[Rd] Error message for infinite probability parameters in rbinom() and rmultinom()

[Duncan Murdoch]

On 08/04/2023 5:53 p.m., Martin Maechler wrote:
>>>>>> Christophe Dutang
>>>>>>      on Sat, 8 Apr 2023 14:21:53 +0200 writes:
> 
>      > Dear all,
> 
>      > Using rmultinom() in a stochastic model, I found this function returns an error message 'NA in probability' for an infinite probability.
> 
>      > Maybe, a more precise message will be helpful when debugging.
> 
>      >> rmultinom(1, 3:5, c(1/2, 1/3, Inf))
>      > Error in rmultinom(1, 3:5, c(1/2, 1/3, Inf)) : NA in probability vector
>      >> rmultinom(1, 3:5, c(1/2, 1/3, NA))
>      > Error in rmultinom(1, 3:5, c(1/2, 1/3, NA)) : NA in probability vector
> 
> Thank you.
> 
> I agree the first ('Inf') should not do what it currently does,
> and probably the 2nd one should neither give an error.
> 
> 
> Note that in rmultinom,  the 'prob' is allowed to be *NOT*
> scaled to sum(.) = 1.
> 
> Therefore 'Inf' makes sense as the limit (of a sequence) of (a)
> very large number(s).
> 
> I claim that
> 
>    rmultinom(1, 3, c(1/2, 1/3, Inf))
> 
> should give the same as
> 
>    rmultinom(1, 3, c(1/2, 1/3, 1e300))
> 
> even without a warning,

That case makes sense, but is it worth the effort?  Certainly

     rmultinom(1, 3, c(1/2, Inf, Inf))

can't give a useful answer because we don't know the relative size of 
the two infinities.  I imagine the first NA comes from computing 
prob/sum(prob), which is c(0, 0, NaN).

Duncan Murdoch

 > and OTOH,  an NA in prob may return NA (and signal a warning)
 > instead of an error.
> 
> 
>      > For rgeom() or rbinom(), we got a warning for infinite probability :
> 
> Yes, but there, prob must be in [0,1] ... so that's somewhat differnt.
> 
>      >> rbinom(1, 3, Inf)
>      > [1] NA
>      > Warning message:
>      > In rbinom(1, 3, Inf) : NAs produced
>      >> rbinom(1, 3, NA)
>      > [1] NA
>      > Warning message:
>      > In rbinom(1, 3, NA) : NAs produced
>      >> rgeom(1, Inf)
>      > [1] NA
>      > Warning message:
>      > In rgeom(1, Inf) : NAs produced
>      >> rgeom(1, NA)
>      > [1] NA
>      > Warning message:
>      > In rgeom(1, NA) : NAs produced
> 
> 
>      > Maybe, it could be better to harmonize the behavior for infinite probability.
> 
>      > Kind regards, Christophe
> 
> 
>      >> sessionInfo()
>      > R version 4.2.3 (2023-03-15)
>      > Platform: aarch64-apple-darwin20 (64-bit)
>      > Running under: macOS Ventura 13.2.1
> 
>      > Matrix products: default
>      > BLAS:   /Library/Frameworks/R.framework/Versions/4.2-arm64/Resources/lib/libRblas.0.dylib
>      > LAPACK: /Library/Frameworks/R.framework/Versions/4.2-arm64/Resources/lib/libRlapack.dylib
> 
>      > locale:
>      > [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
> 
>      > attached base packages:
>      > [1] stats     graphics  grDevices utils     datasets  methods   base
> 
>      > loaded via a namespace (and not attached):
>      > [1] compiler_4.2.3 tools_4.2.3
> 
>      > -------------------------------------------------
>      > Christophe DUTANG
>      > LJK, Ensimag, Grenoble INP, UGA, France
>      > Web: http://dutangc.free.fr
> 
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> 
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