[Rd] Undefined behavior of head() and tail() with n = 0

[William Dunlap]

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
<maechler at stat.math.ethz.ch> wrote:
>>>>>> Florent Angly <florent.angly at 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
>
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>
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