Re: [R] %in% not working

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From: Peter Dalgaard BSA (p.dalgaard@biostat.ku.dk)
Date: Fri 01 Nov 2002 - 19:26:09 EST 

Message-id: <x265vhbsri.fsf@biostat.ku.dk>

Neil Klepeis <nklepeis@uclink4.berkeley.edu> writes:

> I'm at a loss on this one:
>
> > c(1.253) %in% seq(1.1,1.3,by=0.001)
> [1] FALSE
> > c(1.252) %in% seq(1.1,1.3,by=0.001)
> [1] TRUE
> > c(1.254) %in% seq(1.1,1.3,by=0.001)
> [1] TRUE

"Never trust exact equality with floating point operations."

Finite decimal fractions are not finite binary fractions and if you
compute them in two different ways you'll very likely get differences
in the last few bits because of rounding errors.

> seq(1.1,1.3,by=0.001)[154]
[1] 1.253
> seq(1.1,1.3,by=0.001)[154]-1.253
[1] 2.220446e-16

It's not different in principle from the fact that in 3-digit decimal
arithmetic 1/3 * 3 = 0.333 * 3 = 0.999 

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907
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