title: R: how to sum columns grouped by a factor?
body: <p>If I have a table like this:</p>
  
  <pre><code>user,v1,v2,v3
  a,1,0,0
  a,1,0,1
  b,1,0,0
  b,2,0,3
  c,1,1,1
  </code></pre>
  
  <p>How to I turn it into this?</p>
  
  <pre><code>user,v1,v2,v3
  a,2,0,1
  b,3,0,3
  c,1,1,1
  </code></pre>