title: Merge data frames based on rownames in R
body: <p>How can I merge the columns of two data frames, containing a distinct set of columns but <em>some</em> rows with the same names? The fields for rows that don't occur in both data frames should be filled with zeros:</p>
  
  <pre><code>&gt; d
      a   b   c   d   e   f   g   h   i  j
  1 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10
  2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9  1
  &gt; e
     k  l  m  n  o  p  q  r  s  t
  1 11 12 13 14 15 16 17 18 19 20
  3 21 22 23 24 25 26 27 28 29 30
  &gt; de
      a   b   c   d   e   f   g   h   i  j  k  l  m  n  o  p  q  r  s  t
  1 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10 11 12 13 14 15 16 17 18 19 20
  2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9  1  0  0  0  0  0  0  0  0  0  0
  3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0  0 21 22 23 24 25 26 27 28 29 30
  </code></pre>