Component finding in graph BFS - c++

Question

I have written the program in which I am making the adjacency list of a graph, which is working perfectly:
class graph {
private:
Q q;
int v; //no of vertixes
struct node* array; //array of vertixes
public:
...
};
Now I want to find the number of components in that graph. I have written this code, but it gives me a segmentation fault error at run time when it try to run findingcomponent() function.
void findingcomponents()
{
int compcount = 0;
int noOfcomp = 0;
node* curr;
for (int i = 0; i < v; i++)
{
curr = array[i].head;
if (curr->next != NULL)
{
while (curr)
{
//while loop for the connected nodes traversel
while (curr)
{
compcount++;
//loop for inner loop traversal and for enque data
if (curr->next->visited != true) //if the next loop of this parent loop is not visited.
{
q.EnQ(curr->next->nodeno);
curr->next->visited = true;
curr = curr->next;
}
else
{
curr = curr->next;
}
}
curr = array[q.deQ()].head;
}
noOfcomp++;
}
}
cout << endl << "There Are : " << noOfcomp << "components in this graph.";
}
Additional infos:
The nodes are defined as follows:
struct node {
int nodeno;
bool visited;
struct node* next;
struct node* head;
};
I use a Q class for managing a queue.
class Q
{
private:
node* front;
node* roar;
public:
...
int deQ()
{
node * temp = new node;
if (front == NULL){
cout << endl << "\t Queue is Empty";
return NULL;
}
else
{
temp = front;
front = front->next;
return temp->nodeno;
}
}
...
};

Answer 1

First problem:
When you look at the head of the adjacency list, here:
curr = array[i].head;
you can't be sure that the list is not empty (i.e. head is nullptr).
So when you write on the next line:
if (curr->next != NULL)
You might dereference a null pointer, which is undefined behaviour and hence might trigger a rutime error (when you're lucky). This happens whenever you have a node which has no edge.
Next problem:
In the inner while loop, you can be sure that curr != nullptr. This time dereferencing is ok, but you can't be sure that curr->next is not null. So when you write
if (curr->next->visited != true)
you might again dereference a null pointer and have undfined behaviour (for example a segfault, if you're lucky).
With the dataset that you've provided, this happens at the first node: the loop goes through the adjacency list, and on the last element the error is triggered.
Other potential problem
I have not analysed in depth your algorithm, as there is important code missing. But it is not clear to me how it is ensured that all enqueued values are dequeued, and that q.deQ() is not called when not called when the queue is empty.
Well, with the additional code provided, in your deQ() you return 0 (NULL) if the queue is empty. But you also return 0 when encountering the node 0. So you don't process correctly the end of the queue, risking to loop for ever.
Therefore you have to change this function to:
int deQ()
{
node * temp = new node;
if (front == nullptr) {
cout << endl << "\t Queue is Empty";
return -1; // <======!!!! Normally you should declare a constant for this
}
else {
temp = front;
front = front->next;
return temp->nodeno;
}
}
Wrapping it together: correction of your function
void findingcomponents()
{
int compcount = 0;
int noOfcomp = 0;
node* curr;
for (int i = 0; i < v; i++) {
curr = array[i].head;
if (curr == nullptr) { // make sure you don't hit isolated nodes
cout << "isolated node";
}
else if (curr->next != nullptr) {
while (curr) { //while loop for the connected nodes traversel
while (curr) {
compcount++;
//loop for inner loop traversal and for enque data
if (curr->next != nullptr && curr->next->visited != true) //!!!!!!if the next loop of this parent loop is not visited.
{
q.EnQ(curr->next->nodeno);
curr->next->visited = true;
}
curr = curr->next; //!!! this was called at the end of the if, and in an else: just call it systematically.
}
int nodeid = q.deQ(); //!!! here deque and
if (nodeid == -1) //!!! check if a node was found in the quue
curr = nullptr; //!!! if not
else curr = array[nodeid].head;
}
noOfcomp++;
}
}
cout << endl << "There Are : " << noOfcomp << "components in this graph.";
}
Please verify however if the number of components is correct. I suspect that you increase the counter in a wrong manner. But you have now 3 hours left to address it.