edge_id.m

function y = edge_id(A,x,y)
    n = -1;
    i=1;

    [mx, ~] = size(A);

    while n == -1 && i <= mx
        if (A(i,1) == x && A(i,2) == y) || (A(i,1) == y && A(i,2) == x)
            n = i;
        end
        i=i+1;
    end

    y = n;
end