On 07-Aug-09 11:07:08, Duncan Murdoch wrote: > Martin Maechler wrote: >>>>>>> William Dunlap <wdun...@tibco.com> >>>>>>> on Thu, 6 Aug 2009 15:06:08 -0700 writes: >> >> -----Original Message----- From: >> >> r-help-boun...@r-project.org >> >> [mailto:r-help-boun...@r-project.org] On Behalf Of >> >> Giovanni Petris Sent: Thursday, August 06, 2009 3:00 PM >> >> To: milton.ru...@gmail.com Cc: r-h...@r-project.org; >> >> daniel.gerl...@geodecapital.com Subject: Re: [R] Why is 0 >> >> not an integer? >> >> >> >> >> >> I ran an instant experiment... >> >> >> >> > typeof(0) [1] "double" > typeof(-0) [1] "double" > >> >> identical(0, -0) [1] TRUE >> >> >> >> Best, Giovanni >> >> > But 0.0 and -0.0 have different reciprocals >> >> >> 1.0/0.0 >> > [1] Inf >> >> 1.0/-0.0 >> > [1] -Inf >> >> > Bill Dunlap TIBCO Software Inc - Spotfire Division wdunlap >> > tibco.com >> >> yes. {finally something interesting in this boring thread !} >> ---> diverting to R-devel >> >> In April, I've had a private e-mail communication with John >> Chambers [father of S, notably S4, which also brought identical()] >> and Bill about the topic, >> where I had started suggesting that R should be changed such >> that >> identical(-0. , +0.) >> would return FALSE. >> Bill did mention that it does so for (newish versions of) S+ >> and that he'd prefer that, too, >> and John said >> >> >> I agree on having a preference for a bitwise comparison for >> >> identical()---that's what the name means after all. But since >> >> someone implemented the numerical case as the C == it's probably >> >> going to be more hassle than it's worth to change it. But we >> >> should make the implementation clear in the documentation. >> >> so in principle, we all agreed that R's identical() should be >> changed here, namely by using something like memcmp() instead >> of simple '==' , however we haven't bothered to actually >> *implement* this change. >> >> I am currently testing a patch which would lead to >> identical(0, -0) return FALSE. >> > I don't think that would be a good idea. Other expressions besides > "-0" > calculate the zero with the negative sign bit, e.g. the following > sequence: > > pos <- 1 > neg <- -1 > zero <- 0 > y <- zero*pos > z <- zero*neg > identical(y, z) > > I think most R users would expect the last expression there to be > TRUE based on the previous two lines, given that pos and neg both > have finite values. In a simple case like this y == z would be a > better test to use, but if those were components of a larger > structure, identical() is all we've got, and people would waste a > lot of time tracking down why structures differing only in the > sign of zero were not identical, even though every element tested > equal. > > Duncan Murdoch >> Martin Maechler, ETH Zurich
My own view of this is that there may in certain cirumstances be an interest in distinguishing between 0 and (-0), yet normally most users will simply want to compare the numerical values. Therefore I am in favour of revising identical() so that it can so distinguish; but also of taking the opportunity to give it a parameter say identical(x,y,sign.bit=FALSE) so that the default behaviour would be to see 0 and (-0) as identical, but with sign.bit=TRUE it would see the difference. However, I put this forward in ignorance of a) Any difficulties that this may present in re-coding identical(); b) Any complications that may arise when applying this new form to complex objects. Ted. -------------------------------------------------------------------- E-Mail: (Ted Harding) <ted.hard...@manchester.ac.uk> Fax-to-email: +44 (0)870 094 0861 Date: 07-Aug-09 Time: 14:49:51 ------------------------------ XFMail ------------------------------ ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
