R help - Apr 2009 - 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

A few further oddities (no longer having to do with modular arithmetic):
> (0.3 * 3) == 0.9
[1] FALSE
> (0.2 * 6) == 1.2
[1] FALSE

This is bad behavior, right? I feel like I'm going crazy.

I'm using R version 2.8.1 (2008-12-22).

Thanks,
Peter

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
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and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
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Gaverstraat 4
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tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be 
www.inbo.be 

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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
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-----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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That explains it, thanks. all.equal() gets it right. I always knew I had to be
careful with rounding errors, just not with something like (0.2*6)...
I guess I'll just have to avoid using '%%' or '%/%' with
non-integers, at least if I need it to sometimes come out even.
Thanks again,
Peter
> That is more or less FAQ 7.31:
>
http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f