R accuracy

Hello,

I am trying to test the precision of R on datasets from The Statistical Reference Datasets Project http://www.itl.nist.gov/div898/strd/index.html and I don't manage to understand how R is storing its results.

For example, I calculate a mean on the michelso dataset (100 values) and find:

m=mean(michel)
m
      V1 
299.8524 

print(m,digits=15)
      V1 
299.8524 

print(m,digits=22)
                V1 
299.85239999999993 

The certified value of the mean is 299.85240, so I try

print(m-299.8524)
           V1 
-5.684342e-14 

print(m-299.8524,digits=15)
                   V1 
-5.68434188608080e-14 

Does it have a sens to print with more than 15 signifiant digits?
Depends, e.g. you mightg want to see that the result is only 
-5.684342e-14 off, hence not identical() to, but all.equal() with what 
you expected.
Why is the difference not equal to zero?
Floating point calculations on a digital computer are involved...

Uwe Ligges
I am using R 2.0.1 under Windows XP.

Regards,

Anthony Landrevie 

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R accuracy

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