From thomas@biostat.washington.edu  Thu May  1 17:28:48 1997
From: thomas@biostat.washington.edu (Thomas Lumley)
Date: Thu, 1 May 1997 09:28:48 -0700 (PDT)
Subject: R-alpha: ==NULL
In-Reply-To: <9705011547.AA01745@>
Message-ID: <Pine.SUN.3.95.970501092039.11571A-100000@avon>

On Thu, 1 May 1997, Martin Maechler wrote:

> >>>>> "Tom" == Thomas Lumley <thomas@biostat.washington.edu> writes:
	^^^^^ Aaargh! Not "Tom", please.
> 
>     Tom> Any comparison with NULL generates an error Error: comparison is
>     Tom> possible only for vector types whereas in S(-PLUS) it gives NA,
>     Tom> which seems more sensible.
> 
>     Tom> Along similar lines, comparison with a length 0 vector returns
>     Tom> logical(0) in R but NA in S.
> 
> Is n't  logical(0)  more  logical than  NA ?

I didn't (previously) make any comment on this -- I only said that NA was
more logical than an error message.  However, the advantage of returning
NA is that NA | TRUE is TRUE, NA & FALSE is FALSE, which doesn't happen
with logical(0). Also, from a compatibility point of view one of them is
tested with is.na(), the other with length(), so it can matter which one
you use.  Of course no-one should deliberately write code where it
matters, but these things happen. 

It seems in fact that logical(0) | TRUE causes R to freeze (R0.49,
sparc solaris).


Thomas Lumley
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R-alpha: ==NULL
Thomas Lumley