R help - Jan 2009 - comparing the previous and next entry of a vector to a criterium

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:

a <- c(1,2,3,3,2,1,6,3,2)
b <- c(NA,a[-length(a)])
c <- c(a[-1],NA)
a[b==1 & c==3]
[1] 2 6


hth,

Kingsford Jones


On Sun, Jan 25, 2009 at 3:02 PM, J?rg Gro? <joerg at licht-malerei.de>
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?
>
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