Message-ID: <497CEE8D.9030406@comcast.net>
Date: 2009-01-25T22:58:21Z
From: Marc Schwartz
Subject: comparing the previous and next entry of a vector
In-Reply-To: <63CBE05F-D7CA-461E-9F14-B38063131B5A@licht-malerei.de>
on 01/25/2009 04:29 PM J?rg Gro? wrote:
> Hi,
>
> I have a quit abstract problem, hope someone can help me here.
>
> I have a vector like this:
>
>
> x <- c(1,2,3,4,5,2,6)
> x
>
> [1] 1 2 3 4 5 2 6
>
> now I want to get the number where the previous number is 1 and the next
> number is 3
> (that is the 2 at the second place)
>
> I tried something with tail(x, -1) ...
> with that, I can check the next number, but how can I check the
> previous number?
How about this:
InBetween <- function(x, val1, val2)
{
unlist(sapply(2:(length(x) - 1),
function(i) if ((x[i - 1] == val1) & (x[i + 1] == val2)) x[i]))
}
> InBetween(x, 1, 3)
[1] 2
> InBetween(x, 4, 2)
[1] 5
It will return NULL if not found.
You might want to reinforce it with some error checking as well.
HTH,
Marc Schwartz