title: How to combine 2 variables and ignore NAs
body: <p>I have some data like these</p>
  
  <pre><code>var1   var2
  10     NA
  101    NA
  NA     86
  11     NA
  NA     11
  NA     61
  </code></pre>
  
  <p>If one variable is NA then the other one is not, and vice-versa.</p>
  
  <p>How can I combine them into a single variable:</p>
  
  <pre><code>var3
  10
  101
  86
  11
  11
  61
  </code></pre>
  
  <p>I can do it easily with a loop, but it is quite slow, so I would like to find an easier way. I thought about assigning 0 to the values that are NA and then just adding the variables together...is there a better way ?</p>