[R] strange behaviour with equality after simple subtraction
mike.prager at noaa.gov
Fri Jan 26 17:40:34 CET 2007
"martin sikora" <martin.sikora at upf.edu> wrote:
> today while trying to extract data from a list for subsequent analysis, i
> stumbled upon this funny behavior on my system:
> > x<-c(0.1,0.9)
> > 1-x
>  0.1
> > x
>  0.1
> > x==1-x
>  FALSE
> > x>1-x
>  TRUE
Not at all strange, an expected property of floating-point
arithmetic and one of the most frequently asked questions here.
> print(0.1, digits=17)
> print(1 - 0.9, digits=17)
A simple description of the issue is at
In most cases, it suffices to test for approximate difference or
relative difference. The former would look like this
if (abs(x - x) < eps)) ...
with "eps" set to something you think is an insignificant
difference, say 1.0e-10.
Mike Prager, NOAA, Beaufort, NC
* Opinions expressed are personal and not represented otherwise.
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