[R] Integer / floating point question

[Erik Iverson]

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