How to catch integer(0)?

Question

That is R's way of printing a zero length vector (an integer one), so you could test for a being of length 0:

R> length(a)
[1] 0

It might be worth rethinking the strategy you are using to identify which elements you want, but without further specific details it is difficult to suggest an alternative strategy.

If it's specifically zero length integers, then you want something like

is.integer0 <- function(x)
{
  is.integer(x) && length(x) == 0L
}

Check it with:

is.integer0(integer(0)) #TRUE
is.integer0(0L)         #FALSE
is.integer0(numeric(0)) #FALSE

---

You can also use assertive for this.

library(assertive)
x <- integer(0)
assert_is_integer(x)
assert_is_empty(x)
x <- 0L
assert_is_integer(x)
assert_is_empty(x)
## Error: is_empty : x has length 1, not 0.
x <- numeric(0)
assert_is_integer(x)
assert_is_empty(x)
## Error: is_integer : x is not of class 'integer'; it has class 'numeric'.

Maybe off-topic, but R features two nice, fast and empty-aware functions for reducing logical vectors -- any and all:

if(any(x=='dolphin')) stop("Told you, no mammals!")

Inspired by Andrie's answer, you could use identical and avoid any attribute problems by using the fact that it is the empty set of that class of object and combine it with an element of that class:

attr(a, "foo") <- "bar"

identical(1L, c(a, 1L))
#> [1] TRUE

Or more generally:

is.empty <- function(x, mode = NULL){
    if (is.null(mode)) mode <- class(x)
    identical(vector(mode, 1), c(x, vector(class(x), 1)))
}

b <- numeric(0)

is.empty(a)
#> [1] TRUE
is.empty(a,"numeric")
#> [1] FALSE
is.empty(b)
#> [1] TRUE
is.empty(b,"integer")
#> [1] FALSE

if ( length(a <- which(1:3 == 5) ) ) print(a)  else print("nothing returned for 'a'") 
#[1] "nothing returned for 'a'"

On second thought I think any is more beautiful than length(.):

 if ( any(a <- which(1:3 == 5) ) ) print(a)  else print("nothing returned for 'a'") 
 if ( any(a <- 1:3 == 5 ) ) print(a)  else print("nothing returned for 'a'")