[Rd] round(x, dig) [was "Development version of R fails tests .."]

[suh@rto_@@ggo@o m@iii@g oii y@hoo@com]

I disagree with what is assessed as "correct" in vignette of package 'round'.

With
x <- 9.18665
, what is actually stored in 'x' is a number that is slightly larger than 9.18665. So, as said in the vignette, it is closer to 9.1867 than 9.1866.

sprintf("%.4f", x)
giving
"9.1867"
is correct, as it is a string representing the _exact_ number 9.1867.

For round(x, 4) , that gives number and cannot return 9.1867 exactly, I still think that a binary double precision number closest to 9.1867 should be returned. My principles:
- Input number should be read as is, should be treated as exact.
- What counts is what the exact result should be.


------------
>>>>> Hugh Parsonage
>>>>>     on Sat, 8 Feb 2020 21:12:43 +1100 writes:

    > The only observation I can make is that the change to
    > round() was made in r77727 whereas your R-devel appears to
    > be r77715 (so would not exhibit the fixed behaviour).  My
    > guess is that there was a perpetual installation failure
    > after r77715 but that the test folder was still retrieved
    > and used.


    > On Sat, 8 Feb 2020 at 19:27, Berwin A Turlach <berwin.turlach using gmail.com> wrote:
    >>
    >> G'day all,
    >>
    >> I have daily scripts running to install the patched version of the
    >> current R version and the development version of R on my linux box
    >> (Ubuntu 18.04.4 LTS).
    >>
    >> The last development version that was successfully compiled and
    >> installed was "R Under development (unstable) (2020-01-25 r77715)" on
    >> 27 January.  Since then the script always fails as a regression test
    >> seems to fail.  Specifically, in the tests/ subdirectory of my build
    >> directory I have a file reg-tests-1d.Rout.fail which ends with:
    >>
    >> > ## more than half of the above were rounded *down* in R <= 3.6.x
    >> > ## Some "wrong" test cases from CRAN packages (partly relying on wrong R <= 3.6.x behavior)
    >> > stopifnot(exprs = {
    >> +     all.equal(round(10.7775, digits=3), 10.778, tolerance = 1e-12) # even tol=0, was 10.777
    >> +     all.equal(round(12345 / 1000,   2), 12.35 , tolerance = 1e-12) # even tol=0, was 12.34 in Rd
    >> +     all.equal(round(9.18665, 4),        9.1866, tolerance = 1e-12) # even tol=0, was  9.1867
    >> + })
    >> Error: round(10.7775, digits = 3) and 10.778 are not equal:

    >> Mean relative difference: 9.27902e-05
    >> Execution halted
    >>
    >> This happens while the 32bit architecture is installed,  which is a bit
    >> surprising as I get the following results for the last installed
    >> version of R's development version:
    >>
    >> R Under development (unstable) (2020-01-25 r77715) -- "Unsuffered Consequences"
    >> Copyright (C) 2020 The R Foundation for Statistical Computing
    >> Platform: x86_64-pc-linux-gnu/32 (32-bit)
    >> [...]
    >> > round(10.7775, digits=3)
    >> [1] 10.778
    >>
    >> and
    >>
    >> R Under development (unstable) (2020-01-25 r77715) -- "Unsuffered Consequences"
    >> Copyright (C) 2020 The R Foundation for Statistical Computing
    >> Platform: x86_64-pc-linux-gnu/64 (64-bit)
    >> [...]
    >> > round(10.7775, digits=3)
    >> [1] 10.778
    >>
    >>
    >> On the other hand, the R 3.6.2 version, that I mainly use at the moment,
    >> gives the following results:
    >>
    >> R version 3.6.2 (2019-12-12) -- "Dark and Stormy Night"
    >> Copyright (C) 2019 The R Foundation for Statistical Computing
    >> Platform: x86_64-pc-linux-gnu/32 (32-bit)
    >> [...]
    >> > round(10.7775, digits=3)
    >> [1] 10.777
    >>
    >> and
    >>
    >> R version 3.6.2 (2019-12-12) -- "Dark and Stormy Night"
    >> Copyright (C) 2019 The R Foundation for Statistical Computing
    >> Platform: x86_64-pc-linux-gnu/64 (64-bit)
    >> [...]
    >> > round(10.7775, digits=3)
    >> [1] 10.777
    >>
    >>
    >> So it seems as if the behaviour of round() has changed between R 3.6.2
    >> and the development version.  But I do not understand why this test all
    >> of a sudden failed if the results from the last successfully installed
    >> development version of R suggest that the test should be passed.
    >>
    >> Thanks in advance for any insight and tips.
    >>
    >> Cheers,
    >> Berwin

Note that r77727 was the last of a few commits I made related to
dealing with R's bug report PR#17668:
  https://bugs.r-project.org/bugzilla/show_bug.cgi?id=17668

which itself triggered an involved dialogue, mostly online,
visible at the PR's URL above.

It lead me to also write an R package 'round' (in order to
compare R 3.6.x and later's round() versions, comparing them etc)
with a (not entirely polished) package vignette
that explains how rounding to decimal digits is not at all
trivial and why and how I ended (*) improving R's
round(x, digits) algorithm in R-devel.

The CRAN version of the package
    https://cran.r-project.org/package=round

    install.packages("round")

is not quite current, notably its vignette isn't and so I have
mentioned in the above thread
( https://bugs.r-project.org/bugzilla/show_bug.cgi?id=17668#c8 )
that the latest version of the vignette is also available as

     https://stat.ethz.ch/~maechler/R/Rounding.html

You can install and load the devel version of 'round' by

   remotes::install_gitlab("mmaechler/round")
   require("round")

and then look a bit at the different versions of round(.)  using

   example(roundX)

i.e. using round::roundX(x, digits, version)

For those who read so far:  I'm really interested in getting
critical (constructive) feedback and comments about what I've
written there (in the bugzilla report, and the package vignette).
It seems almost nobody till now has had much interest and time to delve
into the somewhat intriguing issues.

Best regards,
Martin Maechler
ETH Zurich and R Core team