function A=buildlattice(N,K)

%Builds ring lattice with N nodes.  K is the number of neighbours TO EACH SIDE (i.e. half the average degree).

%K must be less than (N-1)/2

%Usage: A=buildlattice(N,K)

A=sparse([],[],[],N,N,2*(N*K));

B=repmat(1,N,2*K);

d=[[-(N-1):-(N-K)] [1:K]];

A=spdiags(B,d,N,N);

A=A+A'; %could do it directed not symetric