Creating a graph with c++ based on user input - c++

Question

I am trying to modify the DFS algorithm in C++ from the geeks4geeks site so that the graph is created according to users input.
Original code:
// C++ program to print DFS traversal from
// a given vertex in a given graph
#include<iostream>
#include<list>
using namespace std;
// Graph class represents a directed graph
// using adjacency list representation
class Graph
{
int V; // No. of vertices
// Pointer to an array containing
// adjacency lists
list<int> *adj;
// A recursive function used by DFS
void DFSUtil(int v, bool visited[]);
public:
Graph(int V); // Constructor
// function to add an edge to graph
void addEdge(int v, int w);
// DFS traversal of the vertices
// reachable from v
void DFS(int v);
};
Graph::Graph(int V)
{
this->V = V;
adj = new list<int>[V];
}
void Graph::addEdge(int v, int w)
{
adj[v].push_back(w); // Add w to v’s list.
}
void Graph::DFSUtil(int v, bool visited[])
{
// Mark the current node as visited and
// print it
visited[v] = true;
cout << v << " ";
// 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);
}
// DFS traversal of the vertices reachable from v.
// It uses recursive DFSUtil()
void Graph::DFS(int v)
{
// Mark all the vertices as not visited
bool *visited = new bool[V];
for (int i = 0; i < V; i++)
visited[i] = false;
// Call the recursive helper function
// to print DFS traversal
DFSUtil(v, visited);
}
int main()
{
// Create a graph given in the above diagram
Graph g(4);
g.addEdge(0, 1);
g.addEdge(0, 2);
g.addEdge(1, 2);
g.addEdge(2, 0);
g.addEdge(2, 3);
g.addEdge(3, 3);
cout << "Following is Depth First Traversal"
" (starting from vertex 2) \n";
g.DFS(2);
return 0;
}
I've changed the main() function to read from cin as follows, leaving the remaining part of the code the same:
int main()
{
int V,A[4][2];
cin>>V;
Graph g(V);
for(int i=0;i<V;i++){
cin>> A[i][0];
cin>>A[i][1];
}
for (int j=0;j<V;j++){
g.addEdge(A[j][0], A[j][1]);
}
g.DFS(2);
return 0;
}
The graph is given in adjacency list, for example with the following input data (first line is the V parameter, remaining lines represent edges from one node to another):
4
1 2
2 3
3 1
4 2
4 1
These are stored in the array sequentially, so once the data is read, I expect that:
A[0][0]=1, A[0][1]=2 (edge 1->2)
A[1][0]=2, A[1][1]=3 (edge 2->3)
...
But the output of the IDE is:
Command terminated by signal 11.
I think this is a segmentation fault and it means that I am trying to access memory I should not but I don't know how to fix this. Any ideas?

Answer 1

The problem with your reading function is that you can read only one edge per node. So a part of the edges is ignored. Consider this refactoring:
int main()
{
int V,A[2];
cin>>V;
Graph g(V);
while ( cin>> A[0]>>A[1] ) {
if (A[0]<0 || A[1]<0 || A[0]>=V || A[1]>=V)
cout << A[0]<<"->"<<A[1]<<" refers to a non-existent node"<<endl;
else g.addEdge(A[0], A[1]);
}
g.DFS(2);
return 0;
}
As you see, I've added a validation on the data read in order to avoid obvious errors. Running it on your test data will show you that there's a problem with your node identifications: you go from 1 to 4 in the test data, while your code expects from 0 to 3 (because the graph is implemented as an array of V adjacency lists and you shall not go out of range).
Here an online demo.