push_back in 2D vector c++ code example

Example 1: how to make a 2d vector in c++

// Create a vector containing n 
//vectors of size m, all u=initialized with 0
vector<vector<int> > vec( n , vector<int> (m, 0));

Example 2: 2d vector

#include <bits/stdc++.h>
using namespace std;
int main() 
{
  int rows = 2;
  int cols = 2;
  int val = 1;
  vector< vector<int> > v(rows, vector<int> (cols, val));  /*creates 2d vector “v[rows][cols]” and initializes all elements to “val == 1” (default value is 0)*/
  v[0][0] = 5;
  v[1][1] = 4;
  cout << v[0][0] << endl; //Output: 5cout << v[1][0] << endl; //Output: 1return 0;}

Example 3: 2d vector push back

std::vector<std::vector<int>> normal;
for(int i=0; i<10; i++)
{
  	//push a vector each time you loop a row
    normal.push_back(std::vector<int>());
    for(int j=0; j<20; j++)
    {
      	//push an item each time you loop a column
        normal[i].push_back(j);    
    }
}