# How Are Graphs Represented in Data Structure?

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Larry Thompson

Graphs are an important data structure used in computer science and mathematics to represent relationships between objects. They are widely used in various applications such as social networks, routing algorithms, and recommendation systems. In this article, we will explore how graphs are represented in data structures.

## Graph Representations

There are two commonly used ways to represent a graph: adjacency matrix and adjacency list. Each representation has its own advantages and is suitable for different scenarios.

An adjacency matrix is a 2-dimensional array where each cell represents the connection between two vertices. It is often used for dense graphs where the number of edges is close to the maximum possible edges. In an adjacency matrix, each row represents a source vertex, and each column represents a destination vertex.

To represent an edge between two vertices (i,j) in an adjacency matrix, we set the cell value to 1 if there is an edge or 0 if there isn’t. For example, consider a graph with 4 vertices:

```0 1 1 0
1 0 1 1
1 1 0 0
0 1 0 0
```

In the above adjacency matrix, row index represents the source vertex and column index represents the destination vertex. For example, cell (0,2) contains the value “1”, indicating that there is an edge from vertex 0 to vertex 2.

An adjacency list representation uses an array of linked lists or arrays to store the connections between vertices. It is often used for sparse graphs where the number of edges is significantly less than the maximum possible edges. In an adjacency list, each vertex maintains a list of its neighboring vertices.

Let’s consider the same example graph:

```0: 1 -> 2
1: 0 -> 2 -> 3
2: 0 -> 1
3: 1
```

In the above adjacency list representation, each line represents a vertex, followed by an arrow (->) and the neighboring vertices. For example, vertex 0 is connected to vertices 1 and 2.

## Choosing the Right Representation

The choice between adjacency matrix and adjacency list depends on various factors. If the graph is dense and the number of edges is close to the maximum possible edges, an adjacency matrix can be more efficient in terms of space complexity and random access time. On the other hand, if the graph is sparse and memory efficiency is a concern, an adjacency list can be a better choice as it only stores connections that exist.

## Conclusion

In this article, we explored two common ways to represent graphs in data structures: adjacency matrix and adjacency list. Each representation has its own strengths and weaknesses, making them suitable for different types of graphs. By understanding these representations, you will be better equipped to choose the appropriate one for your specific needs.