What Is a Adjacency Matrix in Data Structure?


Larry Thompson

A Adjacency Matrix is a fundamental data structure used in graph theory to represent the connections between vertices in a graph. It provides a concise way to store and retrieve information about the relationships between different elements in a graph. In this article, we will delve into the details of what an adjacency matrix is and how it is used.

What is a Graph?

Before we dive into adjacency matrices, let’s first understand what a graph is. In computer science, a graph is a collection of nodes (also known as vertices) that are connected by edges.

These nodes can represent various entities, such as cities in a transportation network or web pages on the internet. The edges represent the relationships or connections between these nodes.

Understanding Adjacency Matrices

An adjacency matrix is a square matrix that represents the connections between vertices in a graph. It provides an efficient way to store information about which vertices are adjacent to each other. The rows and columns of the matrix correspond to the vertices of the graph, and each cell in the matrix indicates whether there exists an edge between two vertices.

Let’s consider an example to illustrate how adjacency matrices work. Suppose we have a simple undirected graph with 4 vertices labeled A, B, C, and D. The adjacency matrix for this graph would be:

  • A B C D
  • A 0 1 1 0
  • B 1 0 1 1
  • C 1 1 0 0
  • D 0 1 0 0

In this example, there is an edge between vertex A and vertex B (indicated by the “1” in cell [1,2]), as well as between vertex B and vertex C (indicated by the “1” in cell [2,3]). The absence of an edge is represented by a “0” in the corresponding cell.

Advantages of Adjacency Matrices

Adjacency matrices offer several advantages when it comes to representing graphs. Firstly, they provide a compact way to store graph information, especially when the graph is sparse (i.e., has relatively few edges compared to the total number of possible edges). This is because only the cells corresponding to existing edges need to be stored.

Secondly, adjacency matrices allow for efficient retrieval of information about graph connectivity. Checking whether two vertices are adjacent can be done in constant time by simply accessing the corresponding cell in the matrix.

Disadvantages of Adjacency Matrices

While adjacency matrices have their benefits, they also come with some drawbacks. One major disadvantage is their space complexity.

For large graphs with many vertices, the size of the matrix can become prohibitively large. This can lead to significant memory consumption and reduced performance.

Additionally, updating an adjacency matrix can be computationally expensive. Adding or removing an edge requires modifying multiple cells in the matrix, which takes time proportional to the number of vertices in the graph.


In summary, an adjacency matrix is a powerful data structure for representing graph connectivity. It provides a concise and efficient way to store information about relationships between vertices in a graph.

Despite its limitations in terms of space complexity and update operations, adjacency matrices remain widely used due to their simplicity and effectiveness.

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