title: invalid vector <T> subscript
body: <p>I am trying to make a two dimensional vector with where the 0 and 1 coordinates are both complex numbers, defining the vector seems to be working fine, but when i try to access it, i get an error that pops up in a new tab with a load of gibberish, but somewhere in there it says invalid vector  subscript. The (abridged) code is</p>
  
  <pre><code>#include &lt;iostream&gt;     
  #include &lt;vector&gt;
  #include &lt;complex&gt;
  using namespace std;
  int main() 
  {
  vector&lt;vector&lt;complex&lt;double&gt;&gt;&gt; rho;
  for(int g = 0; g &lt; 4; ++g){
      for(int h = 0; h &lt; 4; ++h){
          rho.push_back(vector&lt;complex&lt;double&gt;&gt;(2));
          rho.at(g).at(h) = 0;
          cout &lt;&lt; rho.at(g).at(h)&lt;&lt;endl;
      }
  }
  }
  </code></pre>
  
  <p>Any help would be greatly appreciated :) xx</p>