[R] NA and NaN question

[Prof Brian Ripley]

Pascal A. Niklaus wrote:
> Hi all,
> 
> I ran into a problem in some of my code that could be traced back to 'mean' 
> sometimes returning NA and sometimes NaN, depending on the value of na.rm:
> 
>> mean(c())
> [1] NA
> 
>> mean(c(NA),na.rm=T)
> [1] NaN
> 
> However, I don't understand the reasoning behind this and would appreciate and 
> explanation. 
> 
> I understand that the mean of an empty vector is not definied, 

Not so, it is well-defined as 0/0 = NaN.

> but I don't 
> understand why it matters whether the vector was empty from the beginning 

You didn't try that case:  mean(numeric(0)) is also NaN.  The issue is that

 > typeof(c())
[1] "NULL"

is not numeric (not evan a vector), and so mean() of it is undefined.

 > or only after removing the NAs.

Speculation (and wrong).


> Pascal Niklaus
> 
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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