[R] statistics for ragged arrays: loop to vector
Elena Moltchanova
Elena.Moltchanova at ktl.fi
Mon Oct 22 13:44:10 CEST 2001
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?
Thanks for any comments
Elena Moltchanova
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