In addition, signed zeroes only exist for floating point numbers - the bit patterns for as.integer(0) and as.integer(-0) are identical. Bill Dunlap TIBCO Software wdunlap tibco.com
On Thu, Jan 26, 2017 at 1:53 AM, Martin Maechler <maech...@stat.math.ethz.ch> wrote: >>>>>> Florent Angly <florent.an...@gmail.com> >>>>>> on Wed, 25 Jan 2017 16:31:45 +0100 writes: > > > Hi all, > > The documentation for head() and tail() describes the behavior of > > these generic functions when n is strictly positive (n > 0) and > > strictly negative (n < 0). How these functions work when given a zero > > value is not defined. > > > Both GNU command-line utilities head and tail behave differently with > +0 and -0: > > http://man7.org/linux/man-pages/man1/head.1.html > > http://man7.org/linux/man-pages/man1/tail.1.html > > > Since R supports signed zeros (1/+0 != 1/-0) > > whoa, whoa, .. slow down -- The above is misleading! > > Rather read in ?Arithmetic (*the* reference to consult for such issues), > where the 2nd part of the following section > > || Implementation limits: > || > || [..............] > || > || Another potential issue is signed zeroes: on IEC 60659 platforms > || there are two zeroes with internal representations differing by > || sign. Where possible R treats them as the same, but for example > || direct output from C code often does not do so and may output > || ‘-0.0’ (and on Windows whether it does so or not depends on the > || version of Windows). One place in R where the difference might be > || seen is in division by zero: ‘1/x’ is ‘Inf’ or ‘-Inf’ depending on > || the sign of zero ‘x’. Another place is ‘identical(0, -0, num.eq = > || FALSE)’. > > says the *contrary* ( __Where possible R treats them as the same__ ): > We do _not_ want to distinguish -0 and +0, > but there are cases where it is inavoidable > > And there are good reasons (mathematics !!) for this. > > I'm pretty sure that it would be quite a mistake to start > differentiating it here... but of course we can continue > discussing here if you like. > > Martin Maechler > ETH Zurich and R Core > > > > and the R head() and tail() functions are modeled after > > their GNU counterparts, I would expect the R functions to > > distinguish between +0 and -0 > > >> tail(1:5, n=0) > > integer(0) > >> tail(1:5, n=1) > > [1] 5 > >> tail(1:5, n=2) > > [1] 4 5 > > >> tail(1:5, n=-2) > > [1] 3 4 5 > >> tail(1:5, n=-1) > > [1] 2 3 4 5 > >> tail(1:5, n=-0) > > integer(0) # expected 1:5 > > >> head(1:5, n=0) > > integer(0) > >> head(1:5, n=1) > > [1] 1 > >> head(1:5, n=2) > > [1] 1 2 > > >> head(1:5, n=-2) > > [1] 1 2 3 > >> head(1:5, n=-1) > > [1] 1 2 3 4 > >> head(1:5, n=-0) > > integer(0) # expected 1:5 > > > For both head() and tail(), I expected 1:5 as output but got > > integer(0). I obtained similar results using a data.frame and a > > function as x argument. > > > An easy fix would be to explicitly state in the documentation what n = > > 0 does, and that there is no practical difference between -0 and +0. > > However, in my eyes, the better approach would be implement support > > for -0 and document it. What do you think? > > > Best, > > > Florent > > > > PS/ My sessionInfo() gives: > > R version 3.3.2 (2016-10-31) > > Platform: x86_64-w64-mingw32/x64 (64-bit) > > Running under: Windows 7 x64 (build 7601) Service Pack 1 > > > locale: > > [1] LC_COLLATE=German_Switzerland.1252 > > LC_CTYPE=German_Switzerland.1252 > > LC_MONETARY=German_Switzerland.1252 LC_NUMERIC=C > > LC_TIME=German_Switzerland.1252 > > > attached base packages: > > [1] stats graphics grDevices utils datasets methods base > > > ______________________________________________ > > R-devel@r-project.org mailing list > > https://stat.ethz.ch/mailman/listinfo/r-devel > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
