R help - Apr 2006 - as.character.factor when the factor contains "NA"

as.character.factor contains this line (where cx=levels(x)[x]):
  if ("NA" %in% levels(x)) cx[is.na(x)] <- "<NA>"

Is it possible that this is no longer the desired behavior?  These
two results don't seem very consistent:
> as.character(as.factor(c("AB", "CD", NA)))
[1] "AB" "CD" NA  
> is.na(.Last.value)[3]
[1] TRUE
> as.character(as.factor(c("NA", "CD", NA)))
[1] "NA"   "CD"  
"<NA>"
> is.na(.Last.value)[3]
[1] FALSE

I'm using R-2.3.0 on Redhat Linux, but I don't think the behavior
is new (maybe since character NA's were introduced?).

-- David Brahm (brahm at alum.mit.edu)

Hi

I did not see any response so I try to express my opinion that the 
behaviour is quite ok.

In your first example there is no "NA" level so clearly the only NA 
stands for "not available". However in your second example there is 
"NA" as a level which stands for e.g. North America or New Account. 
Therefore

fac<-as.factor("NA", "CD", NA)

retains NotAvailable on third place and 
as.character(fac)

gives you 2 different NA values, one with meaning and one which 
represents NotAvailable.

if you want NA to stays NA in your arguments you can try
> is.na(as.character(factor(c("NA", "CD", NA),
exclude=NULL)))
[1] FALSE FALSE  TRUE
> is.na(as.character(factor(c("AB", "CD", NA),
exclude=NULL)))
[1] FALSE FALSE  TRUE
>
HTH
Petr



On 28 Apr 2006 at 18:15, Brahm, David wrote:

Date sent:      	Fri, 28 Apr 2006 18:15:36 -0400
From:           	"Brahm, David" <David.Brahm at
geodecapital.com>
To:             	"R-help" <r-help at stat.math.ethz.ch>
Subject:        	[R] as.character.factor when the factor contains "NA"
> as.character.factor contains this line (where cx=levels(x)[x]):
>   if ("NA" %in% levels(x)) cx[is.na(x)] <-
"<NA>"
> 
> Is it possible that this is no longer the desired behavior?  These two
> results don't seem very consistent:
> 
> > as.character(as.factor(c("AB", "CD", NA)))
> [1] "AB" "CD" NA  
> > is.na(.Last.value)[3]
> [1] TRUE
> 
> > as.character(as.factor(c("NA", "CD", NA)))
> [1] "NA"   "CD"   "<NA>"
> > is.na(.Last.value)[3]
> [1] FALSE
> 
> I'm using R-2.3.0 on Redhat Linux, but I don't think the behavior
is
> new (maybe since character NA's were introduced?).
> 
> -- David Brahm (brahm at alum.mit.edu)
> 
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Petr Pikal
petr.pikal at precheza.cz