Multiple array using vector of vector

Question

I want to insert multiple array using Vector of vector.
For that I/P is like this:

2   : Number of array ( can be vary )    

3  1 2 3  :  3( size of array)  1 2 3( elements in array )      

4  4 5 6 7  : 4( size of array) 4 5 6 7 ( elements in array)       

Then I need to perform some operation on it.
All the I/P is entered by user. Only its range is given.

2 <= No of array <= 10 


1 <= Elements in array <= 1000000 

For above I tried following ways :

int main()
{
    int no_of_array, size;
    long int num;
    int j = 0; 

    cin >> no_of_array;

1st way :

vector < vector<int> > array_vector[no_of_array];
   while( no_of_array -- )
   {
        cin >>size;
        for( int i = 0; i < size ; i++ )
            {
                cin >> num;
                array_vector[j].push_back({num});
            }
            j++;
    }
  cout<< vector_array[1][2];    // Error 

2nd way :

vector< vector<int> > array_vector;
while( no_of_array --)
{
    cin >> size;
    for( int i = 0 ; i < size ; i++ )
    {
        cin >> num;
        array_vector[j].push_back(num);
    }
    j++; 
 }   
cout<< vector_array[1][2];     // Error       

3rd way : ( Working )

vector<vector<int> > array_vector;
for( int i = 0 ; i < no_of_array ; i++ )
{
    cin >> size;
    vector<int> v;
    for( int j = 0 ; j < size ; j++ )
    {

        cin >> num;
        v.push_back(num);
    }
    array_vector.push_back(v);
}    
cout<<array_vector[1][2];    // No Error         

Somewhere I read 3rd way of vector of vector is not suitable for competitive programming. Why ??
What mistake I did while traversing the elements with 1st two approach ??

Answer 1

You can try the following code

int main(int argc,char* argv[])
{
    unsigned int num;
    int elm;
    unsigned int size;

    std::cout << "Enter the number of rows" << std::endl;
    std::cin >> num;
    std::vector<std::vector<int> > a(num,std::vector<int>(0,0));

    for(unsigned int i=0;i<num;i++)
    {
        std::cout  << "Enter the size of " << i+1 << "array:" << std::endl;
        std::cin >> size;

        for(unsigned int j=0;j<size;j++)
        {
            std::cout << "Enter the element" <<std::endl;
            std::cin >> elm;

            a[i].push_back(elm);
        }
    }

    for(unsigned int i=0;i<a.size();i++)
    {
        for(unsigned int j=0;j<a[i].size();j++)
        {
            std::cout << "Element at [" <<i<<"][" <<j <<"] is " << a[i][j] <<std::endl;
        }
    }

    return 0;
}