R help - Oct 2013 - work with observations in column

Hi,
May be this helps:
?vec1<- 1:10
library(zoo)
rollmean(vec1,2)
#[1] 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5
vec2<- c(5,8,9,13,20)
?diff(vec2)
#[1] 3 1 4 7

A.K.





Hello, 

How can I calculate half sum of 2 observ (l(i) + l(i+1))/2 ? 
How compute f(i) - f(i+1) ? 

please help me.