title: How do I get R to recognise that there is a missing value in my dataframe, by inserting a "NA" into the empty cell?
body: <p>I've created the following data frame, DF, in R.</p>
  
  <pre><code>Data_point &lt;- c('a','a','b','b','','c')
  Measurement &lt;- c(2,4,6,8,4,-10.4)
  
  DF &lt;- data.frame(Data_point, Measurement)
  </code></pre>
  
  <p>However DF is not showing any NAs in R, even though there is a missing value in the "Data_point" column. How can I get R to identify that there is a missing value in the "Data_point" column?</p>