title: Sum by Key Value in r
body: <p>I have a data set that looks like this. You can see that column X2 has key values. I would like to sum the pair values from column X1 by the key values in column X2. I have been trying to use ddply or aggregate but I am running into problems. Does anyone have a simple way to do this? </p>
  
  <pre><code>&gt; X1=rnorm(30,mean=20, sd=10)
  &gt; X2=rep(1:15,2)
  &gt;df= data.frame(X1,X2)
            X1 X2
  1  10.065545  1
  2   6.938315  2
  3   5.657782  3
  4  11.371521  4
  5  37.037992  5
  6  13.443674  6
  7   8.868314  7
  8   5.944946  8
  9  18.493563  9
  10 16.059931 10
  11 22.520693 11
  12 17.172936 12
  13 28.676676 13
  14 27.200911 14
  15 30.560696 15
  16 22.435021  1
  17 31.143627  2
  18 19.234473  3
  19 29.329251  4
  20 18.420183  5
  21 13.184905  6
  22  7.798989  7
  23 12.910870  8
  24 12.513204  9
  25 25.760775 10
  26 19.479005 11
  27 13.696628 12
  28 11.016639 13
  29 35.126859 14
  30 17.632507 15
  </code></pre>