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

An undirected graph is graph, i.e, a set of objects (called vertices or nodes) that are connected together, where all the edges are bidirectional. An undirected graph is sometimes called an undirected network. In contrast, a graph where the edges point in a direction is called a directed graph.

Undirected Graph : Bidirectional

Examples:

Input:
Enter the number of Edges :
5
Output:
The Generated Random Graph is :
1 -> { 5 2  }
2 -> { 1  }
3 -> { Isolated Vertex! }      
4 -> { 6  }
5 -> { 1  }
6 -> { 4 10  }
7 -> { Isolated Vertex! }      
8 -> { 10  }
9 -> { Isolated Vertex! }      
10 -> { 8 6  }

Input:
Enter the number of Edges :
10
Output:
The Generated Random Graph is :
1 -> { 4 9  }
2 -> { 8  }
3 -> { 8  }
4 -> { 1 6  }
5 -> { 6  }
6 -> { 10 5 8 9 4  }
7 -> { 8  }
8 -> { 7 3 6 2  }
9 -> { 1 6  }
10 -> { 6  }

Approach:

• Take the input of the number of edges for the random Directed Acyclic Graph.
• Build a connection between two random vertices iteratively.
• Assert “Isolated Vertex” if degree of a certain vertex is zero.

Implementation:

Java

// Java program to Generate a Random Undirected Graph
// for a GivenNumber of Edges

// importing generic packages
import java.util.*;
import java.util.Random;

public class RandomUG {
    static int maxVertex = 10;

    // Function to generate random graph
    public static void generateRandomGraphs(int e)
    {
        int i = 0, j = 0, count = 0;

        int[][] edge = new int[e][2];
        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;

            // using rand to pick a random integer in range
            // of (1 - maxVertex)
            i++;
        }

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

                // print “Isolated vertex” for the vertex
                // having zero degree.
                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 = 6;
        System.out.println("Enter the number of Edges : " +e);

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

Output

Enter the number of Edges : 6
The Generated Random Graph is :
1 -> { 8  }
2 -> { 2  }
3 -> { Isolated Vertex! }
4 -> { 5  }
5 -> { 4 6  }
6 -> { 5  }
7 -> { 9  }
8 -> { 1 10  }
9 -> { 7  }
10 -> { 8  }