different ways of making 2d vector and list

Question

So I have this snippet

vector<int> *adj;
adj = new vector<int>[n];

and this another common way

vector<vector<int> adj(n);

can former way of using pointer be used to make 2d array ? And will it work the same way like latter does?

Also when is former way used if in case it can be used to make 2d vectors.

Can I use former way for making 2d list too?

Incase for those of you who are wondering former method is wrong way of making list it's on geeks for geeks http://www.geeksforgeeks.org/topological-sorting/

Answer 1

There are may ways to create a multidimensional vectors but I use this method which utilizes struct,

Here is an example for 2X2 table

#include<iostream>
#include<vector>

struct table
{
    std::vector<int> column;
};

int main()
{
    std::vector<table> myvec;
    // values of the column for row1
    table t1;
    t1.column = {1,2};
    // Push the first row
    myvec.push_back(t1);
    // values of the column for row2
    table t2;
    t2.column = { 3, 4};
    // push the second row
    myvec.push_back(t2);
    // now let us see whether we've got a 2x2 table
    for(auto row : myvec)
    {
        auto values = row.column; 
        for(auto value : values) std::cout<< value << " ";
        std::cout<<"\n";
    }
    // Now we will try to get a particular value from the column index of a particular row
    table row = myvec[1]; // 2nd row
    std::cout<<"The value present at 2nd row and 1st column is: "<<row.column[0] <<"\n";
}

which gives me,

1 2 
3 4 
The value present at 2nd row and 1st column is: 3

You can easily change it to different dimensions.

Note: I have posted this answer to my knowledge if it is wrong somebody correct me. Thank you