title: Include levels of zero count in result of table()
body: <p>I have a vector 'y' and I count the different values using <code>table</code>:</p>
  <pre><code>y &lt;- c(0, 0, 1, 3, 4, 4)
  table(y)
  # y
  # 0 1 3 4 
  # 2 1 1 2 
  </code></pre>
  <p>However, I also want the result to include the fact that there are zero 2's and zero 5's. Can I use <code>table()</code> for this?</p>
  <p>Desired result:</p>
  <pre><code># y
  # 0 1 2 3 4 5 
  # 2 1 0 1 2 0
  </code></pre>