[r] How to catch integer(0)?

Let's say we have a statement that produces integer(0), e.g.

 a <- which(1:3 == 5)

What is the safest way of catching this?

You can easily catch integer(0) with function identical(x,y)

x = integer(0)
identical(x, integer(0))
[1] TRUE

foo = function(x){identical(x, integer(0))}
foo(x)
[1] TRUE

foo(0)
[1] FALSE

another option is rlang::is_empty (useful if you're working in the tidyverse)

The rlang namespace does not seem to be attached when attaching the tidyverse via library(tidyverse) - in this case you use purrr::is_empty, which is just imported from the rlang package.

By the way, rlang::is_empty uses user Gavin's approach.

rlang::is_empty(which(1:3 == 5))
#> [1] TRUE

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 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!")

isEmpty() is included in the S4Vectors base package. No need to load any other packages.

a <- which(1:3 == 5)
isEmpty(a)
# [1] TRUE

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'")