public static void Main(string[] args)
{
#region create graph: https://en.wikipedia.org/wiki/Prim%27s_algorithm
Graph graph = new Graph();
graph.AddEdges(0, 1, 2);
graph.AddEdges(0, 3, 1);
graph.AddEdges(1, 3, 2);
graph.AddEdges(2, 3, 3);
#endregion
foreach (Edge e in PrimsAlgorithm(graph))
{
Console.WriteLine(String.Format("Edge ({0}, {1})", e.vertexU, e.vertexV));
}
Console.WriteLine("Press any key...");
Console.ReadKey();
}
public static List<Edge> PrimsAlgorithm(Graph graph)
{
List<Edge> minSpanningTree = new List<Edge>();
List<Edge> unselectedEdges = new List<Edge>(graph.Edges);
List<int> unselectedVertices = new List<int>(graph.Vertices);
List<int> selectedVertices = new List<int>();
// random selection of vertex
selectedVertices.Add(new Random().Next(0, graph.Vertices.Count()));
unselectedVertices.RemoveAt(selectedVertices[0]);
while (unselectedVertices.Count > 0)
{
int indexMinEdge = -1;
// search the minimum-weight edge
for (int i = 0; i < unselectedEdges.Count; i++)
{
if ((selectedVertices.IndexOf(unselectedEdges[i].vertexU) != -1) && (unselectedVertices.IndexOf(unselectedEdges[i].vertexV) != -1) ||
(selectedVertices.IndexOf(unselectedEdges[i].vertexV) != -1) && (unselectedVertices.IndexOf(unselectedEdges[i].vertexU) != -1))
{
indexMinEdge = indexMinEdge == -1 ? i : (unselectedEdges[i].weight < unselectedEdges[indexMinEdge].weight ? i : indexMinEdge);
}
}
// add a new vertex (U or V) to the selected list and remove from list unselected
if (selectedVertices.IndexOf(unselectedEdges[indexMinEdge].vertexU) != -1)
{
selectedVertices.Add(unselectedEdges[indexMinEdge].vertexV);
unselectedVertices.Remove(unselectedEdges[indexMinEdge].vertexV);
}
else
{
selectedVertices.Add(unselectedEdges[indexMinEdge].vertexU);
unselectedVertices.Remove(unselectedEdges[indexMinEdge].vertexU);
}
// add a new edge to the tree and delete from the list of unselected
minSpanningTree.Add(unselectedEdges[indexMinEdge]);
unselectedEdges.RemoveAt(indexMinEdge);
}
return minSpanningTree;
}
