Martin, I agree with you that +0 and -0 should generally be treated as equal, and R does a fine job in this respect. The Wikipedia article on signed zero (https://en.wikipedia.org/wiki/Signed_zero) echoes this view but also highlights that +0 and -0 can be treated differently in particular situations, including their interpretation as mathematical limits (as in the 1/-0 case). Indeed, the main question here is whether head() and tail() represent a special case that would benefit from differentiating between +0 and -0.
We can break down the discussion into two problems: A/ the discrepancy between the implementation of R head() and tail() and the documentation of these functions (where the use of zero is not documented and thus not permissible), B/ the discrepancy between the implementation of R head() and tail() and their GNU equivalent (which allow zeros and differentiate between -0 and +0, i.e. head takes "0" and "-0", tail takes "0" and "+0"). There are several possible solutions to address these discrepancies: 1/ Leave the code as-is but document its behavior with respect to zero (zeros allowed, with negative zeros treated like positive zeros). Advantages: This is the path of least resistance, and discrepancy A is fixed. Disadvantages: Discrepancy B remains (but is documented). 2/ Leave the documentation as-is but reflect this in code by not allowing zeros at all. Advantages: Discrepancy A is fixed. Disadvantages: Discrepancy B remains in some form (but is documented). Need to deprecate the usage of +0 (which was not clearly documented but may have been assumed by users). 3/ Update the code and documentation to differentiate between +0 and -0. Advantages: In my eyes, this is the ideal solution since discrepancy A and (most of) B are resolved. Disadvantages: It is unclear how to implement this solution and the implications it may have on backward compatibility: a/ Allow -0 (as double). But is it supported on all platforms used by R (see ?Arithmetic)? William has raised the issue that negative zero cannot be represented as an integer. Should head() and tail() then strictly check double input (while forbidding integers)? b/ The input could always be as character. This would allow to mirror even more closely GNU tail (where the prefix "+" is used to invert the meaning of n). This probably involves a fair amount of work and careful handling of deprecation. On 26 January 2017 at 16:51, William Dunlap <wdun...@tibco.com> wrote: > 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
