[R] erratic behavior of match()?
murdoch at stats.uwo.ca
Thu Apr 19 23:06:24 CEST 2007
On 4/19/2007 4:29 PM, Bernhard Klingenberg wrote:
> Thank you! Is floating point arithmetic also the reason why
> 1 %% 0.1
> gives the "surprising" answer 0.1 (because 0.1 cannot be written as a
> fraction with denominator a power of 2, e.g. 1%%0.5 correctly gives 0).
> This seems to go a bit against the statement in the help for '%%', which
> states "For real arguments, '%%' can be subject to catastrophic loss of
> accuracy if 'x' is much larger than 'y', and a warning is given if this
> is detected."
I don't see the contradiction. The statement is talking about one way
to get imprecise results; you may have found another.
However, I'm not sure if you can blame %% in your example: the loss of
precision probably came from the translation of "0.1" to the internal
representation. I think "0.1" ends up a little bit larger than 0.1
after string conversion and rounding, so 1 %/% 0.1 should give 9, and 1
%% 0.1 should give something very close to 0.1, as you saw.
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