Calculate Newman's assortativity coefficient for a mixing matrix

assortativity_coefficient.R

# calculate the assortativity coefficient for a mixing matrix of a graph
# ref: MEJ Newman, 'Mixing patterns in networks', Phys Rev E 67, 026126 (2003)
#
# define assortativity coefficient as
#        trace (m) - sum (m^2)
# ac = -------------------------
#           1 - sum (m^2)
# 
# where m is the mixing matrix of a graph

assortcoeff <- function(m) {
        tr <- sum(diag(m))
        sumsq <- sum (rowSums(m)*colSums(m))
        (tr - sumsq) / (1 - sumsq)
}

This solution should generalize for both directed and undirected graphs.