[R] Car.proper C[] matrix

[Uwe Ligges]

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Uwe Ligges


Jason Gasper wrote:
> I am hoping someone can help translate some WinBUGS code into R code. I 
> would like to use R to create the C[] matrix required for a car.proper 
> model in WinBUGS, but I am having a difficult time negotiating the 
> coding. The C matrix provides normalized weights for each pair of 
> spatial areas.   So the WinBUGS example is as follows:
> 
> # of the weight matrix with elements Cij. The first J1 elements of the 
> C[] vector contain the
> # weights for the J1 neighbours of area i=1; the (J1+1) to J2 elements 
> of the C[] vector contain
> # the weights for the J2 neighbours of area i=2; etc.
> # To set up this vector, we need to define a variable cumsum, which 
> gives the values of J1,
> # J2, etc.; we then set up an index matrix pick[,] with N columns 
> corresponding to the
> # i=1,...,N areas, and with the same number of rows as there are 
> elements in the C[] vector
> # (i.e. sumNumNeigh). The elements C[ (cumsum[i]+1):cumsum[i+1] ] 
> correspond to
> # the set of weights Cij associated with area i, and so we set up ith 
> column of the matrix pick[,]
> # to have a 1 in all the rows k for which cumsum[i] < k <= cumsum[i+1], 
> and 0's elsewhere.
> # For example, let N=4 and cumsum=c(0,3,5,6,8), so area i=1 has 3 
> neighbours, area i=2 has 2
> # neighbours, area i=3 has 1 neighbour and area i=4 has 2 neighbours. 
> The the matrix pick[,] is:
> # pick
> # 1, 0, 0, 0,
> # 1, 0, 0, 0,
> # 1, 0, 0, 0,
> # 0, 1, 0, 0,
> # 0, 1, 0, 0,
> # 0, 0, 1, 0,
> # 0, 0, 0, 1,
> # 0, 0, 0, 1,
> #
> # We can then use the inner product (inprod(,)) function in WinBUGS and 
> the kth row of pick to
> # select which area corresponds to the kth element in the vector C[]; 
> likewise, we can use inprod(,)
> # and the ith column of pick to select the elements of C[] which 
> correspond to area i.
> 
> Basically I want to do this in R to speed things up a little. Has anyone 
> written a function for this conversion?
> 
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