[R] Difference between 32-bit and 64-bit version

[Duncan Murdoch]

On 03/06/2015 11:56 AM, Thierry Onkelinx wrote:
> Dear all,
> 
> I'm a bit puzzled by the difference in an object when created in R 32-bit
> and R 64-bit.
> 
> Consider the code below. test.rda is available at
> https://drive.google.com/file/d/0BzBrlGSuB9n-NFBWeC1TR093Sms/view?usp=sharing
> 
> # Run in R 3.2.0 Windows 32-bit, lme4 1.1-8
> library(lme4)
> load("test.rda")
> coef.32 <- coef(test)
> save(coef.32, file = "32bit.rda")
> 
> # Run in R 3.2.0 Windows 64-bit, lme4 1.1-8
> library(lme4)
> load("~/test.rda")
> coef.64 <- coef(test)
> save(coef.64, file = "64bit.rda")
> 
> 
> # Compare the results
> # Run in R 3.2.0 Windows 32-bit, lme4 1.1-8
> # Run in R 3.2.0 Windows 64-bit, lme4 1.1-8
> library(lme4)
> load("32bit.rda")
> load("64bit.rda")
> identical(coef.32, coef.64) # FALSE
> identical(coef.32$fRow, coef.64$fRow) # FALSE
> identical(coef.32$fLocation, coef.64$fLocation) # TRUE
> identical(coef.32$fSubLocation, coef.64$fSubLocation) # TRUE
> 
> The first comparison is FALSE, because the second is FALSE. But why is the
> second FALSE and the third and fourth TRUE?
> 
> My goal is the calculate a SHA1 hash on the coef(test) to track if the
> coefficients of test have changed. I'd like to get the same hash on a
> 32-bit and 64-bit system. A simple hack would be to calculate the hash on
> round(coef(test), 20). Is that a good or bad idea?
> 
> identical(round(coef.32$fRow, 20), round(coef.64$fRow, 20)) # TRUE

Different math libraries round differently, so small differences are
expected.  This is FAQ 7.31.  In many cases the 32 bit calculations are
more accurate, because they tend to use more 80 bit extended precision
intermediate values, but that is not guaranteed.

Rounding before comparing makes sense, but I would use signif() instead
of round(), I would choose a relatively small number of significant
digits, and I would expect to see a few false positives:  if the true
value is 0 but some "random" noise is added, I'd expect values rounded
by signif() to be unequal.

Duncan Murdoch

> 
> Best regards,
> 
> ir. Thierry Onkelinx
> Instituut voor natuur- en bosonderzoek / Research Institute for Nature and
> Forest
> team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
> Kliniekstraat 25
> 1070 Anderlecht
> Belgium
> 
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> than asking him to perform a post-mortem examination: he may be able to say
> what the experiment died of. ~ Sir Ronald Aylmer Fisher
> The plural of anecdote is not data. ~ Roger Brinner
> The combination of some data and an aching desire for an answer does not
> ensure that a reasonable answer can be extracted from a given body of data.
> ~ John Tukey
> 
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