How to Generate a Random Directed Acyclic Graph for a Given Number of Edges in Java?

A Directed Acyclic Graph is a directed graph with no directed cycles. In a directed graph, the edges are connected so that each edge only goes one way. A directed acyclic graph means that the graph is not cyclic, or that it is impossible to start at one point in the graph and traverse the entire graph. Each edge is directed from an earlier edge to a later edge.

To generate a random DAG(Directed Acyclic Graph) for a given number of edges.

Directed Acyclic Graph

Examples:

Input:
Enter the number of Edges :
20
Output:  
The Generated Random Graph is :
1 -> { Isolated Vertex! }
2 -> { Isolated Vertex! }
3 -> { 18  }
4 -> { 5  }
5 -> { 16 8  }
6 -> { Isolated Vertex! }
7 -> { Isolated Vertex! }
8 -> {  }
9 -> { Isolated Vertex! }
10 -> { Isolated Vertex! }
11 -> { Isolated Vertex! }
12 -> {  }
13 -> { Isolated Vertex! }
14 -> { 18  }
15 -> { Isolated Vertex! }
16 -> {  }
17 -> { 19 3 5 4  }
18 -> {  }
19 -> {  }
20 -> { 12  }

Input:
Enter the number of Edges :
30
Output:  
The Generated Random Graph is :
1 -> { 12 8 7 16 5 11  }       
2 -> { 16  }
3 -> {  }
4 -> { 10  }
5 -> {  }
6 -> { 7  }
7 -> { 5  }
8 -> { 7 12 20  }
9 -> { 16 12  }
10 -> { 3  }
11 -> { 17 14  }
12 -> { 4 3  }
13 -> { 12 5  }
14 -> { 15 17  }
15 -> {  }
16 -> { 20  }
17 -> { 20 13  }
18 -> {  }
19 -> { 12 11  }
20 -> { 18  }

Approach:

• Take the input of the number of edges for the random Directed Acyclic Graph.
• Build a connection between two random vertex and check if any cycle is generated due to this edge.
• If any cycle is found, this edge is discarded and a random vertex pair is generated again.

Implementation:

Java

// Java program to Generate a Random Directed
// Acyclic Graph for a Given Number of Edges

import java.io.*;
import java.util.*;
import java.util.Random;

public class RandomDAG {

    // The maximum number of vertex for the random graph
    static int maxVertex = 20;

    // Function to check for cycle, upon addition of a new
    // edge in the graph
    public static boolean checkAcyclic(int[][] edge, int ed,
                                       boolean[] check, int v)
    {
        int i;
        boolean value;

        // If the current vertex is visited already, then
        // the graph contains cycle

        if (check[v] == true)

            return false;

        else {

            check[v] = true;

            // For each vertex, go for all the vertex
            // connected to it
            for (i = ed; i >= 0; i--) {

                if (edge[i][0] == v)

            return checkAcyclic(edge, ed, check, edge[i][1]);

            }
        }

        // In case, if the path ends then reassign the
        // vertexes visited in that path to false again
        check[v] = false;

        if (i == 0)
            return true;
        return true;
    }

    // Function to generate random graph
    public static void generateRandomGraphs(int e)
    {

        int i = 0, j = 0, count = 0;
        int[][] edge = new int[e][2];
        boolean[] check = new boolean[21];
        Random rand = new Random();

        // Build a connection between two random vertex
        while (i < e) {

            edge[i][0] = rand.nextInt(maxVertex) + 1;
            edge[i][1] = rand.nextInt(maxVertex) + 1;

            for (j = 1; j <= 20; j++)
                check[j] = false;

            if (checkAcyclic(edge, i, check, edge[i][0]) == true)

                i++;

            // Check for cycle and if found discard this
            // edge and generate random vertex pair again
        }

        System.out.println("The Generated Random Graph is :");

        // Print the Graph
        for (i = 0; i < maxVertex; i++) {

            count = 0;
            System.out.print((i + 1) + " -> { ");

            for (j = 0; j < e; j++) {

                if (edge[j][0] == i + 1) {
                    System.out.print(edge[j][1] + " ");
                    count++;
                }

                else if (edge[j][1] == i + 1) {
                    count++;
                }

                else if (j == e - 1 && count == 0)
                    System.out.print("Isolated Vertex!");
            }

            System.out.print(" }\n");
        }
    }

    public static void main(String args[]) throws Exception
    {
        int e = 4;

        System.out.println("Enter the number of Edges :"+ e);

        // Function to generate a Random Directed Acyclic
        // Graph
        generateRandomGraphs(e);
    }
}

Output

Enter the number of Edges :4
The Generated Random Graph is :
1 -> { Isolated Vertex! }
2 -> { 10  }
3 -> {  }
4 -> { Isolated Vertex! }
5 -> {  }
6 -> { 11  }
7 -> { Isolated Vertex! }
8 -> { Isolated Vertex! }
9 -> { Isolated Vertex! }
10 -> { 5  }
11 -> {  }
12 -> { Isolated Vertex! }
13 -> { Isolated Vertex! }
14 -> { Isolated Vertex! }
15 -> { 3  }
16 -> { Isolated Vertex! }
17 -> { Isolated Vertex! }
18 -> { Isolated Vertex! }
19 -> { Isolated Vertex! }
20 -> { Isolated Vertex! }