how many nodes in graph are reachable? - c++

Question

I want to print the number of nodes which are reachable from a particular node. I read graph and stored in adjacency list and performed bfs.i had tried the following code.it works with certain graph.can you traced out what's wrong with this?
#include <vector>
#include <iostream>
#include <list>
#include<queue>
using namespace std;
int BFS(int s)
{
const int V=100;
int r=0;
vector<list<int> > a(V);
int visited[V]={0};
queue<int> Q;
visited[s]=1;
Q.push(s);
++r;
while(!Q.empty())
{
int x=Q.front();
Q.pop(); // pop here. we have x now
++r;
vector<list<int> >::iterator it1=a.begin()+x;
list<int> it2=*it1;
list<int>::iterator iter=it2.begin();
while(iter!=it2.end())
{
if(visited[*iter]==0)
{
visited[*iter]=1;
Q.push(*iter);
}
++iter;
}
visited[x]=2; // set visited here.
}
return r;
}
void printAsGrid(int V)
{
// Create a local 2D projection grid
int size = V.size();
double *grid = new double[size*size];
memset(grid, 0.0, size*size*sizeof(double));
// Get the edge connection and weight
int index;
for (index = 0; index < size; index++) {
list<Edge>::const_iterator eit;
for (eit = V[index].edges.begin();
eit != V[index].edges.end(); eit++) {
int to = (*eit).to;
double w = (*eit).weight;
// record weight in the projection grid
grid[(index*size)+to] = w;
}
}
// print header
out << " |";
for (index = 0; index < size; index++)
out << " " << index;
out << endl;
out << "---+";
for (index = 0; index < size; index++)
out << "-----";
out << endl;
// print content
out.setf(ios::fixed);
out.setf(ios::showpoint);
for (index = 0; index < size; index++) {
out << " " << index << " |";
for (int j = 0; j < size; ++j)
out << setw(5) << setprecision(1) << grid[(index*size)+j];
out << endl;
}
// delete grid before exit
delete [] grid;
}
int main()
{
int s;
int V,total_neighbors, id, weight;
//number of vertices
cout<<"enter the no.of vertices:";
cin>>V;
vector< list< int > > graph(V + 1);
for(int i= 0; i<V;i++) {
cout<<"Enter no.of neighbours of"<<i<<":";
cin>>total_neighbors;
cout<<"Enter the neighbours of"<<i<<":";
for(int j = 0; j <total_neighbors; j++) {
cin>>id;
graph[i].push_back(id);
}
}
vector<list<int> >::iterator i;
int c=0;
for (vector<std::list<int> >::iterator i=graph.begin(); i !=graph.end(); ++i){
cout<<"vertices connected to node "<<c <<" are ";
//cout<<*i;
std::list<int> li = *i;
for(std::list<int>::iterator iter = li.begin(); iter!= li.end(); ++iter){
cout<<*iter<<" ";
}
cout<<endl;
c++;
}
int f;
cin>>f;
s=BFS(f);
cout<<s<<" ";
return 0;
}
adjacencyList 0 -> 1 -> 2
adjacencyList 1 -> 2 -> 4
adjacencyList 2 -> 4
adjacencyList 3 -> 5
adjacencyList 4
adjacencyList 5 -> 3
returns 2 but actual answer is 3

Answer 1

You are doing r++ two times. You can do it only when you are pushing the node or when just popping the node. Otherwise for the source node, it will count twice. Also initialise r to 0. Also initialise the visited array manually to be on safe side.
Also you have taken vector<list<int> >::iterator it1=a.begin()+x; whereas a is empty. vector<list<int> > a(V); just initialises the vector of list but doesn't put values inside it. And hence the list which you are trying to traverse is empty. You are getting 2 because you are making r++ two times . One when source is inserted and other when source is removed which gives you 2.
See this code :
#include <vector>
#include <iostream>
#include <list>
#include<queue>
using namespace std;
int BFS(vector<list<int> > graph, int s)
{
const int V=100;
int r=0;
int visited[V]={0};
for(int i = 0; i < V; i++) visited[i] = 0;
queue<int> Q;
visited[s]=1;
Q.push(s);
while(!Q.empty())
{
int x=Q.front();
cout << x << endl;
Q.pop(); // pop here. we have x now
++r;
vector<list<int> >::iterator it1=graph.begin()+x;
list<int> it2=*it1;
list<int>::iterator iter=it2.begin();
while(iter!=it2.end())
{
if(visited[*iter]==0)
{
visited[*iter]=1;
Q.push(*iter);
}
++iter;
}
visited[x]=2; // set visited here.
}
return r;
}
int main()
{
int s;
int V,total_neighbors, id, weight;
//number of vertices
cout<<"enter the no.of vertices:";
cin>>V;
vector< list< int > > graph(V + 1);
for(int i= 0; i<V;i++) {
cout<<"Enter no.of neighbours of"<<i<<":";
cin>>total_neighbors;
cout<<"Enter the neighbours of"<<i<<":";
for(int j = 0; j <total_neighbors; j++) {
cin>>id;
graph[i].push_back(id);
}
}
vector<list<int> >::iterator i;
int c=0;
for (vector<std::list<int> >::iterator i=graph.begin(); i !=graph.end(); ++i){
cout<<"vertices connected to node "<<c <<" are ";
//cout<<*i;
std::list<int> li = *i;
for(std::list<int>::iterator iter = li.begin(); iter!= li.end(); ++iter){
cout<<*iter<<" ";
}
cout<<endl;
c++;
}
int f;
cin>>f;
s=BFS(graph,f);
cout<<s<<" ";
return 0;
}