title: Aggregate data in R
body: <p>I'm looking for a dead simple example on how to use <code>aggregate</code> and calculate means in R.</p>
  
  <p>Say, I have the following data frame:</p>
  
  <pre><code>A      B
  100    85
  200    95
  300    110
  400    105
  </code></pre>
  
  <p>And I want to calculate the mean values for some ranges with the following result:</p>
  
  <pre><code>RANGE         MEAN
  100-200       90
  300-400       107.5
  </code></pre>
  
  <p>How would I go about doing this, <code>cast()</code> or <code>aggregate()</code>?</p>