Why c++ Lambda Expression Not working

I am very much new to this feature, I was just reading about Lambda expression in c++ and tried to implement it in a simple program.

int main()
{
  std::string name;
  int a = 5;
  std::cout << "What is your name? ";
  getline (std::cin, name);
  for([&](){a = 7;};a > 0; a--)
  {
    std::cout << "Hello, " << name << "!\n";
  }
}

but its not working as I thought it will.. My assumption : [&](){a = 7;} this will change value of variable a to 7 from 5 but its not.. Is there anything wrong in code?? Or just my assumption is incorrect?