title: dplyr: Find date for which an event occurs
body: <p>First question. I am fairly new at R. I have the following data frame.</p>
  
  <pre><code>Source: local data frame [865,264 x 10]
  
     page_views       date dayofweek daytype   caseID dateDecision dateArgument dateRearg
  1         169 2008-01-30 Wednesday       0 2007-001   2007-10-10   2007-10-01          
  2         211 2008-01-16 Wednesday       0 2007-001   2007-10-10   2007-10-01          
  3         203 2008-01-17  Thursday       0 2007-001   2007-10-10   2007-10-01          
  4         177 2008-01-14    Monday       0 2007-001   2007-10-10   2007-10-01          
  5         224 2008-01-15   Tuesday       0 2007-001   2007-10-10   2007-10-01          
  6         152 2008-01-12  Saturday       1 2007-001   2007-10-10   2007-10-01          
  7         149 2008-01-13    Sunday       1 2007-001   2007-10-10   2007-10-01          
  8         220 2008-01-10  Thursday       0 2007-001   2007-10-10   2007-10-01          
  9         169 2008-01-11    Friday       0 2007-001   2007-10-10   2007-10-01          
  10        189 2008-01-18    Friday       0 2007-001   2007-10-10   2007-10-01          
  ..        ...        ...       ...     ...      ...          ...          ...       ...
  Variables not shown: caseName (chr), term (int)
  </code></pre>
  
  <p>I would like to find the first, earliest chronologically, date for each caseID for which page_views is greater than zero. I would like to create a new column with this date. The results should have one row for each caseID. </p>
  
  <p>I am hoping that I can do this using dplyr, but I am open to other solutions. Using dplyr it seems group_by(caseID) and some kind of filter are the place to start, but I have had no luck. </p>
  
  <p>I have searched stackoverflow and other places and haven't found anything that comes close. </p>