That is not strange behaviour. It's a case of FAQ 7.31:
http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
HTH,
Thierry
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
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tel. + 32 54/436 185
thierry.onkel...@inbo.be
www.inbo.be
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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.
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-Oorspronkelijk bericht-
Van: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] Namens
Žroutík
Verzonden: donderdag 12 februari 2009 16:20
Aan: r-help@r-project.org
Onderwerp: [R] trunc/floor a number -- strange bahaviour
Hi everybody,
given a fresh rgui.exe load on winxp OS, I enter (a minimal exaple)
n - 12.357531
Then the following command:
n - (n - floor(n))*10; n
gives the following outputs:
[1] 3.57531
[1] 5.7531
[1] 7.531
[1] 5.31
[1] 3.1
[1] 1 === still as expected
[1] 10=== not expected, count with me: 1 - floor(1) is zero, times
10 gives 0, not 10
[1] 10=== should stay forever zero (0)
[1] 10
[1] 9.98
[1] 9.82
[1] 9.999821
[1] 9.998212
The sama happens with trunc().
e.g. (a minimal exaple)
n - 0.245
n - (n - trunc(n))*10; n
[1] 2.45
[1] 4.5
[1] 5
[1] 1.776357e-13 = zero expected!!!
[1] 1.776357e-12
And I'm asking what the heck?! and where is the bug in my examples? Any
suggestion well appreciated.
p.s. The expression with floor() and trunc() are to be implemented in a
function which gives a value equal precision order of the given number. e.g.
12.345 would have (-3), 12.1 would have (-1), 12 would have (0) and e.g.
12000 would have the order of the precision (4). Basically, it is the order
of the last given non-zero digit.
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