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

[Rainer M Krug]

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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)

Centre of Excellence for Invasion Biology
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