create.MEM.model <- function(coord, ngroups, nsites)
#
# This function reads a file containing the Cartesian coordinates of  
# the different groups of sites and constructs the staggered matrix 
# of MEM spatial eigenvectors. Cartesian coordinates can be obtained 
# from Lat-Lon data using the function geoXY() of library SoDA.
#
# coord:   File containing the Cartesian coordinates of the sites
# ngroups: Number of groups of sites
# nsites:  A vector containing the number of sites per group
#
# The library PCNM required by this function is available on the R-Forge
# page   http://r-forge.r-project.org/R/?group_id=195
# 
# Example of use: 
#
# cartesian = matrix(rnorm(70),35,2)
# ngr = 6
# nsites.per.group = c(6,7,6,5,5,6)
# result = create.MEM.model(cartesian, ngr, nsites.per.group)
#
#                                          Pierre Legendre, May 2010
{
library(PCNM)
n = nrow(coord)
if(sum(nsites) != n) stop("Vector nsites does not sum to nrow(coord)")
out = matrix(0,n,n)
end = 0
end.mem = 0
for(k in 1:ngroups) {
	start = end+1
	end = end+nsites[k]
	res = PCNM(dist(coord[start:end,]), dbMEM=TRUE)
	MEM = as.matrix(res$vectors)
	n.mem = ncol(MEM)
	out[start:end,(end.mem+1):(end.mem+n.mem)] = MEM
	end.mem = end.mem+n.mem
	}
out = out[,1:end.mem]
if(is.null(rownames(coord))) {
	rownames(out) <- rownames(out,do.NULL = FALSE, prefix = "Site.")
	} else {
	rownames(out) <- rownames(coord)
	}
colnames(out) <- colnames(out,do.NULL = FALSE, prefix = "MEM.")
out
}