Re: [R] Integer / floating point question

From: Erik Iverson <iverson_at_biostat.wisc.edu>
Date: Fri, 16 May 2008 10:45:43 -0500

Marc -

Marc Schwartz wrote:
> 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

Yes, that is one of the solutions. However, can I be sure that, say,

2.0 %% 1 == 0

The help page for '%%' addresses this a bit, but then caveats it with 'up to rounding error', which is really my question. Is there ever 'rounding error' with 2.0 %% 1 as opposed to 2 %% 1?

>

>> 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?

A bit, and this is the source of my confusion. Can I always assume that 2.0 == 2 when the class of each is 'numeric'?

>
> Marc Schwartz

---

R-help_at_r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Fri 16 May 2008 - 16:51:31 GMT