[R] comparing the previous and next entry of a vector
marc_schwartz at comcast.net
Sun Jan 25 23:58:21 CET 2009
on 01/25/2009 04:29 PM Jörg Groß wrote:
> 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)
>  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)
> InBetween(x, 4, 2)
It will return NULL if not found.
You might want to reinforce it with some error checking as well.
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