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

[Ivan Calandra]

Hi,
It is a know issue, reported in the FAQ 7.31.
Ivan

Le 10/28/2010 13:08, Enrico R. Crema a écrit :
> 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:
>
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA1
>>
>> 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
>>>
>>> ______________________________________________
>>> 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.
>>
>> - -- 
>> Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
>> Biology, UCT), Dipl. Phys. (Germany)
>>
>> Centre of Excellence for Invasion Biology
>> Natural Sciences Building
>> Office Suite 2039
>> Stellenbosch University
>> Main Campus, Merriman Avenue
>> Stellenbosch
>> South Africa
>>
>> Tel:        +33 - (0)9 53 10 27 44
>> Cell:       +27 - (0)8 39 47 90 42
>> Fax (SA):   +27 - (0)8 65 16 27 82
>> Fax (D) :   +49 - (0)3 21 21 25 22 44
>> Fax (FR):   +33 - (0)9 58 10 27 44
>> email:      Rainer at krugs.de
>>
>> Skype:      RMkrug
>> -----BEGIN PGP SIGNATURE-----
>> Version: GnuPG v1.4.10 (GNU/Linux)
>> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/
>>
>> iEYEARECAAYFAkzJVOEACgkQoYgNqgF2egpVXACeNO9GyRh9XKLg1g4EhcFs4Qa5
>> WXgAmgOdNB/xgmRS4UsMaEtunxS6NB/u
>> =azme
>> -----END PGP SIGNATURE-----
> ______________________________________________
> 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.
>

-- 
Ivan CALANDRA
PhD Student
University of Hamburg
Biozentrum Grindel und Zoologisches Museum
Abt. Säugetiere
Martin-Luther-King-Platz 3
D-20146 Hamburg, GERMANY
+49(0)40 42838 6231
ivan.calandra at uni-hamburg.de

**********
http://www.for771.uni-bonn.de
http://webapp5.rrz.uni-hamburg.de/mammals/eng/mitarbeiter.php