title: a simple way to sum the value of certain rows in R
body: <p>I have a dataframe like this:</p>
  
  <pre><code>count  date
  1233 2012-10-12     
  3232 2012-10-12     
  2334 2012-10-12     
  3330 2012-10-13    
  1445 2012-10-13     
  3455 2012-10-13     
  7632 2012-10-13     
  </code></pre>
  
  <p>I'd like to know the sum of count of each day, is any simple way to do that?
  Also, how to select the data of certain date with certain logic?</p>