DATA STRUCTURES PROGRAM • GRAPHS

Count Connected Components in an Undirected Graph

Learn how to count connected components in an undirected graph using a clear C program.

IntermediateConnected componentsDFS forestUndirected graph

PROBLEM UNDERSTANDING

Input and expected output

Sample input

No input required

Sample output

Components = 2

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>

void mark(int vertex, int graph[6][6], int visited[6])
{
    visited[vertex] = 1;
    for (int neighbour = 0; neighbour < 6; neighbour++)
        if (graph[vertex][neighbour] && !visited[neighbour]) mark(neighbour, graph, visited);
}

int main(void)
{
    int graph[6][6] = {
        {0,1,0,0,0,0}, {1,0,1,0,0,0}, {0,1,0,0,0,0},
        {0,0,0,0,1,0}, {0,0,0,1,0,1}, {0,0,0,0,1,0}
    };
    int visited[6] = {0}, components = 0;
    for (int vertex = 0; vertex < 6; vertex++)
        if (!visited[vertex]) { mark(vertex, graph, visited); components++; }
    printf("Components = %d\n", components);
    return 0;
}

CURRENT STEP

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SELECTED LINE

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EXPECTED OUTPUT FOR THE SAMPLE

Components = 2

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Start a new DFS whenever the vertex loop finds an unvisited component.
Display the computed result.

Start a new DFS whenever the vertex loop finds an unvisited component.

EFFICIENCY

Time and space complexity

Time complexity

O(V²) with matrix

Auxiliary space

O(V)

DEBUGGING CHECKLIST

Common mistakes

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Use the correct format specifier for every variable.

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Initialize variables before using their values.

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Check braces, semicolons and input order carefully.

Try it yourself

Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.