How do I represent a graph given as an adjacency list in C#?

Question

I am going to program various graph algorithms, and as input, I have given graphs on the form of adjacency lists.

Here's an example:

1 2 3 4

2 1 3 4

3 1 2 4

4 1 2 3 5

5 4 6

6 5

The graph has 6 vertices, represented by 6 rows (and the first entry of each row denotes the number of the vertex representing that row). The remaining entries in a row denote the vertices that the vertex corresponding to that row are adjacent to.

What is a good way of representing this in C#? The differing number of entries in each row seem to rule out an array, so I found these lists of lists.

I am going to manipulate the graph, e.g. contract edges, and I am looking for a data structure in which graph manipulations are both possible and efficient.

https://stackoverflow.com/a/73504595/4393351 – KushalSeth Aug 26 '22 at 17:28 — KushalSeth, Aug 26 '22 at 17:28

score 7 · Accepted Answer · answered Oct 30 '15 at 16:06

Looks like dictionary with a list of integers as a value structure cold be useful:

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Dictionary<int, List<int>> graph = new Dictionary <int, List<int>>();
        graph[1] = new List<int> {2, 3, 4};
        graph[2] = new List<int> {1, 3, 4};
        graph[3] = new List<int> {1, 2, 4};
        graph[4] = new List<int> {1, 2, 3, 5};
        graph[5] = new List<int> {4, 6};
        graph[6] = new List<int> {5};
    }
}

score 4 · Answer 2 · answered Oct 30 '15 at 15:55

4

When I had a similar task, I found it easy to have the following classes:

    class Edge
    {
    public Node From;
    public Node To;
    }

    class Node
    {
         public List<Edge> EdgesIn;
         public List<Edge> EdgesOut;
    }

    class Graph
    {
         public List<Node> Nodes;
    }

answered Oct 30 '15 at 15:55

michaelb

451
2
11

Thank you. Does this mean I create an object Node and an object Edge for each node and edge in my graph? I wonder if I have a large graph, say the Facebook graph with users as nodes, and edges as users being friends, will it be hard performancewise to keep track of that many objects? – HowDoICSharply Oct 31 '15 at 19:51

score 1 · Answer 3 · answered Oct 30 '15 at 15:47

1

If you'll choose coding of custom classes for working with graphs this info may be helpful. But all info here on java

answered Oct 30 '15 at 15:47

isxaker

8,446
12
60
87

score 1 · Answer 4 · answered Jan 13 '17 at 11:57

I found that a convenient way of representing graphs is the GraphSON format. You can check it out here: GraphSON format. An example of it:

{
   "graph": {
       "mode":"NORMAL",
       "vertices": [
           {
               "_id": "1",
               "_type": "vertex"
           },
           {
               "_id": "2",
               "_type": "vertex"
           },
           {
               "_id": "3",
               "_type": "vertex"
           }
       ],
       "edges": [
           {
               "weight": 1,
               "_id": "11",
               "_type": "edge",
               "_outV": "2",
               "_inV": "1"
           },
           {
               "weight": 0.5,
               "_id": "12",
               "_type": "edge",
               "_outV": "3",
               "_inV": "2"
           }
       ]
   }
}

score 0 · Answer 5 · answered Aug 26 '22 at 17:27

Adjacency List working example in C#. Converting the Edges to the Adjacency List in C#

            int[][] edges = new int[][]
            {
                new int[] { 0, 1 },
                new int[] { 1, 2 },
                new int[] { 2, 0 }
            };

            var graph = new Dictionary<int, List<int>>();

            for (int i = 0; i < edges.Length; i++)
            {
                var pair = edges[i];

                if (!graph.ContainsKey(pair[0])) graph.Add(pair[0], new List<int>());
                if (!graph.ContainsKey(pair[1])) graph.Add(pair[1], new List<int>());

                graph[pair[0]].Add(pair[1]);
                graph[pair[1]].Add(pair[0]);
            }

How do I represent a graph given as an adjacency list in C#?

5 Answers5

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