The following program reads a set of edges that define an undirected graph and creates a representation of this graph with an adjacency matrix, giving a[i][j] and a[j][i] the value of 1 if there is an edge from i to j or from j to i in the graph, or the value of 0 if it does not exist. Also, we assume that the number of vertices V is a constant known at compilation time. Otherwise, there should be dynamic memory allocation for the array that represents the adjacency matrix.

You should know that the implementation of the algorithm does not take into account issues of data input validation or proper management of dynamic memory (e.g. avoiding memory leaks) because it is only necessary to highlight the logic of the algorithm.

#include <iostream>
#include <cstdlib>

using namespace std;

const int V = 100;

main () {
  bool adj[V][V] = { false };

  int i;
  for (i = 0; i < V; i++)
    adj[i][i] = true;

  int j;
  while (cin >> i >> j)
    adj[i][j] = adj[j][i] = true;

  return EXIT_SUCCESS;