Why is my for loop giving error : X does not name a type

Question

Can anybody tell me that when I use for loop to get the number of nodes stored in level 'it', then why it doesn't work?

And please do tell me other ways I can access a vector with a range based for loop.

// A simple representation of graph using STL 
#include<iostream>
#include<vector>
using namespace std;

// A utility function to add an edge in an 
// undirected graph. 
void addEdge(vector<int> adj[], int u, int v) 
{ 
    adj[u].push_back(v); 
    adj[v].push_back(u); 
} 


void printNodes(vector<int> adj[], int n) 
{ int count=0;
        for (auto x : adj[n]){
            count++;
        } 
        cout<<count;

} 

// Driver code 
int main() 
{ 

int V,x,y;
cin>>V;

    vector<int> adj[V+1]; 

    for(int i=0;i<V-1;i++){
        cin>>x>>y;
    addEdge(adj, x, y); 
}
    int it;
    cin>>it;
    printNodes(adj, it); 
    return 0; 
} 

Answer 1

First of all, there are some things missing in your approach:

1. You are adding edges in the graph and there is no input for it, i have assumed that V is for vertices only.
2. Whenever you want to check for the number of nodes in a particular level you should always mention a starting point. Because different starting points may account to different outputs in a graph which is not rooted like a tree.
3. If the number of vertices is V, then the vector should be vector adj[V+1], if you want your vertices to be 1-indexed.

So here is the final code:

#include<iostream>
#include<vector>
#include<queue>
using namespace std;


int nodes_at_level[10];
// Taken from hackerearth.....


    int level[10]; //To determine the level of each node
    bool vis[10]; //Mark the node if visited 

    void bfs(int s,vector<int> adj[]) {
        queue <int> q;
        q.push(s);
        level[ s ] = 0 ;  //Setting the level of the source node as 0
        nodes_at_level[level[s]]++;
        vis[ s ] = true;
        while(!q.empty())
        {
            int p = q.front();
            q.pop();
            for(int i = 0;i < adj[ p ].size() ; i++)
            {
                if(vis[ adj[ p ][ i ] ] == false)
                {
            //Setting the level of each node with an increment in the level of parent node
                    level[ adj[ p ][ i ] ] = level[ p ]+1;

                    nodes_at_level[level[ adj[ p ][ i ] ]]++;

                     q.push(adj[ p ][ i ]);
                     vis[ adj[ p ][ i ] ] = true;
                }
            }
        }
    }

void addEdge(vector<int> adj[], int u, int v)
{
    adj[u].push_back(v);
    adj[v].push_back(u);
}

//to print the number of nodes in the level 'it'
void printNodes(vector<int> adj[], int n)
{
    int count = 0;
    count = nodes_at_level[n];
    cout << count;
}

// Driver code 
int main()
{
    int V, x, y ,E;
    cin >> E;
    cin >> V;

    vector<int> adj[V+1];

    for (int i = 0; i < E; i++) {
        cin >> x >> y;
        addEdge(adj, x, y);
    }
    bfs(1,adj);  //assuming the start of the graph to be 1 

    int it;
    cin >> it;
    printNodes(adj, it);
    return 0;
}

I have seen the hackerearth question yet. But just tried to solve your problem.

However if you want the for loop anyhow, then try this... i am just storing the vertices according to their levels ..

vector<int> nodes_at_level[10];
//^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

// Taken from hackerearth.....


    int level[10];              //To determine the level of each node
    bool vis[10];               //Mark the node if visited 

    void bfs(int s,vector<int> adj[]) {
        queue <int> q;
        q.push(s);
        level[ s ] = 0 ;  //Setting the level of the source node as 0
        nodes_at_level[level[s]].push_back(s);
        vis[ s ] = true;
        while(!q.empty())
        {
            int p = q.front();
            q.pop();
            for(int i = 0;i < adj[ p ].size() ; i++)
            {
                if(vis[ adj[ p ][ i ] ] == false)
                {
                    //Setting the level of each node with an increment in the level of parent node
                    level[ adj[ p ][ i ] ] = level[ p ]+1;

                    nodes_at_level[level[ adj[ p ][ i ] ]].push_back(adj[p][i]);
                    //^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                     q.push(adj[ p ][ i ]);
                     vis[ adj[ p ][ i ] ] = true;
                }
            }
        }
    }

//to print the number of nodes in the level 'it'
void printNodes(vector<int> adj[], int n)
{
    int count=0;
        for (auto x : nodes_at_level[n]){
            count++;
        } 
        cout<<count;
}

Hope it might help.