[R] sensitivity logical operators in R

[Ken Knoblauch]

Marc Jekel <feuerwald <at> gmx.de> writes:
> Another question for the community that really frightened me today. The 
> following logical comparison produces a "false" as output:
> 
> t = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,1,-1,-1,1))
> tt = sum((c(.7,.69,.68,.67,.66)-.5)*c(1,-1,1,1,-1))
> 
> t == tt
> 
> This is really strange behavior. Most likely this has something to do 
> how R represents numbers internally and the possible sensitivity of a 
> computer? Does anyone know when this strange behavior occurs and how to 
> fix it?
> 
> Thank you all! This list is pleasure!!!
> 
> Marc
> 

But, try

all.equal(tt, t)
[1] TRUE

and see the R FAQ 7.31

-- 
Ken Knoblauch
Inserm U846
Stem-cell and Brain Research Institute
Department of Integrative Neurosciences
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