R-alpha: x[NA]

[Thomas Lumley]

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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