Hi all, i am new is C++.

i want to create a global 2D matrix and here is the code is use:
Code:
#include <iostream>
#include <vector>

using namespace std;

vector<vector<int> >mat; // 2D matrix

void func(void){
     
     for(int i=0; i<5; i++)
            for(int j=0; j<5; j++)
                    mat[i][j] = 0;
     
}

int main(void){
    int size_x = 5; // X_dimension
    int size_y = 5; // Y_dimension
    
    mat.resize(size_x, vector<string>(size_y)); //resize 2D matrix
    
    for(int i=0; i<size_x; i++)
            for(int j=0; j<size_y; j++)
                    mat[i][j] = i;
    
    //print matrix
    for(int i=0; i<size_x; i++){
            cout << "\n";
            for(int j=0; j<size_y; j++){
                    cout << mat[i][j] << " ";
            }
    }
    
    func(); // create a zero matrix
     
    //print zero matrix
    cout << "\n\n";
    for(int i=0; i<size_x; i++){
            cout << "\n";
            for(int j=0; j<size_y; j++){
                    cout << mat[i][j] << " ";
            }
    }
    
    cout << "\n\n";
    system("PAUSE");
    return 0;
}
i am doing something wrong but i have no idea how to correct my mistake.

Any help will be appreciated.