How to use dplyr as alternative to aggregate
<p>I have a dataframe <code>times</code> that looks like this:</p>

<pre><code>user     time
A        7/7/2010
B        7/12/2010
C        7/12/2010
A        7/12/2010 
C        7/15/2010
</code></pre>

<p>I'm using <code>aggregate(time ~ user, times, function(x) sort(as.vector(x)))</code> to get this:</p>

<pre><code>user     time
A        c(7/7/2010, 7/12/2010)
B        c(7/12/2010)
C        c(7/12/2010, 7/15/2010)
</code></pre>

<p>The problem is that I have over 20 million entries in <code>times</code> so <code>aggregate</code> is taking a over 4 hours. Is there any alternative using <code>dplyr</code> that will get me the sorted vector of dates?</p>