[R] 0.3 is not 0.3, bug in seq() function?

[Enrico R. Crema]

Well...thanks to everybody for suggestions and reading...I guess I have to study more!

Enrico
On 28 Oct 2010, at 11:48, Rainer M Krug wrote:

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> On 28/10/10 12:23, Enrico R. Crema wrote:
>> Dear List,
>> 
>> I've been running a numerical simulation and I found this odd error in my code where the which command could not identify which rows of a column of data.frame were corresponding to the value 0.3. There are 7 unique values in this column (0.01,0.05,0.1,0.2,0.3,0.4,0.5), and this does not work only for 0.3. So I looked at the column and manually tried to use the which() command, and the results were all FALSE despite I could see those number. So I recreated my sequence of number and tested:
>> 
>> seq(0.1,0.5,0.1)[3]==0.3
>> 
>> which gave me FALSE!!! All the other numbers (0.1,0.2,0.4,0.5) give me TRUE, but 0.3 was not working. So I did:
>> 
>> seq(0.1,0.5,0.1)[3]-0.3
>> 
>> which gave me 5.551115e-17. If you run a similar sequence like:
>> 
>> seq(0.2,0.6,0.1)[2]==0.3
>> 
>> this will still give me FALSE. No, for my own purpose, I fixed the problem in this way:
>> 
>> zerothree=seq(0.1,0.5,0.1)[3]
>> which(data[,1]==zerothree)
>> 
>> but I guess this bug is a bit of problem...Apologies if it is the wrong place to post this bug, and apologies also if this was a known issue. My version of R is :
>> 
>> platform       x86_64-pc-linux-gnu          
>> arch           x86_64                       
>> os             linux-gnu                    
>> system         x86_64, linux-gnu            
>> status                                      
>> major          2                            
>> minor          10.1                         
>> year           2009                         
>> month          12                           
>> day            14                           
>> svn rev        50720                        
>> language       R                            
>> version.string R version 2.10.1 (2009-12-14)
> 
> This is not a bug, but based on the internal representation of numbers
> in the binary format. Numbers like 0.3 can not be represented as exactly
> 0.3, wherefore errors accumulate in a sequence.
> 
> I can't provide you a link for more details, but the topic comes up
> every now and then in this mailing list.
> 
> Cheers,
> 
> Rainer
> 
>> 
>> 
>> Many Thanks,
>> 
>> Enrico
>> 
>> ______________________________________________
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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.
> 
> 
> - -- 
> Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
> Biology, UCT), Dipl. Phys. (Germany)
> 
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