[R] Integer / floating point question

[Gabor Grothendieck]

2, 2.0, 2e0 are all double while 2L is an integer.

On Fri, May 16, 2008 at 10:56 AM, Erik Iverson <iverson at biostat.wisc.edu> wrote:
> Dear R-help -
>
> I have thought about this question for a bit, and come up with no
> satisfactory answer.
>
> Say I have the numeric vector t1, given as
>
> t1 <- c(1.0, 1.5, 2.0, 2.5, 3.0)
>
> I simply want to reliably extract the unique integers from t1, i.e., the
> vector c(1, 2, 3).  This is of course superficially simple to carry out.
>
> However, my question is related to R FAQ 7.31, "Why doesn't R think these
> numbers are equal?" The first sentence of that FAQ reads, "The only numbers
> that can be represented exactly in R's numeric type are integers and
> fractions whose denominator is a power of 2."
>
> All the methods I've devised to do the above task seem to ultimately rely on
> the fact that identical(x.0, x) == TRUE, for integer x.
>
> My assumption, which I'm hoping can be verified, is that, for example, 2.0
> (when, say, entered at the prompt and not computed from an algorithm) is an
> integer in the sense of FAQ 7.31.
>
> This seems to be the case on my machine.
>
>> identical(2.0, 2)
> [1] TRUE
>
> Apologies that this is such a trivial question, it seems so obvious on the
> surface, I just want to be sure I am understanding it correctly.
>
> Erik Iverson
> iverson at biostat.wisc.edu
>
> sessionInfo()
> R version 2.7.0 (2008-04-22)
> i686-pc-linux-gnu
>
> locale:
> LC_CTYPE=en_US.UTF-8;LC_NUMERIC=C;LC_TIME=en_US.UTF-8;LC_COLLATE=en_US.UTF-8;LC_MONETARY=C;LC_MESSAGES=en_US.UTF-8;LC_PAPER=en_US.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_US.UTF-8;LC_IDENTIFICATION=C
>
> attached base packages:
> [1] grDevices datasets  tcltk     splines   graphics  utils     stats
> [8] methods   base
>
> other attached packages:
> [1] fortunes_1.3-4  debug_1.1.0     mvbutils_1.1.1  SPLOTS_1.3-47
> [5] Hmisc_3.4-3     chron_2.3-21    survival_2.34-1 erik_0.0-1
>
> loaded via a namespace (and not attached):
> [1] cluster_1.11.10 grid_2.7.0      lattice_0.17-6
>
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