Disallow indexing by factors

[richierocks]

Since factors sometimes behave like character vectors and sometimes behave like integers, it is a bad idea to pass a factor into a square bracket index. For example

(x <- c(d = 1, c = 4, b = 9, a = 16))
##  d  c  b  a
##  1  4  9 16
(index <- factor(letters[1:4]))
## [1] a b c d
## Levels: a b c d
x[as.character(index)]
##  a  b  c  d
## 16  9  4  1
x[as.integer(index)]
##  d  c  b  a
##  1  4  9 16

Can you guess what x[index] returns?

If it took some thinking about, then I think you'll agree that factor indexing is problematic.

It would be useful if strict mode caused x[index] to throw an error.