My collection of maximal matched edges comes back empty (using QuickGraph in C#)

Question

I'm trying to use QuickGraph to, well, find a maximal matching in my bipartite graph, but the MatchedEdges collection they return to me is empty. I know there are matchings because I tested it with a K7,7 (complete bipartite) graph. So, I'm confused on what I did wrong.

Here's my code (I wrote Vertex and Edge in place of my actual classes for readability):

    public void findMatchings(List<Friend> setA, List<Friend> setB, List<Edge> candidateEdges) {
        // we need a graph and two sets of vertices
        IMutableVertexAndEdgeListGraph<Vertex, Edge> graph = new AdjacencyGraph<Vertex, Edge>();

        foreach (Vertex vertex in setA) {
            graph.AddVertex(vertex);
        }

        foreach (Vertex vertex in setB) {
            graph.AddVertex(vertex);
        }

        foreach (Edge candidate in candidatesEdges) {
            graph.AddEdge(candidate);
        }

        // sets a and b must be distinct, and their union must be equal to the set of all vertices in the graph
        // (though they're conceptually the same set, to you or me, I created new instances of everything so they should be practically different.)
        IEnumerable<Vertex> vertexSetA = setA;
        IEnumerable<Vertex> vertexSetB = setB;

        // These functions are used to create new vertices and edges during the execution of the algorithm.  
        // All added objects are removed before the computation returns
        VertexFactory<Vertex> vertexFactory = newVertex; //newVertex() is defined below
        EdgeFactory<Vertex, Edge> edgeFactory = (source, target) => new Edge(source, target);

        // computing the maximum bipartite match
        var maxMatch = new MaximumBipartiteMatchingAlgorithm<Vertex, Edge>(
                graph, vertexSetA, vertexSetB, vertexFactory, edgeFactory);

        Console.WriteLine(maxMatch.MatchedEdges.Count);

    }

    //This is in the same class as the above function:
    static Vertex newVertex() {
        return new Vertex("");
    }

    //This is the constructor in the Edge class:
    public Edge(Vertex source, Vertex target) {
        this.source = source;
        this.target = target;
    }

maxMatch.MatchedEdges.Count always comes back as 0. That's the problem.

I'm hopeful that there will be an easy solution to this, like I shouldn't be using new AdjacencyGraph() or something, but I'm also open to suggestions to other ways to find maximal matchings in bipartite graphs.

Thanks!

BTW, this link is what I used to write my stuff: Maximum Bipartite Matching in QuickGraph

score 1 · Answer 1 · answered Jul 06 '16 at 07:51

After creating an instance of MaximumBipartiteMatchingAlgorithm, you need to call the its Compute method so a matching is computed. In terms of your example, this means adding:

maxMatch.Compute();

Also each call to the newVertex() method should return a unique string which differs from the strings identifying the vertices in the input to MaximumBipartiteMatchingAlgorithm.

FYI: I personally found that sometimes maxMatch.MatchedEdges contains too many edges: some vertices are covered twice. Also I've seen maxMatch.MatchedEdges contain edges not in the input to MaximumBipartiteMatchingAlgorithm but created internally by the algorithm.

My collection of maximal matched edges comes back empty (using QuickGraph in C#)

1 Answers1