How to test when condition returns numeric(0) in R

You could use ?length:

isEmpty <- function(x) {
    return(length(x)==0)
}

input <- c(3, 12);

if (!isEmpty(setdiff(input, 1:9))) {
    stop ("not valid")
}

Here's another option identical(x, numeric(0)). Here's an example (basically took everything from sgibb and replaced the key line as I'm lazy):

isEmpty <- function(x) {
    return(identical(x, numeric(0)))
}

input <- c(3, 12)

if (!isEmpty(setdiff(input, 1:9))) {
    stop ("not valid")
}