title: Using Dplyr fill empty cells using corresponding values in another column
body: <p>A dataframe:</p>
  
  <pre><code>a = c("a", "", "c", "", "e")
  b = c(1, 2, 3, 4, 5)
  c = c("g", "h", "i", "j", "k")
  
  df = data.frame(a,b,c)
  </code></pre>
  
  <p>How can I fill column "a" with corresponding values in column "c" when "a" is empty using dplyr. I know how to do it using data.table.</p>
  
  <p>Thank you.</p>