title: Splitting matrices and vectors in GNU Octave to get rows not extracted
body: <p>In GNU Octave, I can use the following code to get every 5th row of X.  </p>
  
  <p>How do I get another matrix <code>X_2</code> that consists of the rows that have not been extracted or left behind in <code>X</code>?</p>
  
  <pre><code>X = [1;2;3;4;5;6;7;8;9;10;11]
  ix = ( 5 : 5 : size(X,1) )
  X_1 = X( ix , : )
  </code></pre>
  
  <p>I want <code>X_2</code> to have everything but 5 and 10.  Is this possible?</p>