[R] how to get empty sequence for certain bounds

[Marc Schwartz]

On Wed, 2006-11-15 at 11:35 -0500, Tamas K Papp wrote:
> Hi,
> 
> I have encountered this problem quite a few times and thought I would
> ask.
> 
> Let's say that I have two endpoints, a and b, which are integers.  If
> a <= b, I would like to get a:b, but if a > b, then numeric(0), for
> example:
> 
> myseq(3,5) => 3:5
> myseq(3,3) => 3
> myseq(3,2) => numeric(0)
> 
> The operator : just gives decreasing sequences in the latter case, and
> I could not coax seq into doing this either (of course the
> documentation is correct, it never claims that I could).  Should I
> just write my own function, or is there a standard R way to do this?
> 
> Thanks,
> 
> Tamas


seq(a, b, length = ifelse(a <= b, b - a + 1, 0))

Thus:

a <- 3
b <- 5

> seq(a, b, length = ifelse(a <= b, b - a + 1, 0))
[1] 3 4 5

a <- 5
b <- 3

> seq(a, b, length = ifelse(a <= b, b - a + 1, 0))
integer(0)


However, is that better than:

a <- 3
b <- 5

> if (a <= b) a:b else numeric(0)
[1] 3 4 5

a <- 5
b <- 3

> if (a <= b) a:b else numeric(0)
numeric(0)


?


Of course, you could define your own operator:


"%:%" <- function(a, b) {if (a <= b) a:b else numeric(0)}

a <- 3
b <- 5

> a %:% b
[1] 3 4 5

a <- 5
b <- 3

> a %:% b
numeric(0)


HTH,

Marc Schwartz