degree of connection between 2 nodes in a social graph - c++

Question

I'm trying to find out the degree of connection between 2 entities in a social graph where
1 hop : 1st Degree
2 hop : 2nd Degree
3 hop : 3rd Degree
And so on.
The vertices are the entities and the edges are the friendship between the two entities. Given such a graph I want to analyse the graph and answer the query as to what is the type of connection between the entities.It can be disconnected graph.In case of no connection it'll return 0.
It takes the input as-
Number_of_vertices Number_of_Edges
Edge 1
Edge 2
(So on.)
Query
Output
The degree of connection
Example
Input
5 4
Abhs Krax // Edge 1
Harry Idrina // Edge 2
Harry Jigma // Edge 3
Harry Krax // Edge 4
Abhs Jigma // Query
Output
Degree : 3
I've used BFS to find out the depth between 2 nodes, but my program works only for degree 1. It fails to test the next subsequent member of the queue thus stuck at testing only the 1st member of the queue. What did I miss in my code? The problem is in Connection() function which I couldn't trace.
#include <iostream>
#include <list>
#include <string>
using namespace std;
class Vertex // Each vertex of the graph is represented by the object of the Vertex class
{
public:
// Fields in every vertex node
string name;
std::list<Vertex*> adjacencyList;
bool status;
int depth;
// Constructor which initializes the node
Vertex(string id)
{
name = id;
adjacencyList = list<Vertex*>();
status = false;
depth =0;
}
// Function to add edges by pushing the vertices to its adjacency list
void addEdge(Vertex *v)
{
adjacencyList.push_back(v);
}
};
class Graph{
public:
// Fields of the Graph node
int N;
std::list<Vertex> vertexList;
// Functions to be implemented
int Connection(Vertex,Vertex);
Graph(int n){ // Constructor
N = n;
vertexList = list<Vertex>();
}
/* This function first checks whether the vertex has been already added
to Vertex List of the Graph. If not found it would create the vertex
node and push the node into Vertex List. Then the edges are added by
updating the adjacency list of respective vertices. */
void addEdge(string to, string from ){
if(find(to))
{
Vertex entity_1 = Vertex(to); // New vertex node creation
vertexList.push_back(entity_1); // Pushing to the Vertex List
}
if(find(from))
{
Vertex entity_2 = Vertex(from);
vertexList.push_back(entity_2);
}
Vertex *v1 = &(*(find_it(to)));
Vertex *v2 = &(*(find_it(from)));
v1->addEdge(v2); // Updating respective adjacency list
v2->addEdge(v1);
}
// Function to check whether the vertex is already added in the Vertex List
int find(string check)
{
list<Vertex>::iterator it;
it = find_it(check);
if(it==vertexList.end())
return 1;
else
return 0;
}
// Function which returns pointer to a Vertex in the Vertex List
list<Vertex>::iterator find_it(string check)
{
list<Vertex>::iterator it;
for (it = vertexList.begin(); it != vertexList.end(); it++)
if((check.compare(it->name))==0)
break;
return it;
}
};
int main()
{
int numVertices,numEdges,i,result;
string to,from,queryTo,queryFrom;
cin>>numVertices>>numEdges;
Graph G = Graph(numVertices); // Creating the Graph object
for( i=0;i<numEdges;i++)
{
cin>>to>>from;
G.addEdge(to,from); // Adding Edges to Graph
}
cin>>queryTo>>queryFrom;
// The function you have to write is called here where the address of vertex
// node is passed.
result = G.Connection((*(G.find_it(queryTo))),(*(G.find_it(queryFrom))));
if(!result)
cout<<"No Connection";
else
cout<<"Degree : "<<result;
return 0;
}
int Graph::Connection(Vertex v1,Vertex v2)
{
// Mark all the vertices as not visited
Vertex s=Vertex("xx");
int i=0;
//list<Vertex>::iterator it;
Vertex *temp=&(*(vertexList.begin()));
while(!temp)
temp->status = false,++temp;
// Create a queue for BFS
list<Vertex> queue;
// Mark the current node as visited and enqueue it
v1.status=true;
queue.push_back(v1);
// it will be used to get all adjacent vertices of a vertex
int depth;
while (!queue.empty())
{
depth=0;
// Dequeue a vertex from queue and print it
s = queue.front();
queue.pop_front();
// Get all adjacent vertices of the dequeued vertex s
// If a adjacent has not been visited, then mark it visited
// and enqueue it
temp=s.adjacencyList.front();
while(temp!=NULL)
{
++depth;
// If this adjacent node is the destination node, then return true
if ((v2.name.compare(temp->name))==0)
{
v2.depth=depth;
return v2.depth;
}
// Else, continue to do BFS
if(temp->status==false)
{
temp->status = true;
queue.push_back(*temp);
}
++temp;
}
}
return 0;
}

Answer 1

I'm going to assume that the degree of connection you are trying to compute is the shortest path distance between nodes (with uniform edge costs). You can use Floyd-Warshall to pre-process the graph and answer queries in O(1) time. It's really simple to implement.
int a[N][N]; /* adjacency matrix where N is max node count */
/* build graph here and init distances between distinct nodes to +INFINITY */
/* n is number of nodes */
for(int k = 0; k < n; k++)
for(int i = 0; i < n; i++)
for(int j = 0; j < n; j++)
dist[i][j] = min(dist[i][j], dist[i][k]+dist[k][j]);
To answer a query (x,y) you print dist[x][y].
Your BFS solution looks unnecessarily complicated. Use a vector<in> g[N] to represent your graph. Add an edge x->y with g[x].push_back(y). BFS would look like:
queue<int> Q;
Q.push(s); /* start node */
for(int i = 0; i < n; i++)
{ dist[i] = INFINITY;
}
dist[s] = 0; /* distance to s is set to 0 */
while(Q.empty() == false)
{ int x = Q.front(); Q.pop();
for(int i = 0; i < g[x].size(); i++)
{ int y = g[x][i];
/* process edge x->y */
if(dist[y] == INFINITY)
{ dist[y] = dist[x] + 1;
Q.push(y);
}
}
}
distance between s and any other node t is dist[s][t].

Answer 2

Your code also crashes in a segmentation fault, if you try to find a connection with degree 1. Given your graph try to find "Harry Krax".
I think the mistake is using a pointer Vertex * temp = temp=s.adjacencyList.front(); and later trying to access the next Vertex by ++temp;.
This is not how std::list in combination with pointers work.
If you want to access the next Vertex with ++temp, than you might want to use iterators std::list<x>::iterator temp.
What you are trying to do works with arrays like int a[N], because the elements of an array are adjacent in memory.
With int * aptr = a. ++aptr says that temp is to move to another location in memory that is the size of one int further away.
std::list<x> does not do this. Here the elements can be scatter at different places in memory. (Simplified) it's value and pointers to the previous and next element are stored.