Confusing error message for [[.factor (PR#14209)

Full_Name: George Russell
Version: 2.10.0 and 2.11.0 Under development (unstable) (2010-02-08 r51108)
OS: Windows
Submission from: (NULL) (217.111.3.131)

c("a","b")[[c(TRUE,FALSE)]]

Error in `[[.default`(factor(c("a", "b")), c(TRUE, FALSE)) : 
  recursive indexing failed at level 1
  

I don't see that.  I get this:

 >  c("a","b")[[c(TRUE,FALSE)]]
Error in c("a", "b")[[c(TRUE, FALSE)]] :
  recursive indexing failed at level 1

which differs because c("a", "b") is not a factor.

I find this error message confusing, though after reading the HELP carefully I
think I know what is going on. Would not something like "[[ does not work with
logical index vectors" be more appropriate?
  

No, because it sometimes does work with logical index vectors:

 > x <- 1:2
 > x[[TRUE]]
[1] 1

(Here the TRUE is treated as 1.  I think it only works when the logical 
vector contains TRUE values, FALSE will fail, just as x[[0]] fails.)

The problem is that you were asking for the FALSE entry of the TRUE 
entry of the object, and since you had a simple vector, recursive 
indexing fails, and that's what the message says.  I imagine you had 
meant to type c("a", "b")[c(TRUE, FALSE)], but how can R know you meant 
that?

Duncan Murdoch