[Rd] Issues of R_pretty in src/appl/pretty.c
Suharto Anggono Suharto Anggono
suharto_anggono at yahoo.com
Fri Aug 18 17:44:06 CEST 2017
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
Geänderte 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
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