title: left_join two data frames and overwrite
body: <p>I'd like to merge two data frames where <code>df2</code> overwrites any values that are <code>NA</code> or present in <code>df1</code>.  <a href="https://stackoverflow.com/questions/16042380/merge-data-frames-and-overwrite-values">Merge data frames and overwrite values</a> provides a <code>data.table</code> option, but I'd like to know if there is a way to do this with <code>dplyr</code>.  I've tried all of the <code>_join</code> options but none seem to do this.  Is there a way to do this with <code>dplyr</code>?</p>
  
  <p>Here is an example:</p>
  
  <pre><code>df1 &lt;- data.frame(y = c("A", "B", "C", "D"), x1 = c(1,2,NA, 4)) 
  df2 &lt;- data.frame(y = c("A", "B", "C"), x1 = c(5, 6, 7))
  </code></pre>
  
  <p>Desired output:</p>
  
  <pre><code>  y x1
  1 A  5
  2 B  6
  3 C  7
  4 D  4
  </code></pre>