R devel - Sep 1997 - R-alpha: x[NA]

Just curious. Current logic in both R and Splus has
> (1:3)[c(T,F,NA)]
[1]  1 NA

Does anyone know *why*? It makes sense for integer indexing, but for
logicals it seems to have some strange consequences - if you divide a
dataframe according to some criterion, you get as many NA-filled
records as there are missing values of the criterion added to *both*
the group that satisfies the criterion and to those that do not.

One reason could be that since NA is logical by default, you would be
getting awkward consequences of the type x[NA] == real(0), but
x[c(NA,1)] == c(NA,5.3), but why is NA logical by default, then? 

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907
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On 22 Sep 1997, Peter Dalgaard BSA wrote:
> Just curious. Current logic in both R and Splus has
> 
> > (1:3)[c(T,F,NA)]
> [1]  1 NA
> 
<snip>
> One reason could be that since NA is logical by default, you would be
> getting awkward consequences of the type x[NA] == real(0), but
> x[c(NA,1)] == c(NA,5.3), but why is NA logical by default, then? 
The alternative would be worse.  If one element of the index is not
logical then the whole index is converted, so
R> x<-1:4 
R> x[c(F,T,F,T)]
[1] 2 4
R> x[c(F,T,F,NA)]
[1]  2 NA
but
R> x[c(F,T,F,4)]
[1] 1 4

That is, the presence of any NA would result in selecting only element
as.numeric(T)==1.

BTW, while you obviously can't mix logical and numerical references I
think it's unfortunate that you can't mix numeric and name-based
references like
R> names(x)<-letters[1:4]
R> x
a b c d 
1 2 3 4 
R> x[c("a",2)]
Error: subscript out of bounds

If you want a real example, suppose you had a model frame from which you
wanted to extract the response (in position 1) and the variables whose
names were in a vector nn
	mf[,c(1,nn)] 
appears to be the obvious solution.


Thomas Lumley
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