[R] unexpected behavior using round to 2 digits on randomly generated numbers

[David Winsemius]

On Nov 28, 2010, at 4:20 PM, Cory Rieth wrote:

> Hello!
>
> I stumbled upon something odd that took a while to track down, and I  
> wanted to run it by here to see if I should submit a bug report. For  
> randomly generated numbers (from a variety of distributions)  
> rounding them to specifically 2 digits and then multiplying them by  
> 100 produces strange results on about 8% of cases. The problematic  
> numbers display as I would have expected, but do not logically match  
> the as.integer counterpart (additionally they will not be used  
> correctly by functions such as rep()). I realize there are easy  
> workarounds, but I wouldn't have expected this result, and it only  
> occurs rounding to 2 decimals, i.e. changing digits to 3 and  
> multiplying by 1000 after rounding gives the expected result.
>
> x<-runif(100)				#generate some random numbers
> y<-round(x,digits=2)*100  #round them all to two decimals, then  
> multiply them all by 100. I expected the results to all be integers
> sum(y!=as.integer(y))		#but on about 8% of the numbers they do not  
> match the integer version
> x[which(y!=as.integer(y))]	# a list of the problem numbers from the  
> original distribution. They seem to be more common but not exclusive  
> to .54 to .57
> y[which(y!=as.integer(y))]  #the numbers still display as would be  
> expected, i.e. they are integers
> as.integer(y[which(y!=as.integer(y))])  # and sometimes display as  
> the same number they are not logically identical to
>
> Thanks, and sorry if I came across something that is known, or it is  
> meant to behave this way, I couldn't find anything.

It's one of the FAQ and probably the most F-ly of the FAQ's. #21 or  
#31 if I remember (vaguely)  ... the one about why seq(0.1, 1, by=0.1)  
==  (1:11)/10  returns 2 FALSE's.

-- 
David.


> Cory Rieth
>
> R.version() output:
> platform       x86_64-apple-darwin9.8.0
> arch           x86_64
> os             darwin9.8.0
> system         x86_64, darwin9.8.0
> status
> major          2
> minor          12.0
> year           2010
> month          10
> day            15
> svn rev        53317
> language       R
> version.string R version 2.12.0 (2010-10-15)
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
West Hartford, CT