From: Duncan Murdoch <murdoch.duncan_at_gmail.com>
Date: Wed, 20 Jul 2011 07:52:21 -0400

On 11-07-19 8:01 PM, Mike Marchywka wrote:
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>> Date: Tue, 19 Jul 2011 06:13:01 -0700
>> From: jeroen.ooms_at_stat.ucla.edu
>> To: r-devel_at_r-project.org
>> Subject: [Rd] Randomness not due to seed
>>
>> I am working on a reproducible computing platform for which I would like to
>> be able to _exactly_ reproduce an R object. However, I am experiencing
>> unexpected randomness in some calculations. I have a hard time finding out
>> exactly how it occurs. The code below illustrates the issue.
>>
>> mylm1<- lm(dist~speed, data=cars);
>> mylm2<- lm(dist~speed, data=cars);
>> identical(mylm1, mylm2); #TRUE
>>
>> makelm<- function(){
>> return(lm(dist~speed, data=cars));

>> }
>>
>> mylm1<- makelm();
>> mylm2<- makelm();
>> identical(mylm1, mylm2); #FALSE
>>
>> When inspecting both objects there seem to be some rounding differences.
>> Setting a seed does not make a difference. Is there any way I can remove
>> this randomness and exactly reproduce the object every time?
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> I don't know if anyone had a specific answer for this

I think Bill Dunlap's answer addressed it: the claim appears to be false.

Duncan Murdoch

but in general floating point is not
> something for which you want to make bitwise equality tests. You can check the Intel
> website for some references but IIRC the FPU can start your calculation with bits or
> settings ( flushing denorms to zero for example) left over from the last user although I can't document that.
>
> for example, you can probably find more like this suggesting that changes in alignmnet
> and rounding in preamble code can be significant,
>
> http://software.intel.com/en-us/articles/consistency-of-floating-point-results-using-the-intel-compiler/
>
> and of course if your algorithm is numerically sensitive results could change a lot. Now
> its also possible you have unitiliazed or corrupt memory, but you would need to
> consider that you will not get bit wise reproduvibility. You can of course go to java
> if you really want that LOL.
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>> --
>> View this message in context: http://r.789695.n4.nabble.com/Randomness-not-due-to-seed-tp3678082p3678082.html
>> Sent from the R devel mailing list archive at Nabble.com.
>>
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