Unexpected behavior using -which() in R when the search term is not found

Question

I have been using the R which function to remove rows from a data frame. I recently discovered that if the search term is NOT in the data.frame, the result is an e

Answer 1

Because of this:

which(LETTERS == '-1')
## integer(0)

and this:

(1:2)[integer(0)]
integer(0)

Instead of #4, use this:

LETTERS[LETTERS != "R"]