R help - Oct 2003 - is.na(v)<-b (was: Re: Beginner's query - segmentation fault)

Note this behaviour:
> a<-"a"
> a<-NA
> mode(a)
[1] "logical"
> a<-"a"
> is.na(a) <- T
> mode(a)
[1] "character"

However after either way of assigning NA to a, is.na(a) is true,
and it prints as NA, so I can't see it's ever likely to matter. [Why
do I say these things? Expect usual flood of examples where it 
does matter.]

Also if a is a character vector, a[2] <- NA coerces the NA to
as.character(NA); again, just as one would hope/expect.

I have to echo Richard O'K's remark: if <- NA can ever go wrong,
is that not a bug rather than a feature?
 

Simon Fear
Senior Statistician
Syne qua non Ltd
Tel: +44 (0) 1379 644449
Fax: +44 (0) 1379 644445
email: Simon.Fear at synequanon.com
web: http://www.synequanon.com
 
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On Wed, 8 Oct 2003, Simon Fear wrote:
> Note this behaviour:
> 
> > a<-"a"
> > a<-NA
> > mode(a)
> [1] "logical"
> > a<-"a"
> > is.na(a) <- T
> > mode(a)
> [1] "character"
> 
> However after either way of assigning NA to a, is.na(a) is true,
> and it prints as NA, so I can't see it's ever likely to matter.
[Why
> do I say these things? Expect usual flood of examples where it 
> does matter.]
> 
> Also if a is a character vector, a[2] <- NA coerces the NA to
> as.character(NA); again, just as one would hope/expect.
> 
> I have to echo Richard O'K's remark: if <- NA can ever go wrong,
> is that not a bug rather than a feature?
I don't think it can ever `go wrong', but it can do things other than
the
user intends.  The intention of is.na<- is clearer, and so perhaps user 
error is less likely?  That is the thinking behind the function, anyway.

-- 
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)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595