[R] Modular Arithmetic Error?

ONKELINX, Thierry Thierry.ONKELINX at inbo.be
Fri Apr 17 16:31:30 CEST 2009


That is more or less FAQ 7.31:
http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-the
se-numbers-are-equal_003f 


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium 
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be 
www.inbo.be 

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens iuhz7j202 at sneakemail.com
Verzonden: vrijdag 17 april 2009 16:15
Aan: r-help at r-project.org
Onderwerp: [R] Modular Arithmetic Error?

Hi,

I'm using the '%%' operator in some code, and am running into the
following erroneous outcome:

  > 1.2 %% 0.2
  [1] 0.2

Unless I'm very mistaken, the result should be 0 (indeed, 12 %% 2 does
result in 0). Furthermore:

  > 1.20000000000000001 %% 0.2
  [1] 0.2
  > (1.2+1e17) %% .2
  [1] 0
  Warning message:
  probable complete loss of accuracy in modulus 

(Warning comes up only when it is giving a closer-to-correct answer)
What makes it even stranger is this result:

  > (1.2 %% 0.2) == 0.2
  [1] FALSE

Does anybody know what's going on here? This seems like very bad
behavior.

Thanks,
Peter

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