title: how to pushback on a vector of vectors?
body: <p>I am taking 20 lines of input. I want to separate the contents of each line by a space and put it into a vector of vectors. How do I make a vector of vectors? I am having have struggles pushing it back...</p>
  
  <p>My input file:</p>
  
  <pre><code>Mary had a little lamb
  lalala up the hill
  the sun is up
  </code></pre>
  
  <p>The vector should look like something like this.</p>
  
  <pre><code>ROW 0: {"Mary","had", "a","little","lamb"}
  ROW 1: {"lalala","up","the","hill"}
  </code></pre>
  
  <p>This is my code....</p>
  
  <pre><code>string line; 
  vector &lt;vector&lt;string&gt; &gt; big;
  string buf;
  for (int i = 0; i &lt; 20; i++){
      getline(cin, line);
      stringstream ss(line);
  
      while (ss &gt;&gt; buf){
          (big[i]).push_back(buf);
      }
  }
  </code></pre>