title: Replace NAs in one variable with values from another variable
body: <p>How can I replace the "NAs" of a variable VAR1 with with the values of the second variable VAR2 to create a third variable VAR3 in R? The data looks like this:</p>
  
  <pre><code> VAR1:    VAR2:
   1        NA
   3        NA
   NA       1 
   NA       3
   2        NA 
   NA       1
  </code></pre>
  
  <p>Afterwards it should look likes this: </p>
  
  <pre><code> VAR1:    VAR2:    VAR3:
   1        NA       1   
   3        NA       3
   NA       1        1
   NA       3        3
   2        NA       2
   NA       1        1  
  </code></pre>