no matching function for call to 'begin(std::vector*&)'

Question

I was implementing cycle detection in an undirected graph using breadth-first search.

However, I seem to be getting the following error:

./Solution.cpp:23:27: error: no matching function for call to 'begin(std::vector*&)' for(auto nbr: adj)

I understand there have been similar questions asked on the platform before but being a beginner, I am still quite confused as to how to fix this in my code specifically. Here's the code:

class Solution {
  public:
    // Function to detect cycle in an undirected graph.
    bool cyclicUsingDFS(int src, unordered_map<int,bool>& visited, vector<int> adj[])
    {
        queue<int> q;
        unordered_map<int,bool> parent;
        
        q.push(src);
        visited[src]=true;
        parent[src]=-1;
        
        while(!q.empty())
        {
            int frontnode = q.front();
            q.pop();
            
            for(auto nbr: adj)
            {
                if(!visited[nbr])
                {
                    q.push(nbr);
                    visited[nbr]=true;
                    parent[nbr]=frontnode;
                }
                if(visited[nbr] && nbr!=parent[frontnode]);
                {
                    return true;
                }
            }
        }
         return false;
    }
    
    bool isCycle(int V, vector<int> adj[]) 
    {
        unordered_map<int, bool> visited;
        for(int i=0; i<V; i++)
        {
            if(!visited[i])
            {
                cyclicUsingDFS(i, visited,adj);
            }
        }
    }
};

Thanks :)

`vector adj[]` is almost never what you want to receive as an argument in modern C++ - it is really a `vector* adj` which means you have no idea how many elements it refers to — UnholySheep, Jul 13 '23 at 13:52
What is that supposed to be: `vector adj[]` ? This is C-style array of `std::vector` of int, which decays to a pointer. You try to run a range loop over it, and it simply cannot tell what is the "beginning" or "ending" of your pointer. Go for "vector of vector" if you need 2-d array. — pptaszni, Jul 13 '23 at 13:53
`vector adj[]` i know only 1 site that promotes this kind of data structure. Forget that it exists. Learn C++ from a [book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) and/or proper courses. Don't trust so-called tutorial sites you find on the internet. Most of them are not worth what you pay for them — 463035818_is_not_an_ai, Jul 13 '23 at 13:54

score 0 · Answer 1 · answered Jul 13 '23 at 13:59

In this range-based for loop

for(auto nbr: adj)

the variable adj represenets a pointer due to the function declaration

bool cyclicUsingDFS(int src, unordered_map<int,bool>& visited, vector<int> adj[])

where the last parameter is adjusted by the compiler to

bool cyclicUsingDFS(int src, unordered_map<int,bool>& visited, vector<int> *adj)

So the function begin called in the range based for loop can not be applied to a pointer and the compiler issues an error.

You could simplify the code if instead of an array of vectors you used vector of vectors.

no matching function for call to 'begin(std::vector*&)'

1 Answers1