R help - Nov 2006 - how to get empty sequence for certain bounds

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

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

On Wed, 15 Nov 2006, 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?
This has been discussed fairly recently, so see the list archives.

myseq <- function(m, n) m - 1 + seq_len(max(n-m+1, 0))

looks about right.  Note that this is a little strange, as you want to 
include the left endpoint sometimes and not others.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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

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

seems a bit simpler and more transparent.

-- Bert

Bert Gunter
Genentech Nonclinical Statistics
South San Francisco, CA 94404