En fråga om vektorer i c++

#include <iostream>
#include <vector>

int main()
{
  std::vector<int> v;
  v.resize(3);
  
  v[0] = 5;
  v[1] = 7;
  v[2] = 2;
  
  std::cout<<"Vector original"<<std::endl;

  for (int i=0; i<3; i++)
    std::cout<<"v["<<i<<"] = "<<v[i]<<std::endl;
  
  v.back() = 9;
  
  std::cout<<"\nVector modified"<<std::endl;
  
  for (int i=0; i<3; i++)
    std::cout<<"v["<<i<<"] = "<<v[i]<<std::endl;
}

Vector original
v[0] = 5
v[1] = 7
v[2] = 2

Vector modified
v[0] = 5
v[1] = 7
v[2] = 9

// Från http://www.cplusplus.com/reference/stl/vector/back/
#include <iostream>
#include <vector>
using namespace std;

int main ()
{
  vector<int> myvector;

  myvector.push_back(10);

  while (myvector.back() != 0)
  {
    myvector.push_back ( myvector.back() -1 );
  }

  cout << "myvector contains:";
  for (unsigned i=0; i<myvector.size() ; i++)
    cout << " " << myvector[i];

  cout << endl;

  return 0;
}