Determine if a graph contains a cycle.

thereIsACycle.m

function yes = thereIsACycle(G)
    % G is an n-by-n association matrix.
    % An edge is defined by anything in that
    % matrix that is not 0 and not positive infinity.
    % Elements along the diagonal are ignored.
    % Returns 1 if there is a cycle. 0 otherwise.
    
    yes = 0;
    n = size(G, 1);
    seen = zeros(1, n);

    queue = [1];
    while length(queue) > 0
        x = queue(1);
        queue = queue(2:end);

        if seen(x) == 0
            seen(x) = 1;
            for i = 1:n
                if i~=x & G(x,i) ~= 0 & isinf(G(x,i)) == 0
                    queue = [queue i];
                end
            end
        else
            yes = 1;
            break;
        end
    end
end