title: Match two data frames in R considering two variables and not change the rows not matched
body: <p>Hi everybody I have a little problem matching two data frames in R when they have two common variables to be matched. The first data frame is like this:</p>
  
  <pre><code>Class  Count  V1  V2 V3
  E       124   1   2   2
  E       123   2   0   0
  L       100   5   5   5
  L       111   1   1   1
  E       120   3   3   3
  </code></pre>
  
  <p>Second data frame has this form:</p>
  
  <pre><code>Class  Count Code
  E       124  1241
  L       111  1234 
  </code></pre>
  
  <p>I would like to have a new data frame considering <code>Class</code> and <code>Count</code> variables for the match. The resulting data frame would be like this:</p>
  
  <pre><code>    Class  Count   V1    V2 V3
      E       124   1241   2   2
      E       123   2      0   0
      L       100   5      5   5
      L       111   1234   1   1
      E       120   3      3   3
  </code></pre>
  
  <p>Where only the elements that have matched were replaced with <code>Code</code> variable in <code>V1</code> variable. The rest of elements are the same and I don't have <code>NA</code> and other changes in my first data frame. I wait it is possible to make in R. Thanks in advance.</p>