I =[...
1 1 1 2 3 3
1 1 2 2 3 3
4 4 4 5 5 3
4 4 4 4 5 5];
N = unique(I);
n = numel(N);
out = cell(n,2);
for ii = 1:n
out(ii,:) = {ii, unique(I(bwdist(I == ii,'chessboard')==1))};
end
or
N = unique(I);
n = numel(N);
out = cell(n,2);
o3 = ones(3);
for ii = 1:n
lo = I == ii;
out(ii,:) = {ii, unique(I(imdilate(lo,o3) + lo == 1))};
end
out_as_table = cell2table(out,'v',{'number','neighbours'})
