[R] R accuracy

[Uwe Ligges]

Anthony Landrevie wrote:

> 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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> 
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