title: Fill the null values in R dataframe with default but distinct values
body: <p>I have a R dataframe where one of the column has no values. I want to fill this values with distinct value. </p>
  
  <pre><code>id  var1   var2   var3
  A   25      26     abc
  B   28      35     abk
  C   55      46      
  D   64      az     lft
  J   cd      yz      
  M   dg      ck     ja
  ......................
  .........
  </code></pre>
  
  <p>I want to fill those null values with a default but distinct values.</p>
  
  <p>Expected output:</p>
  
  <pre><code>   id  var1   var2   var3
  A   25      26     abc
  B   28      35     abk
  C   55      46     NA1 
  D   64      az     lft
  J   cd      yz     NA2
  M   dg      ck     ja
  so now the other null value will be filled with NA3
  </code></pre>
  
  <p>Solution using R and pandas both would be nice to compare.</p>