[Jung-support] createLaplacianMatrix

[Charles F. <cf...@us...>]

Hi,

Here is some sample untested code for createLaplacianMatrix it basically
combines the diagonal with the negative adjacancy matrix.

Charles

    /**
     * Returns the Laplacian matrix of the specified graph, defined as
     * the diagonal matrix minus the adjacency matrix.
     * 
     * @return SparseDoubleMatrix2D
     */
    public static SparseDoubleMatrix2D createLaplacianMatrix(Graph G, NumberEdgeValue nev)
    {
        if (nev == null)
            nev = new ConstantEdgeValue(1);
        int numVertices = G.getVertices().size();
        SparseDoubleMatrix2D matrix = new SparseDoubleMatrix2D(numVertices,
                numVertices);
        Indexer id = Indexer.getIndexer(G);
        for (Iterator v_iter = G.getVertices().iterator(); v_iter.hasNext();)
        {
            Vertex v = (Vertex) v_iter.next();
            matrix.set(id.getIndex(v), id.getIndex(v), v.degree());
            for (Iterator o_iter = v.getOutEdges().iterator(); o_iter.hasNext(); )
            {
                Edge e = (Edge)o_iter.next();
                Vertex w = e.getOpposite(v);
                int j = id.getIndex(w);
                matrix.set(i, j, matrix.getQuick(i,j) - nev.getNumber(e).doubleValue());
            }
        }
        return matrix;
    }

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