# Example Masked Triangle Count

From Efficient Java Matrix Library

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Many Graph operations can be performed using linear algebra and this connection is the subject of much recent research. EJML now has basic "Graph BLAS" capabilities as this example shows.

External Resources:

## Example Code

```
/**
* Example using masked matrix multiplication to count the triangles in a graph.
* Triangle counting is used to detect communities in graphs and often used to analyse social graphs.
*
* More about the connection between graphs and linear algebra can be found at:
* https://github.com/GraphBLAS/GraphBLAS-Pointers.
*
* @author Florentin Doerre
*/
public class ExampleMaskedTriangleCount {
public static void main( String[] args ) {
// For the example we will be using the following graph:
// (0)--(1)--(2)--(0), (2)--(3)--(4)--(2), (5)
var adjacencyMatrix = new DMatrixSparseCSC(6, 6, 24);
adjacencyMatrix.set(0, 1, 1);
adjacencyMatrix.set(0, 2, 1);
adjacencyMatrix.set(1, 2, 1);
adjacencyMatrix.set(2, 3, 1);
adjacencyMatrix.set(2, 4, 1);
adjacencyMatrix.set(3, 4, 1);
// Triangle Count is defined over undirected graphs, therefore we make matrix symmetric (i.e. undirected)
adjacencyMatrix.copy().createCoordinateIterator().forEachRemaining(v -> adjacencyMatrix.set(v.col, v.row, v.value));
// In a graph context mxm computes all path of length 2 (a->b->c).
// But, for triangles we are only interested in the "closed" path which form a triangle (a->b->c->a).
// To avoid computing irrelevant paths, we can use the adjacency matrix as the mask, which assures (a->c) exists.
var mask = DMaskFactory.builder(adjacencyMatrix, true).build();
var triangleMatrix = CommonOpsWithSemiRing_DSCC.mult(adjacencyMatrix, adjacencyMatrix, null, DSemiRings.PLUS_TIMES, mask, null, null);
// To compute the triangles per vertex we calculate the sum per each row.
// For the correct count, we need to divide the count by 2 as each triangle was counted twice (a--b--c, and a--c--b)
var trianglesPerVertex = CommonOps_DSCC.reduceRowWise(triangleMatrix, 0, Double::sum, null);
CommonOps_DDRM.apply(trianglesPerVertex, v -> v/2);
System.out.println("Triangles including vertex 0 " + trianglesPerVertex.get(0));
System.out.println("Triangles including vertex 2 " + trianglesPerVertex.get(2));
System.out.println("Triangles including vertex 5 " + trianglesPerVertex.get(5));
// Note: To avoid counting each triangle twice, the lower triangle over the adjacency matrix can be used TRI<A> = A * L
}
}
```