Segmentation Fault encountered while implementation of DFS using STL in C++ - c++

Question

Debugging of segmentation fault is one of the key issues I am facing as beginner in C++. I had tried to implement Depth First Search in Directed Graphs using C++ STL in the following lines of code (based on Steven Skienna's Algorithm Design Manual) :
#include <iostream>
#include <list>
#include <cstdio>
using namespace std;
#define TREE 0 /* tree edge */
#define BACK 1 /* back edge */
#define CROSS 2 /* cross edge */
#define FORWARD 3 /* forward edge */
class Graph
{
int V; //no of vertices
int time;
list <int> *adj; //Pointer to an array containeing the adjacency list
public:
Graph(int V); //A constructor
int entry_time[] ,exit_time[] , parent[] ;
bool processed[] , discovered[] ;
void addEdge(int v , int w ) ; // a function to add an edge to graph
void DFS(int v); // print DFS transversal of the complete graph
void initializeGraph () ; // a function used by DFS
void process_edge(int x , int y);
void process_vertex_early(int x);
void process_vertex_late(int x);
int edge_classification(int x , int y);
};
Graph::Graph(int V)
{
this->V = V;
adj = new list<int>[V]; // dynamic allocation of V lists to an array named adj
}
void Graph::addEdge(int v, int w )
{
adj[v].push_back(w); //Add w to v's list
}
void Graph::initializeGraph ()
{
time = 0;
for (int j=0;j<V;j++)
{
processed[j]=discovered[j] = false;
parent[j]=-1;
}
// Recur for all the vertices adjacent to this vertex
}
void Graph::DFS(int v)
{
process_vertex_early(v);
list <int>::iterator i ;
for (i=(adj[v].begin());i!=adj[v].end();++i)
{ cout << *i ;
if (discovered[*i]==false)
{
parent[*i] = v ;
process_edge(v,*i);
DFS(*i);
}
else if (processed[*i]==false)
process_edge(v,*i);
}
process_vertex_late(v);
}
void Graph::process_vertex_early(int v)
{
discovered[v] = true;
time = time +1 ;
entry_time[v] = time ;
printf("discovered vertex %d at time %d\n",v, entry_time[v]);
}
void Graph::process_vertex_late(int v)
{
time = time + 1 ;
exit_time[v] = time;
processed[v] = true;
//printf("processed vertex %d at time %d\n",v, exit_time[v]);
}
int Graph::edge_classification (int x , int y )
{
if (parent[y]==x) return (TREE);
if (discovered[y] && !processed[y]) return (BACK);
//cout << " Warning : self loop " << x << y ;
}
void Graph::process_edge(int x , int y)
{
int type ;
type = edge_classification(x,y);
//if (type== BACK) cout << "Back Edge" << x << " -> " << y << endl;
//else if (type== TREE) cout << "Tree Edge" << x << " -> " << y << endl;
//else cout << " Not in the type " ;
}
int main()
{
Graph g(4);
g.initializeGraph();
g.addEdge(0,1);
g.addEdge(0,2);
g.addEdge(1,2);
g.addEdge(2,0);
g.addEdge(2,3);
g.addEdge(3,1);
cout << " Following is a DFS transversal \n " ;
g.DFS(0);
return 0;
}
Segmentation Fault occurs after the search operation reaches a depth of one or two. I had tried applying Breadth First Search using a similar syntax which worked . Please help me in debugging this code . Thanks .

Answer 1

Step one is to read all compiler warnings (and compile with warnings switched on).
For example:
int entry_time[] ,exit_time[] , parent[] ;
These arrays are defined with no size - but you are putting data in them. This means you are writing outside the array boundaries which cause Undefined Behavior (such as the crashing and double-frees you are seeing). Either allocate space for these arrays like you do for adj, or use another container (such as vector) which you can resize as needed.
Also edge_classification doesn't always return a value - your compiler should have warned you about this.
Edit: More about std::vector
You can't declare your arrays to be as entry_time[V] because the value of V isn't known at compile time. You could have many different Graph objects with different sizes.
If you change your arrays to std::vector you can then allocate their size in the Graph constructor and let the std::vector class worry about allocating and freeing memory.
For example:
In the class declare entry_time as a std:vector:
std::vector<int> entry_time;
In the constructor, set the size of the entry_time vector.
entry_time.resize(V);
Note that you can use V here as a parameter to resize as this is at run-time so it now has a value.
std::vector has a normal array-like accessor, so you can assign values into the entries of the vector as you would an array. For example, your existing code will still work:
entry_time[v] = time ;