# Program to count Number of connected components in an undirected graph

Given an undirected graph g, the task is to print the number of connected components in the graph.
Examples:

Input: Output:
There are three connected components:
1 – 5, 0 – 2 – 4 and 3

Approach: The idea is to use a variable count to store the number of connected components and do the following steps:

1. Initialize all vertices as unvisited.
2. For all the vertices check if a vertex has not been visited, then perform DFS on that vertex and increment the variable count by 1.

Below is the implementation of the above approach:

## CPP

 `// C++ implementation of the approach` `#include ` `using` `namespace` `std;`   `// Graph class represents a undirected graph` `// using adjacency list representation` `class` `Graph {` `    ``// No. of vertices` `    ``int` `V;`   `    ``// Pointer to an array containing adjacency lists` `    ``list<``int``>* adj;`   `    ``// A function used by DFS` `    ``void` `DFSUtil(``int` `v, ``bool` `visited[]);`   `public``:` `    ``// Constructor` `    ``Graph(``int` `V);`   `    ``void` `addEdge(``int` `v, ``int` `w);` `    ``int` `NumberOfconnectedComponents();` `};`   `// Function to return the number of` `// connected components in an undirected graph` `int` `Graph::NumberOfconnectedComponents()` `{`   `    ``// Mark all the vertices as not visited` `    ``bool``* visited = ``new` `bool``[V];`   `    ``// To store the number of connected components` `    ``int` `count = 0;` `    ``for` `(``int` `v = 0; v < V; v++)` `        ``visited[v] = ``false``;`   `    ``for` `(``int` `v = 0; v < V; v++) {` `        ``if` `(visited[v] == ``false``) {` `            ``DFSUtil(v, visited);` `            ``count += 1;` `        ``}` `    ``}`   `    ``return` `count;` `}`   `void` `Graph::DFSUtil(``int` `v, ``bool` `visited[])` `{`   `    ``// Mark the current node as visited` `    ``visited[v] = ``true``;`   `    ``// Recur for all the vertices` `    ``// adjacent to this vertex` `    ``list<``int``>::iterator i;`   `    ``for` `(i = adj[v].begin(); i != adj[v].end(); ++i)` `        ``if` `(!visited[*i])` `            ``DFSUtil(*i, visited);` `}`   `Graph::Graph(``int` `V)` `{` `    ``this``->V = V;` `    ``adj = ``new` `list<``int``>[V];` `}`   `// Add an undirected edge` `void` `Graph::addEdge(``int` `v, ``int` `w)` `{` `    ``adj[v].push_back(w);` `    ``adj[w].push_back(v);` `}`   `// Driver code` `int` `main()` `{` `    ``Graph g(5);` `    ``g.addEdge(1, 0);` `    ``g.addEdge(2, 3);` `    ``g.addEdge(3, 4);`   `    ``cout << g.NumberOfconnectedComponents();`   `    ``return` `0;` `}`

## Java

 `// Java program for above approach` `import` `java.io.*;` `import` `java.util.*;`   `public` `class` `ConnectedComponentCount` `{` `    `  `    ``private` `int` `v;` `    ``private` `int` `e;` `    ``private` `Map> adjMap;` `    ``private` `static` `Map visited;` `    `  `    ``ConnectedComponentCount(``int` `vertices)` `    ``{` `        ``v = vertices;` `        ``adjMap = ``new` `HashMap>(); ` `        ``visited = ``new` `HashMap();` `    ``}` `    `  `    ``// Function to add edges` `    ``private` `void` `addEdge(``int` `s, ``int` `d)` `    ``{` `        ``adjMap.putIfAbsent(s, ` `                     ``new` `HashSet());` `        ``adjMap.putIfAbsent(d, ` `                     ``new` `HashSet());` `        ``adjMap.get(s).add(d);` `        ``adjMap.get(s).add(s);` `        ``adjMap.get(d).add(s);` `        ``adjMap.get(d).add(d);` `        ``visited.put(s, ``0``);` `        ``visited.put(d, ``0``);` `    ``}` `    `  `    ``// To mark vertices which can be visites` `    ``private` `void` `findDFS(``int` `vertex)` `    ``{` `        `  `        ``// Mark as visited` `        ``visited.put(vertex,``1``);` `      `  `        ``// Print the vertex` `        ``System.out.println(``"vertex "` `+ vertex + ` `                                   ``" visited"``);` `        ``for``(Integer child : adjMap.get(vertex))` `        ``{` `            ``if``(visited.get(child) == ``0``){` `               ``findDFS(child);` `            ``}` `        ``}` `    ``}` `    `  `    ``// Function to print graph` `    ``private` `void` `printGraph()` `    ``{` `        ``for``(HashSet v : ` `                          ``adjMap.values())` `        ``{` `            ``System.out.println(v.toString());` `        ``}` `    ``}` `    `  `    ``// Driver Code` `    ``public` `static` `void` `main(String args[])` `    ``{` `        `  `        ``Scanner sc = ``new` `Scanner(System.in);` `        ``System.out.println("Enter the number ` `                           ``of vertices (V): ");` `        ``int` `vertices = sc.nextInt();` `        `  `        ``System.out.println("Enter the number ` `                                ``of edges (E): ");` `        ``int` `edges = sc.nextInt();` `        `  `        ``// To count total number of` `        ``// components` `        ``int` `ccCount = ``0``;` `        `  `        ``ConnectedComponentCount ccc = ` `                 ``new` `ConnectedComponentCount(` `                                      ``vertices);` `        `  `        ``// Input of edges` `        ``while` `(edges > ``0``)` `        ``{` `            ``System.out.println("Enter the ` `                                ``nodes s & d: ");` `            ``int` `s = sc.nextInt();` `            ``int` `d = sc.nextInt();` `            ``ccc.addEdge(s,d);` `            ``edges-- ;` `        ``}` `        `  `        ``// Function to print graph                      ` `        ``ccc.printGraph();` `        `  `        ``// Traversing every node ` `        ``for``(Integer vertex : visited.keySet())` `        ``{` `            `  `            ``// Check if vertex is already` `            ``// visited  or not` `            ``if``(visited.get(vertex) == ``0``)` `            ``{` `                `  `                ``// Function Call for findDFS` `                ``ccc.findDFS(vertex);` `              `  `                ``// Print Component Found` `                ``System.out.println(``"CC Found"``);` `              `  `                ``// Increase the counter` `                ``ccCount++;` `            ``}` `        ``}` `        `  `        ``// Print number of components                        ` `        ``System.out.println(``"Number of cc component: "` `                                        ``+ ccCount);` `    ``}` `}`   `//The code is contributed by Alfred Skaria`

Output

```2

```

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Improved By : alfiskaria