title: R : Filling in missing values in a column based on other columns
body: <p>I have a large data set where each zipcodes have their corresponding latitude and longitude. In the data set some zipcodes are missing. I need to fill in the missing zipcodes on the basis of their corresponding lat long where that data is not missing.
  In this example I would want rows 4 and 5 to have a and b inserted for zipcode as they have the same lat and long:</p>
  
  <pre><code>zipcode &lt;- c("a","b","c","","")
  lat &lt;- c("1","2","3","1","2")
  lon &lt;- c("6","7","8","6","7")
  data.frame(zipcode,lat,lon)
    zipcode lat lon
  1       a   1   6
  2       b   2   7
  3       c   3   8
  4           1   6
  5           2   7
  </code></pre>
  
  <p>I'd prefer to not install another package unless really necessary.</p>
  
  <p>Thank you</p>