title: Subset of rows containing NA (missing) values in a chosen column of a data frame
body: <p>We have a data frame from a CSV file. The data frame <code>DF</code> has columns that contain observed values and a column (<code>VaR2</code>) that contains the date at which a measurement has been taken. If the date was not recorded, the CSV file contains the value <code>NA</code>, for missing data.</p>
  <pre><code>Var1  Var2 
  10    2010/01/01
  20    NA
  30    2010/03/01
  </code></pre>
  <p>We would like to use the subset command to define a new data frame <code>new_DF</code> such that it only contains rows that have an <code>NA'</code> value from the column (<code>VaR2</code>). In the example given, only Row 2 will be contained in the new <code>DF</code>.</p>
  <p>The command</p>
  <pre class="lang-r prettyprint-override"><code>new_DF&lt;-subset(DF, DF$Var2==&quot;NA&quot;) 
  </code></pre>
  <p>does not work, the resulting data frame has no row entries.</p>
  <p>If in the original CSV file the Value <code>NA</code> are exchanged with  <code>NULL</code>, the same command produces the desired result:</p>
  <pre class="lang-r prettyprint-override"><code>new_DF &lt;- subset(DF, DF$Var2==&quot;NULL&quot;)
  </code></pre>
  <p>How can I get this method working, if for the character string the value <code>NA</code> is provided in the original CSV file?</p>