Re: [R] surprising difference in log()
This is not surprizing at all! The exact result is log(8,2) = 3, but the numerical procedure which calculates the logarithm may produce a result which is a few ULPs different from the exact one, i.e. you can get that log(8,2) = 2.99 and then floor(2.99) = 2. --- Fausto Galli <[EMAIL PROTECTED]> wrote: > > Hello everybody > > My collegue and I noticed a strange behaviour of R > on different > platforms. It's a simple computation, but results > are rather different. > > On Windows XP: > > > floor(log(8,2)) > [1] 3 > > which is what one should expect. > Here's instead the result with Mac OS X (same > version, 2.5.0 > (2007-04-23)) > > > floor(log(8,2)) > [1] 2 > > Is it a "bug" in R or in the operating system? > Anyway, it's quite a surprising one. > > > > > > _ > Fausto Galli > Institute of Finance > University of Lugano > Via G. Buffi 13 > CH-6904 Lugano, Switzerland. > +41 (0)58 666 4497 > http://www.people.lu.unisi.ch/gallif > > __ > 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] surprising difference in log()
On Tue, 26 Jun 2007, Fausto Galli wrote: > > Hello everybody > > My collegue and I noticed a strange behaviour of R on different > platforms. It's a simple computation, but results are rather different. > > On Windows XP: > > > floor(log(8,2)) > [1] 3 > > which is what one should expect. > Here's instead the result with Mac OS X (same version, 2.5.0 > (2007-04-23)) > > > floor(log(8,2)) > [1] 2 > > Is it a "bug" in R or in the operating system? > Anyway, it's quite a surprising one. It is a minor problem in the OS. Given that log(8, 2) is a floating point calculation, it is unreasonable to expect it in general to be exact. For those OSes that have log2, log2(8) is used and one might reasonably expect that to be exactly 3. So either MacOS X is lacking log2 or its log2 is not exact. Taking 'floor' of a floating-point computation without a fuzz is unwise. -- 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.
Re: [R] surprising difference in log()
Sorry, my mistake. I should really leave looking at the help list until AFTER my morning coffee :P On 26-Jun-07, at 11:31 AM, Duncan Murdoch wrote: > On 6/26/2007 10:20 AM, Mike Lawrence wrote: >> According to the description of floor(), the latter result is the >> correct one: >> >> 'floor takes a single numeric argument x and returns a numeric vector >> containing the largest integers *not greater than* the corresponding >> elements of x.' (emphasis added) >> >> floor(3) == 2 >>> True > > 3 is not greater than 3, but it is greater than 2, so the result you > quote above is wrong. You should see > >> floor(3) > [1] 3 > >> floor(3) == 2 > [1] FALSE > > Do you really see the result you posted? > > Duncan Murdoch > > __ > 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. -- Mike Lawrence Graduate Student, Department of Psychology, Dalhousie University Website: http://memetic.ca Public calendar: http://icalx.com/public/informavore/Public "The road to wisdom? Well, it's plain and simple to express: Err and err and err again, but less and less and less." - Piet Hein __ 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] surprising difference in log()
On 6/26/2007 10:20 AM, Mike Lawrence wrote: > According to the description of floor(), the latter result is the > correct one: > > 'floor takes a single numeric argument x and returns a numeric vector > containing the largest integers *not greater than* the corresponding > elements of x.' (emphasis added) > > floor(3) == 2 > >True 3 is not greater than 3, but it is greater than 2, so the result you quote above is wrong. You should see > floor(3) [1] 3 > floor(3) == 2 [1] FALSE Do you really see the result you posted? Duncan Murdoch __ 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] surprising difference in log()
According to the description of floor(), the latter result is the correct one: 'floor takes a single numeric argument x and returns a numeric vector containing the largest integers *not greater than* the corresponding elements of x.' (emphasis added) floor(3) == 2 >True On 26-Jun-07, at 11:09 AM, Fausto Galli wrote: > > Hello everybody > > My collegue and I noticed a strange behaviour of R on different > platforms. It's a simple computation, but results are rather > different. > > On Windows XP: > >> floor(log(8,2)) > [1] 3 > > which is what one should expect. > Here's instead the result with Mac OS X (same version, 2.5.0 > (2007-04-23)) > >> floor(log(8,2)) > [1] 2 > > Is it a "bug" in R or in the operating system? > Anyway, it's quite a surprising one. > > > > > > _ > Fausto Galli > Institute of Finance > University of Lugano > Via G. Buffi 13 > CH-6904 Lugano, Switzerland. > +41 (0)58 666 4497 > http://www.people.lu.unisi.ch/gallif > > __ > 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. -- Mike Lawrence Graduate Student, Department of Psychology, Dalhousie University Website: http://memetic.ca Public calendar: http://icalx.com/public/informavore/Public "The road to wisdom? Well, it's plain and simple to express: Err and err and err again, but less and less and less." - Piet Hein __ 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.
