[R] understanding integer divide (%/%)
I am confused about why the following occurs: version _ platform i386-pc-mingw32 arch i386 os mingw32 system i386, mingw32 status major 2 minor 4.0 year 2006 month 10 day03 svn rev39566 language R version.string R version 2.4.0 (2006-10-03) 1 %/% 0.1 [1] 9 10 %/% 1 [1] 10 This effect led me into an trap when I tried to classify a set of proportions based on the first decimal place by integer dividing by 0.1. Can someone explain why this behavior occurs and give me an insight into how to predict it? Thanks, -- Jeff __ R-help@stat.math.ethz.ch 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.
Re: [R] understanding integer divide (%/%)
This is due to the internal representation of 0.1, which is not exactly 0.1 but very close to it. If you want to do an integer divide, you should only use integers to divide with. Cheers, Thierry ir. Thierry Onkelinx Instituut voor natuur- en bosonderzoek / Reseach Institute for Nature and Forest Cel biometrie, methodologie en kwaliteitszorg / Section biometrics, methodology and quality assurance Gaverstraat 4 9500 Geraardsbergen Belgium tel. + 32 54/436 185 [EMAIL PROTECTED] www.inbo.be Do not put your faith in what statistics say until you have carefully considered what they do not say. ~William W. Watt A statistical analysis, properly conducted, is a delicate dissection of uncertainties, a surgery of suppositions. ~M.J.Moroney -Oorspronkelijk bericht- Van: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Namens Jeffrey Prisbrey Verzonden: woensdag 3 januari 2007 14:21 Aan: r-help@stat.math.ethz.ch Onderwerp: [R] understanding integer divide (%/%) I am confused about why the following occurs: version _ platform i386-pc-mingw32 arch i386 os mingw32 system i386, mingw32 status major 2 minor 4.0 year 2006 month 10 day03 svn rev39566 language R version.string R version 2.4.0 (2006-10-03) 1 %/% 0.1 [1] 9 10 %/% 1 [1] 10 This effect led me into an trap when I tried to classify a set of proportions based on the first decimal place by integer dividing by 0.1. Can someone explain why this behavior occurs and give me an insight into how to predict it? Thanks, -- Jeff __ R-help@stat.math.ethz.ch 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. __ R-help@stat.math.ethz.ch 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.
Re: [R] understanding integer divide (%/%)
Thierry Onkelinx wrote: If you want to do an integer divide, you should only use integers to divide with. I think this should go into ``fortunes''. cheers, Rolf Turner [EMAIL PROTECTED] __ R-help@stat.math.ethz.ch 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.
Re: [R] understanding integer divide (%/%)
On Wed, 3 Jan 2007, ONKELINX, Thierry wrote: This is due to the internal representation of 0.1, which is not exactly 0.1 but very close to it. If you want to do an integer divide, you should only use integers to divide with. This must be more-or-less correct, but it is worth noting that 0.1*10==1 [1] TRUE 1/0.1==10 [1] TRUE 1%/%0.1==10 [1] FALSE so it isn't quite that simple. Interestingly, the results seem to vary by system -- on a G4 Mac I get 1 %/% (1/x) == x for all x from 1 to 50 -thomas Cheers, Thierry ir. Thierry Onkelinx Instituut voor natuur- en bosonderzoek / Reseach Institute for Nature and Forest Cel biometrie, methodologie en kwaliteitszorg / Section biometrics, methodology and quality assurance Gaverstraat 4 9500 Geraardsbergen Belgium tel. + 32 54/436 185 [EMAIL PROTECTED] www.inbo.be Do not put your faith in what statistics say until you have carefully considered what they do not say. ~William W. Watt A statistical analysis, properly conducted, is a delicate dissection of uncertainties, a surgery of suppositions. ~M.J.Moroney -Oorspronkelijk bericht- Van: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Namens Jeffrey Prisbrey Verzonden: woensdag 3 januari 2007 14:21 Aan: r-help@stat.math.ethz.ch Onderwerp: [R] understanding integer divide (%/%) I am confused about why the following occurs: version _ platform i386-pc-mingw32 arch i386 os mingw32 system i386, mingw32 status major 2 minor 4.0 year 2006 month 10 day03 svn rev39566 language R version.string R version 2.4.0 (2006-10-03) 1 %/% 0.1 [1] 9 10 %/% 1 [1] 10 This effect led me into an trap when I tried to classify a set of proportions based on the first decimal place by integer dividing by 0.1. Can someone explain why this behavior occurs and give me an insight into how to predict it? Thanks, -- Jeff __ R-help@stat.math.ethz.ch 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. __ R-help@stat.math.ethz.ch 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. Thomas Lumley Assoc. Professor, Biostatistics [EMAIL PROTECTED] University of Washington, Seattle __ R-help@stat.math.ethz.ch 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.
Re: [R] understanding integer divide (%/%)
On Wed, 3 Jan 2007, Thomas Lumley wrote: On Wed, 3 Jan 2007, ONKELINX, Thierry wrote: This is due to the internal representation of 0.1, which is not exactly 0.1 but very close to it. If you want to do an integer divide, you should only use integers to divide with. This must be more-or-less correct, but it is worth noting that 0.1*10==1 [1] TRUE 1/0.1==10 [1] TRUE 1%/%0.1==10 [1] FALSE so it isn't quite that simple. Interestingly, the results seem to vary by system -- on a G4 Mac I get 1 %/% (1/x) == x for all x from 1 to 50 And even 1 %/% 0.1 == 10 on my Linux boxes. Other things which are going on are the use of extra-precision registers (and potentially the system floor() function). %/% (but not / or *) makes use of a round of iterative refinement. It does 1/0.1 (10) rounds down (10) tmp = 1 - 0.1 *10 (slightly negative) Oops, the answer must be 10 - 1. This is needed for consistency since 1 %% 0.1 [1] 0.1 on MinGW. I think the Windows answer is correct, as 0.1 will be stored as 1/8 * 53-bit binary fraction with leading 1, and according to package gmp as.bigq(0.1) [1] 3602879701896397/36028797018963968 the denominator being 2^55. So 1 - 10 * 0.1 is -2/2^55 0. -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UKFax: +44 1865 272595 __ R-help@stat.math.ethz.ch 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.
