title: Merge three different columns into a date in R
body: <p>Right now, I am having 3 separate columns as year, month, and day in a data file in R. How do I merge these three columns into just one column and make R understand that it is date?</p>
  
  <p>Here is what it looks like right now. </p>
  
  <pre><code>year   mon  day  gnp
  
  1947    1   1   238.1   
  1947    4   1   241.5   
  1947    7   1   245.6   
  1947    10  1   255.6   
  1948    1   1   261.7   
  1948    4   1   268.7
  </code></pre>