[R] Integer / floating point question

[Marc Schwartz]

on 05/16/2008 09:56 AM Erik Iverson 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.

Use modulo division:

 > t1[t1 %% 1 == 0]
[1] 1 2 3

or

 > unique(t1[t1 %% 1 == 0])
[1] 1 2 3

> 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.

Keep in mind that by default and unless specifically coerced to integer, 
numbers in R are double precision floats:

 > is.integer(2)
[1] FALSE

 > is.numeric(2)
[1] TRUE

 > is.integer(2.0)
[1] FALSE

 > is.numeric(2.0)
[1] TRUE


So:

 > identical(2.0, as.integer(2))
[1] FALSE


Does that help?

Marc Schwartz