title: How to update R data.frame column values efficiently?
body: <p>Say I have this table A</p>
  
  <pre><code>                mpg 
  RX4            21.0  
  Wag            21.0  
  Datsun         22.8  
  Drive          21.4   
  Sportabout     18.7   
  Valiant        18.1  
  Duste          14.3   
  Merc           24.4   
  </code></pre>
  
  <p>Now I have a table B</p>
  
  <pre><code>              mpg
  RX4           60.0  
  Wag           60.0  
  Datsun        70.8  
  </code></pre>
  
  <p>What I want to do is to update the mpg value of Table A according to Table B, I can do that easily using hashmap in Java, may I know what is the efficient way of doing that in R?</p>
  
  <p>Thanks very much indeed. </p>