title: How to merge two data frames on common columns in R with sum of others?
body: <p>R Version 2.11.1 32-bit on Windows 7</p>
  
  <p>I got two data sets: data_A and data_B:</p>
  
  <h3>data_A</h3>
  
  <pre><code>USER_A USER_B ACTION
  1      11     0.3
  1      13     0.25
  1      16     0.63
  1      17     0.26
  2      11     0.14
  2      14     0.28
  </code></pre>
  
  <h3>data_B</h3>
  
  <pre><code>USER_A USER_B ACTION
  1      13     0.17
  1      14     0.27
  2      11     0.25
  </code></pre>
  
  <p>Now I want to add the ACTION of data_B to the data_A if their USER_A and USER_B are equal. As the example above, the result would be:</p>
  
  <h3>data_A</h3>
  
  <pre><code>USER_A USER_B ACTION
  1      11     0.3
  1      13     0.25+0.17
  1      16     0.63
  1      17     0.26
  2      11     0.14+0.25
  2      14     0.28
  </code></pre>
  
  <p>So how could I achieve it?</p>