# What Is Adjacency Matrix in Data Structure?

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Heather Bennett

An adjacency matrix is a data structure used to represent the connections between vertices in a graph. In graph theory, a graph consists of a set of vertices (also known as nodes) and a set of edges (also known as links) that connect these vertices. This data structure is widely used in computer science and is particularly efficient for certain types of graph algorithms.

## What is an Adjacency Matrix?

An adjacency matrix is a square matrix that represents the connections between vertices in a graph. The rows and columns of the matrix correspond to the vertices, and each cell contains information about whether there is an edge between two vertices.

Typically, an adjacency matrix is implemented using a two-dimensional array. The value stored in each cell indicates whether there exists an edge between the corresponding row and column vertices. Commonly, the values are boolean (true or false), but they can also be integers, representing weights or distances between vertices.

• Simplicity: One of the main advantages of using an adjacency matrix is its simplicity. It provides an intuitive representation of the connections within a graph.
• Efficient Edge Queries: Checking whether there exists an edge between two vertices becomes very efficient with an adjacency matrix. Simply accessing the corresponding cell value provides immediate information.
• Compact Representation: In graphs with relatively few edges compared to the number of possible edges, an adjacency matrix can be more space-efficient than other representations such as adjacency lists.

• Space Complexity: One major drawback of using an adjacency matrix is its space complexity. It requires O(V^2) space, where V is the number of vertices in the graph.

This can be a significant drawback for large graphs.

• Inefficient for Sparse Graphs: If a graph has many vertices but only a few edges, an adjacency matrix becomes inefficient as it stores information about all possible edges, even if they don’t exist.
• Expensive to Modify: Modifying an adjacency matrix can be expensive in terms of time and space complexity. Adding or removing edges requires updating the entire matrix.

## Example:

Let’s consider a simple undirected graph with 4 vertices: A, B, C, and D. We can represent this graph using an adjacency matrix as follows:

```  A B C D
A 0 1 1 0
B 1 0 1 0
C 1 1 0 1
D 0 0 1 0
```

In this example, cell (A,B) contains a value of ‘1’, indicating that there is an edge between vertex A and vertex B. Similarly, cell (B,C) also contains a ‘1’, indicating an edge between vertex B and vertex C.

An adjacency matrix provides a clear visual representation of the connections between vertices in a graph, making it easier to analyze and reason about the structure of the graph.

## Conclusion

An adjacency matrix is a data structure used to represent connections between vertices in a graph. It offers simplicity and efficient edge queries, making it suitable for certain types of graph algorithms. However, its space complexity and inefficiency for sparse graphs are important considerations when choosing a data structure for representing graphs.