Randomly generate edges and vertices

Question

I'm learning minimum spanning trees and I encountered this question and decided to give it a shot...

Implement the minimum spanning tree algorithm on a randomly generated directed graph network of 100 vertices and 800 edges

public static int[][] getRandomArray(int n){
    int[][] a = new int[n][n];
    Random r = new Random();

    for(int i = 0; i < a.length; i++){
        for(int j = 0; j < a[i].length; j++){
            a[i][j] = r.nextInt();
        }
    }

    return a;
}


public static void main (String [] args)
{
   int x[];
     //TreeSet is used to sort the edges before passing to the algorithm
    TreeSet<Edge>  edges = new TreeSet<Edge>();

    Random random = new Random();

    edges.add(new Edge("0", "1", 2)); 
    edges.add(new Edge("0", "3", 1)); 
    edges.add(new Edge("1", "2", 3)); 
    edges.add(new Edge("2", "3", 5)); 

    System.out.println("Graph");
    KEdges vv = new KEdges();

    for (Edge edge : edges) {
        System.out.println(edge);
        vv.insertEdge(edge);
    }

    System.out.println("Kruskal algorithm");
    int total = 0;
    for (Edge edge : vv.getEdges()) {
        System.out.println(edge);
        total += edge.getEdgeWeight();
    }
    System.out.println("Total weight is " + total);
}


static class Edge implements Comparable<Edge>
{
    String vertexA;
    String vertexB;
    int weight;

    public Edge(String vertexA, String vertexB, int weight)
    {
        this.vertexA = vertexA;
        this.vertexB = vertexB;
        this.weight = weight;
    }

    public String getVertexA()
    {
        return vertexA;
    }

    public String getVertexB()
    {
        return vertexB;
    }

    public int getEdgeWeight()
    {
        return weight;
    }

    public String toString()
    {
        return "("+ vertexA + ", " + vertexB + ") : weight "+ weight ;
    }
    @Override
    public int compareTo(Edge o) {
       return (this.weight < o.weight)? -1 : 1;
    }

}

static class KEdges 
{
    Vector<HashSet<String>>  vertexGroups = new Vector<HashSet<String>>();
    TreeSet<Edge> kruskalEdges = new TreeSet<Edge>();

    public TreeSet<Edge> getEdges()
    {
        return kruskalEdges;
    }

    public HashSet<String> getVertexGroup(String vertex)
    {
        for (HashSet<String> vertexGroup : vertexGroups)
        {
            if (vertexGroup.contains(vertex))
            {
                return vertexGroup;
            }
        }
        return null;
    }



  public void insertEdge(Edge edge)
  {
    String vertexA = edge.getVertexA();
    String vertexB = edge.getVertexB();

    HashSet<String> vertexGroupA = getVertexGroup(vertexA);
    HashSet<String> vertexGroupB = getVertexGroup(vertexB);

    if (vertexGroupA == null)
    {
        kruskalEdges.add(edge);
        if (vertexGroupB == null){
            HashSet<String> htNewVertexGroup = new HashSet<String>();
            htNewVertexGroup.add(vertexA);
            htNewVertexGroup.add(vertexB);
            vertexGroups.add(htNewVertexGroup);
        }
    }
    else{
        if (vertexGroupB == null)
         {
             vertexGroupA.add(vertexB);
             kruskalEdges.add(edge);
         }
        else if (vertexGroupA != vertexGroupB)
         {
        vertexGroupA.addAll(vertexGroupB);
        vertexGroups.remove(vertexGroupB);
        kruskalEdges.add(edge);
         }
    }
  }

}

I'm stuck on how to generate the random edges and vertices... Any suggestions and ideas...

Answer 1

import java.util.Random;

Random random = new Random();

Edge[] edges = new Edge[800];
for(int i = 0; i < edges.length; i++) {
    edges[i] = new Edge(
        Integer.toString(random.nextInt(100)),
        Integer.toString(random.nextInt(100)));
        random.nextInt(300) //weights from 0 to 299
    );
}