Graph components C/C++

[svetlioi]

Hello. I want to count the components in a graph, but something is wrong with my C++ program - the output is sometimes right, but sometimes - wrong. Please help. I'm using BFS.

#include <iostream>
#include <vector>
#include <queue>
using namespace std;
vector< vector<int> > graph(100);
int nodes;
void get(){
    cin>>nodes;
    for(int i=0; i<nodes; i++){
        int neighb; cin>>neighb;
        for(int j=0; j<neighb; j++){
            int nX; cin>>nX;
            graph[i].push_back(nX);

        }
    }
}
vector<int>  vis(100, 0);
void bfs(int start){
    queue<int> prem;
    prem.push(start);
    while(!prem.empty()){
        int seg=prem.front();
        prem.pop();
        int siz=graph[seg].size();
        for(int i=0; i<siz; i++){
            int neighb=graph[seg][i];
            if(vis[neighb]==0&&neighb!=start){
                prem.push(neighb);
                vis[neighb]=vis[seg]+1;
            }
        }
    }
}
void counter(){
    int comps=0;
    for(int i=0; i<nodes; i++){
        if(vis[i]==0) {
            bfs(i);
            comps++;
        }
    }
    cout<<comps<<'\n';
}
int main()
{
get(); counter();
return 0;
}

In the input, I get the list of neighbours(Adjacency list) - first their number and then them.

Example input:
11
2 4 2
2 6 9
2 0 4
1 5
3 2 0 5
3 8 3 4
1 1
0
2 10 5
1 1
1 8

Graph for the example: https://i.stack.imgur.com/M6U3I.png

[salem c]

So is your test case an example of when it works, or when it doesn't work?

What's the simplest test case you can come up with that doesn't work?

With a really simple failing case, you can start to do things like this.

void bfs(int start){
    queue<int> prem;
    prem.push(start);
    while(!prem.empty()){
        int seg=prem.front();
        cout << "Debug:bfs:testing segment=" << seg << endl;
        prem.pop();
        int siz=graph[seg].size();
        for(int i=0; i<siz; i++){
            int neighb=graph[seg][i];
            if(vis[neighb]==0&&neighb!=start){
                cout << "Debug:bfs:new neighbour=" << neighb << endl;
                prem.push(neighb);
                vis[neighb]=vis[seg]+1;
            }
        }
    }
}
void counter(){
    int comps=0;
    for(int i=0; i<nodes; i++){
        if(vis[i]==0) {
            cout << "Debug:counter:testing node=" << i << endl;
            bfs(i);
            comps++;
        }
    }
    cout<<comps<<'\n';
}

You can either add debug cout statements (as above).

Or use a debugger to step through and examine variables without having to keep editing the code.

void get(istream &in){
    in>>nodes;
    for(int i=0; i<nodes; i++){
        int neighb; in>>neighb;
        for(int j=0; j<neighb; j++){
            int nX; in>>nX;
            graph[i].push_back(nX);
        }
    }
}

int main ( ) {
    ifstream inf("test.txt");
    get(inf);
    // get(cin);
}

Also, automatically reading your test file (as opposed to always typing it in) will simplify and speed up your testing.