Graphs¶

In mathematics, a graph is a way of representing relational data. A graph $G(V, E)$ consists of a vertex set $V$, and an edge set $E\subseteq V\times V$.

Often vertices are referred to as nodes.

In a directed graph, there can be an edge $(v_0, v_1)$ as well as an edge $(v_1, v_0)$. This is good for representing asymmetric relations like $v_1$ is better than $v_0$ at some task.

In an undirected graph, the edge $(v_0, v_1)$ is the same as the edge $(v_1, v_0)$. This is good for representing relations like friendship.

In a multigraph, there can be multiple edges between the same vertices. Multigraphs can be directed or undirected.

We’ll primarily consider undirected graphs today.

The neighbors of a vertex $v$ are all vertices that participate in an edge with $v$ $$N(v) = \{ w \in V \mid (v,w) \in E\}$$

The degree of a vertex is the size of its neighborhood set.

Applications of Graphs¶

Graphs come up in a variety of situations studied in scientific computing

Studying social networks (e.g. Facebook)
Studying food webs
Control processes
PDE meshes

and more!

Representing Graphs¶

There are a variety of ways we might represent graphs on a computer

Here are some common representations:

Edge list
Adjacency list
Ajacency matrix
…

We’ll typically treat the vertex set as $[0, 1, ..., N-1]$ where $N$ is the number of vertices in the graph

Edge Lists¶

An edge list is exactly what you might think - a list of edges.

For example:

elist = [(0,1), (1,2), (2,3)]
elist

[(0, 1), (1, 2), (2, 3)]

Adjacency List¶

An adjacency list is a list of lists - every vertex has a list of neighbors

For our example above, we would have

adjlist = [
           [1],
           [0,2],
           [1,3],
           [2]
          ]
adjlist

[[1], [0, 2], [1, 3], [2]]

Adjacency Matrix¶

An adjacency matrix $A$ is a $|V| \times |V|$ matrix, where $$ A[i,j] = \begin{cases} 1 & (i,j) \in E\\ 0 & \text{otherwise} \end{cases} $$ Continuing our example, we would have

import numpy as np
A = np.zeros((4,4))
for e in elist:
    A[e[0], e[1]] = 1
    A[e[1], e[0]] = 1

A

array([[0., 1., 0., 0.],
       [1., 0., 1., 0.],
       [0., 1., 0., 1.],
       [0., 0., 1., 0.]])

the adjacency matrix of an undirected graph is always symmetric (why?)

Often adjacency matrices are sparse, so it makes sense to use a sparse matrix format.

Exercises¶

Assume we are working with undirected graphs

Write a function to return an adjacency list from an edge list
Write a function to return an adjacency matrix from an adjacency list
Write a function to return an edge list from an adjacency matrix

# Your code here

A Graph Class¶

Usually, there is data associated with vertices and/or edges in a graph.

Let’s define a graph class that allows us to store data.

We’ll represent the graph as an edge list.

class Graph(object):
    def __init__(self):
        self.elist = []
        self.edata = dict() # edge data
        self.ndata = dict() # node data
        
    def add_node(self, i, **kwargs):
        """
        add a node to the graph, with data indexed by keywords
        """
        self.ndata[i] = dict(**kwargs)
        
    def add_edge(self, i, j, **kwargs):
        """
        add an edge to the graph, with data indexed by keywords
        """
        self.elist.append((i,j))
        self.edata[(i,j)] = dict(**kwargs)
        
    def node_data(self, i):
        if i in self.ndata.keys():
            return self.ndata[i]
        else:
            raise ValueError("No such node!")
    
    def edge_data(self, i, j):
        if (i,j) in self.edata.keys():
            return self.edata[(i,j)]
        elif (j,i) in self.edata.keys():
            return self.edata[(j,i)]
        else:
            raise ValueError("No such edge!")
        
    def edge_list(self):
        return self.elist
    
    def adjacency_list(self):
        pass
    
    def adjacency_matrix(self):
        pass
        
    

G = Graph()
G.add_node(0, name="dog")
G.add_node(1, name="cat")
G.add_node(2)
G.add_edge(0,1, weight=0.5)

print(G.node_data(2))
print(G.edge_data(0,1))

{}
{'weight': 0.5}

G.edge_list()

[(0, 1)]

Exercise¶

Implement the adjacency_list method
Implement the adjacency_matrix method

Scientific Computing with Python

Graphs

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