get edge between two node set

hi . i have a undirected graph with n nodes and two sets of node . A = [1 2 3 4] , B = [5 6 7 8] . there is no common member in this two set . how should i get number of edges between this two set ? something like :

findedge(G,[1 2 3 4],[5 6 7 8]) ;

p.s: the find edge gives me some unreal answer : it says [3;382;1839] that cant be true ! 4*4 nodes at last can have 16 edges between . and i tested and those numbers is not the edges index of existed edges between two set .

Answer 1

[Alex Karle]

Hi Hamid,

I wrote some code that should solve this problem for you. The details of the code are commented:

    % Let us take this graph as our example
    s = [1 1 1 1 2 1 2 2 3 3 4 5 5 6 7];
    t = [2 4 8 3 4 6 3 7 4 6 5 6 8 7 8];
    G = graph(s,t);
    plot(G)

    % Let us say A = [1,2,3,4], B = [5,6,7,8]
    A = [1,2,3,4];
    B = [5,6,7,8];

    % One way to do this is the following:
    % Assuming this is an unweighted graph, then the adjacency matrix will have a '1'
    % in row 'i' col 'j' if there is an edge 'i' <-> 'j', else there will be a '0'
    adj_matrix = adjacency(G)

    % Note that this is a sparse matrix, so it may not look like a normal matrix, but we can still use
    % matrix indexing on it!

    % adj_matrix(A,B) is equal to adj_matrix([1,2,3,4],[5,6,7,8]), creates a new matrix where the only
    % elements are those that start in A and end in B. Thus, to find the number of edges between A and B
    % we simply need to find the sum of all the '1's in this subsection of the adj_matrix, which can be
    % done with a sum(sum(adj_matrix(A,B))

    count = sum(sum(adj_matrix(A,B)))

If you want more information about the functions used, I would recommend looking at the MATLAB documentation:

adjacency function

array indexing

The reason why "findedge(G,[1 2 3 4],[5 6 7 8])" does not return the desired result is because findedge looks for pairs of edges, like the following:

    findedge(G,[1,2],[3,4],[5,6])

This will look for edges between 1 and 2, 3 and 4, and 5 and 6 as three seperate queries about the edges of the graph. For more documentation on the usage of findedge, look here: findedge.

Hope this helps!

[Hamid Salari]

@alex . thanks man .