From suharto_anggono at yahoo.com  Fri Aug 18 17:44:06 2017
From: suharto_anggono at yahoo.com (Suharto Anggono Suharto Anggono)
Date: Fri, 18 Aug 2017 15:44:06 +0000 (UTC)
Subject: [Rd] Issues of R_pretty in src/appl/pretty.c
References: <973411292.327791.1503071046156.ref@mail.yahoo.com>
Message-ID: <973411292.327791.1503071046156@mail.yahoo.com>

Examples similar to
pretty(c(-1,1)*1e300, n = 1e9, min.n = 1)
with smaller 'n':
pretty(c(-1,1)*1e304, n = 1e5, min.n = 1)
pretty(c(-1,1)*1e306, n = 1e3, min.n = 1)

A report on 'pretty' when working with integers, similar to what led to change of 'seq' fuzz, is https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=15137

--------------------------------------------
On Tue, 15/8/17, Martin Maechler <maechler at stat.math.ethz.ch> wrote:

 Subject: Re: [Rd] Issues of R_pretty in src/appl/pretty.c
 To: "Martin Maechler" <maechler at stat.math.ethz.ch>

@r-project.org
 Date: Tuesday, 15 August, 2017, 3:55 PM

>>>>> Martin Maechler <maechler at stat.math.ethz.ch>
>>>>>? ?? on Mon, 14 Aug 2017 11:46:07 +0200 writes:

>>>>> Suharto Anggono Suharto Anggono via R-devel <r-devel at r-project.org>
>>>>>? ?? on Fri, 11 Aug 2017 17:11:06 +0000 writes:
>>>>> Suharto Anggono Suharto Anggono via R-devel <r-devel at r-project.org>
>>>>>? ?? on Fri, 11 Aug 2017 17:11:06 +0000 writes:

? ? >> See https://stat.ethz.ch/pipermail/r-devel/2017-August/074746.html for the origin of the example here.

? ? >> That
? ? >> pretty(c(-1,1)*1e300, n = 1e9, min.n = 1) gave 20 intervals, far from 1e9, but
? ? >> pretty(c(-1,1)*1e300, n = 1e6, min.n = 1) gave 1000000 intervals
? ? >> (on a machine), made me trace through the code to function 'R_pretty' in https://svn.r-project.org/R/trunk/src/appl/pretty.c .

? ? > thank you.

? ? >> *lo is -1e300, *up is 1e300.
? ? >> cell = fmax2(fabs(*lo),fabs(*up));
? ? >> 'cell' is 1e300.
? ? >> i_small = dx < cell * U * imax2(1,*ndiv) * DBL_EPSILON *3;
? ? >> When *ndiv is (int) 1e9, apparently cell * U * imax2(1,*ndiv) overflows to infinity and 'i_small' is 1 (true). It doesn't happen when *ndiv is (int) 1e6.

[[elided Yahoo spam]]

? ? >> Putting parentheses may avoid the floating point overflow. For example,
? ? >> i_small = dx < cell * (U * imax2(1,*ndiv) * DBL_EPSILON) *3;

? ? > yes... but only if the compiler optimization steps? "keep the parentheses".
? ? > AFAIK, there is no guarantee for that.
? ? > To make sure, I'd replace the above by

? ? > U *= imax2(1,*ndiv) * DBL_EPSILON;
? ? > i_small = dx < cell * U * 3;


? ? >> The part
? ? >> U = (1 + (h5 >= 1.5*h+.5)) ? 1/(1+h) : 1.5/(1+h5);
? ? >> is strange. Because (h5 >= 1.5*h+.5) is 1 or 0, (1 + (h5 >= 1.5*h+.5)) is never zero and 1/(1+h) will always be chosen.

[[elided Yahoo spam]]
? ? > here was as a change (not by me!) adding wrong parentheses
? ? > there (or maybe adding what the previously "missing" parens
? ? > implied, but not what they intended!).
? ? > The original code had been
? ?? 
? ? > U = 1 + (h5 >= 1.5*h+.5) ? 1/(1+h) : 1.5/(1+h5);

? ? > and "of course" was intended to mean

? ? > U = 1 + ((h5 >= 1.5*h+.5) ? 1/(1+h) : 1.5/(1+h5));

? ? > and this what I'll change it to, now.


? ? >> The comment for 'rounding_eps' says "1e-7 is consistent with seq.default()". Currently, seq.default() uses 1e-10 as fuzz.

? ? > Hmm, yes, thank you; this was correct when written,
? ? > but seq.default had been changed in the mean time,
? ? > namely in? svn r51095 | 2010-02-03

? ? > Usually we are cautious / reluctant to change such things w/o
? ? > any bug that we see to fix.
? ? > OTOH, we did have? bug cases we'd wanted to amend for seq() /
? ? > seq.int();
? ? > and I'll look into updating the "pretty - epsilon" also to
? ? > 1e-10.

[[elided Yahoo spam]]

I've committed now what I think has been suggested
above ... to R-devel only :
------------------------------------------------------------------------
r73094 | maechler | 2017-08-15 09:10:27 +0200 (Tue, 15. Aug 2017) | 1 Zeile
Ge?nderte Pfade:
?? M doc/NEWS.Rd
?? M src/appl/pretty.c
?? M src/main/engine.c
?? M tests/reg-large.R
?? M tests/reg-tests-2.Rout.save

pretty(x, n) fix overflow for large n suggested by Suhartu Aggano, R-devel, 2017-08-11



[Rd] Issues of R_pretty in src/appl/pretty.c
Suharto Anggono Suharto Anggono