title: Read null values from file
body: <p>I need to read a data frame from a file containing NULL values. Here's an example file:</p>
  
  <pre>
  charCol floatCol intCol
  a       1.5      10
  b       NULL     3
  c       3.9      NULL
  d       -3.4     4
  </pre>
  
  <p>I read this file into a data frame:</p>
  
  <pre><code>&gt; df &lt;- read.table('example.dat', header=TRUE)
  </code></pre>
  
  <p>But "NULL" entries are not interpreted by R as NULL:</p>
  
  <pre><code>&gt; is.null(df$floatCol[2])
  [1] FALSE
  </code></pre>
  
  <p>How should I format my input file so that R properly treats such entries as NULL?</p>