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

[David Winsemius]

On Nov 28, 2010, at 4:43 PM, David Winsemius wrote:

>
> 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.
         er, make that /.....                             ^(1:10)^
>
> -- 
> 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)
>> ______________________________________________
>> 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.
>
> David Winsemius, MD
> West Hartford, CT
>
> ______________________________________________
> 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.

David Winsemius, MD
West Hartford, CT