statistics for ragged arrays: loop to vector

Dear R users,

I am currently writing a MCMC algorithm for spatial Poisson model. One
problem which I need to solve is as follows:

I have a vector X[1:J] which describes characteristics of cells of a
rectangular grid. Further, for each cell I have a list of adjascent
cells adj[] and the vector listing numbed of adjascent cells num[].

Thus if I want to list cells adjascent to cell i I write

begin.n[i]<-cumsum(c(0,adj)[1:i])+1
end.n[i]<-cumsum(adj[1:i])

and if I want to calculate some function, say mean, over the adjascent
cells I write

f.X[i]<-mean(X[begin.n[i]:end.n[i]])

This works fine, but if I need the function to be calculated for each
cell of the field a couple of times for each MCMC iteration, the amount
of time which is used by the loop is too large. My question is: can this
loop be written in some vector form?