NA and NaN question

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