[R] behaviour of all(NULL == c("a", "b"))

[Matthias Burger]

thanks to Peter Daalgard, Brian Ripley, and Thomas Lumley for their quick, 
accurate and very helpful replies.

My understanding of NULL was incorrect, thanks for making this clear.
As for the question on match.arg I will use pmatch instead.

Best,

   Matthias



Thomas Lumley wrote:
> On Wed, 9 Feb 2005, Matthias Burger wrote:
> 
>>
>> Hi all,
>>
>> I'm a little surprised at
>>
>>> NULL == c("a", "b")
>>
>> logical(0)
>>
>>> all(NULL == c("a", "b"))
>>
>> [1] TRUE
>>
>> Reading the documentation for all() this was not clear for me to be 
>> expected.
> 
> 
> This is related to the question about sum(numeric(0)) that came up a few 
> days ago.
> 
> It is conventional in logic that "for all x in X: P(x)" is true when X 
> is empty.  A reason why this is a useful convention is that it implies
> all(c(x,y)) == all(x) && all(y) is still true when x or y happens to be 
> empty.  The converse holds for any():  any(logical(0)) is FALSE. This 
> surprises fewer people.
> 
> A fairly complete list is
> 
> all(NULL) is TRUE
> any(NULL) is FALSE
> sum(NULL) is 0
> prod(NULL) is 1
> min(NULL) is Inf
> max(NULL) is -Inf
> 
> with the last two giving a warning
> 
>> Originally the question came up when using
>>
>>> match.arg(NULL, c("a", "b"))
>>
>> [1] "a"
>> where I had thought an error would occur.
> 
> 
> I would have assumed "a" would be returned, thinking of match.arg as a 
> way to handle default arguments, but I agree it isn't documented, and 
> looking at the code it isn't clear what the author thought.
> 
> On the other hand, it *is* documented that `arg' must be a character 
> string, and NULL isn't a character string. Perhaps you should just use 
> pmatch() directly, as the main advantage of match.arg() is its ability 
> to look up default arguments, which you don't seem to be using.
> 
> 
>     -thomas

-- 
Matthias Burger                     Project Manager/ Biostatistician
Epigenomics AG    Kleine Praesidentenstr. 1    10178 Berlin, Germany
phone:+49-30-24345-371                          fax:+49-30-24345-555
http://www.epigenomics.com           matthias.burger at epigenomics.com