Creating a graph with c++ based on user input

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.