NA and NaN question

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, but I don't 
understand why it matters whether the vector was empty from the beginning or 
only after removing the NAs.

Pascal Niklaus
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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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. 

note the types:

typeof(c())
typeof(c(NA))
typeof(c(NA)[-na.omit(c(NA))])

now,

mean(NULL)
mean(logical(0))

mean(c())
# NA, because you take the mean of a vector of non-{numeric,logical}
type (see the warning message)
mean(c(NA), na.rm=TRUE)
# NaN, because you take the mean of a zero-length logical vector
mean(c(NA), na.rm=FALSE)
# NA, because you take the mean of a logical vector containing an NA

you can argue that ?mean underspecifies this (it doesn't say anything
about the value for a zero-length logical, numeric, or complex vector,
though you can guess it will be the value of 0/0).

vQ
Thanks, now I understand what's happening. Maybe a line explaining this could 
be added to the help text for mean?

Pascal Niklaus
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

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html and provide commented,
minimal, self-contained, reproducible code.
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. 

note the types:

typeof(c())
typeof(c(NA))
typeof(c(NA)[-na.omit(c(NA))])

wrong, an artifact of some experimenting; should have been:

typeof(na.omit(c(NA)))

vQ