Pointer to an array of vectors in c++ and send it to a function

Question

I want to define an array of vectors in c++. Normally I can do it like this:

vector <pair<int,int> > G[100];

I have a function that read data from a file.

My first problem is that I want to define G with size V that read from a file and G should define in main but calling a function with an uninitialized pointer can't work.

My Second problem is when I define G like this:

vector <pair<int, int> > *G;
G = new vector <pair<int, int>>[10];  

it doesn't read data correctly (it doesn't work correctly).

Here is my code:

void readData(vector <pair<int, int> > *G)
{
    int V, E;
    ifstream file("input.txt");
    file >> V >> E;
    //G = new vector <pair<int, int>>[V]; //my problem
    for (int i = 0; i < E; i++)
    { 
        int u, v, w;
        file >> u >> v >> w; 
        G[u - 1].push_back(make_pair(v - 1, w)); 
        G[v - 1].push_back(make_pair(u - 1, w)); 
    }
    file.close();
}
int main() {
    vector <pair<int, int> > *G;
    G = new vector <pair<int, int>>[10]; // my problem
    MST = new vector <pair<int, int>>;
    readData(G);
}

Example data:

5 6
1 3 4
1 2 3
2 4 6
4 3 5
4 5 20
5 2 21

and this code is working correctly and read data from console:

const int N=10;
vector <pair<int,int> > G[N];
int main() {    
    cin >> V>>E;
    for(int i=0; i<E; i++){
        int u,v,w;
        cin >>u>>v>>w; 
        G[u-1].push_back(make_pair(v-1,w)); 
        G[v-1].push_back(make_pair(u-1,w)); 
    }
}

Answer 1

There is no reason to use a builtin array when using a vector does exactly what you want. For example you could do this:

void readData(std::vector<std::vector<std::pair<int, int>>>& G)
{
    std::size_t V, E;
    std::ifstream file("input.txt");
    file >> V >> E;
    //G = new vector <pair<int, int>>[V]; //my problem
    G.resize(V); // solved!!!
    for (std::size_t i = 0; i < E; i++)
    {
        std::size_t u, v, w;
        file >> u >> v >> w;
        G[u - 1].push_back(std::make_pair(v - 1, w));
        G[v - 1].push_back(std::make_pair(u - 1, w));
    }
    file.close();
}
int main() {
    std::vector<std::vector<std::pair<int, int>>> G;
//    G = new vector <pair<int, int>>[10]; // my problem
//    MST = new vector <pair<int, int>>;
    readData(G);
}

Answer 2

You probably should use a vector or vectors instead of a raw array of vectors.

Example:

#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

void readData(vector<vector <pair<int, int>>> & G)
{
  int V, E;
  ifstream file("input.txt");

  file >> V >> E;
  G.resize(V);

  for (int i = 0; i < E; i++)
  {
    int u, v, w;
    file >> u >> v >> w;
    G[u - 1].push_back(make_pair(v - 1, w));
    G[v - 1].push_back(make_pair(u - 1, w));
  }
  file.close();
}

int main() {
  vector<vector <pair<int, int>>> G;
  readData(G);

  for (auto & v1 : G)
  {
    for (auto & p : v1)
    {
      cout << p.first << ", " << p.second << "\n";
    }

    cout << "\n";
  }
}

Output:

2, 4
1, 3

0, 3
3, 6
4, 21

0, 4
3, 5

1, 6
2, 5
4, 20

3, 20
1, 21

Hints:

• don't use all capital variable names, and use meaningful variable names, eg: data instead of G , nbvectors instead of V and nblines instead of E etc.
• using namespace std; is not very good practice, but I didn't change the OP's code too much.

Answer 3

My first problem is that I want to define G with size V that read from a file and G should define in main but calling a function with an uninitialized pointer can't work.

You don't need to define the size of G. It's possible to simply declare the vector and add your pairs using the push_back function, as in the code below.

vector <pair<int, int>> G;
while (readFile) {
    G.push_back(make_pair(item1, item2));
}

Answer 4

My first problem is that I want to define G with size V that read from a file and G should define in main but calling a function with an uninitialized pointer can't work.

Quick and dirty fix : Pass a reference to the pointer:

void readData(vector <pair<int, int> >*& G)

Then your main can do this:

vector<pair<int, int>>* MST;
readData(MST);

Then readData cand (and will) change the pointer value (because you passed it by reference).

The better way: Don't use bare C-arrays. Use a vector .

void readData(vector<vector<pair<int, int>>>& G)
{
    size_t V, E;
    ifstream file("input.txt");
    file >> V >> E;
    G.resize(V);

    // Your code...
}

main()
{
    vector<vector<pair<int, int>>> MST;
    readData(MST);
}

//G = new vector <pair<int, int>>[V]; //my problem

That's not the real problem. The problem is that your function takes a vector <pair<int, int> > *G and you overwrite that pointer value it in the above line. The same would happen if your function would take an int and you would just overwrite it: In either case, the caller of the function does not see the change.

void foo(int x) {
  x = 10; // Caller will not see this change.
}

The fix is the same as above: Pass a reference instead.

Answer 5

You can rewrite your function to return the vector instead:

vector <pair<int, int> >* readData()
{
    int V, E;
    ifstream file("input.txt");
    file >> V >> E;
    vector <pair<int, int> > *G = new vector <pair<int, int>>[V];
    for (int i = 0; i < E; i++)
    { 
        int u, v, w;
        file >> u >> v >> w; 
        G[u - 1].push_back(make_pair(v - 1, w)); 
        G[v - 1].push_back(make_pair(u - 1, w)); 
    }
    file.close();
    return G;
}
int main() {
    MST = new vector <pair<int, int>>;
    vector <pair<int, int> > *G = readGraph();
}