[R] Integer / floating point question

From: Erik Iverson <iverson_at_biostat.wisc.edu>
Date: Fri, 16 May 2008 09:56:27 -0500


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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