[Rcpp-devel] rpois returns double?

[Christian Gunning]

Thanks all for the details, I appreciate the helpful answers!
-Christian

On Sat, Apr 25, 2015 at 12:46 PM, William Dunlap <wdunlap at tibco.com> wrote:

>  Note that the R function rpois() always returns an integer vector and
> thus
> commonly runs into problems when lambda is near or above 2^31 (the
> smallest positive integral double that cannot represented as a 32-bit
> signed
> integer).
>
>  > set.seed(1)
> > rpois(10, 2^31)
>  [1] 2147454617         NA         NA         NA 2147412285         NA
>       NA
>  [8]         NA 2147469496 2147446621
> Warning message:
> In rpois(10, 2^31) : NAs produced
> > storage.mode(.Last.value)
> [1] "integer"
>
>
>
>  Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
> On Sat, Apr 25, 2015 at 9:33 AM, Michael Weylandt <
> michael.weylandt at gmail.com> wrote:
>
>>
>>  On Apr 24, 2015, at 23:58, Christian Gunning <xian at unm.edu> wrote:
>>
>>   Quick question, mainly out of curiousity.  I get that Rcpp uses the R
>> api on the backend, and in the R api, R::rpois returns a double (and
>> Rcpp::rpois returns NumericVector). The R C code doesn't offer much in the
>> way of explanation of why double is returned, but always returns a floor() (
>> https://svn.r-project.org/R/trunk/src/nmath/rpois.c).
>>
>>  It seems to me that, in this case, upstream could (should?) cast to
>> int.   Since it doesn't, downstream users like Rcpp and me must choose
>> whether to follow upstream convention or to cast.  In this case, is the
>> "best practice" to cast-to-int right away, or to follow along and just use
>> doubles for everything?
>>
>>  I've run into this recently with other RNG code that returns doubles
>> for everything, and I'm wondering if there's an obvious tradeoff that I'm
>> missing, like speed versus type-correctness?  If anyone has any insights,
>> I'd love to hear them.
>>
>>
>>  The set of integers exactly representable by a double is a superset of
>> those which can represented by a 32bit int.
>>
>>
>> http://stackoverflow.com/questions/759201/representing-integers-in-doubles
>>
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>>
>
>


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