title: Replacing values in a data frame with NULL
body: <p>Can you replace using NULL?</p>
  
  <pre><code>#Data Frame
  a = c("tom", "alex", "chris", "jen", "mary") 
  b = c(NA) 
  df = data.frame(a,b) 
  
  #Replace Tom with NULL
  df$c &lt;- gsub('tom', NULL, df$a)
  </code></pre>
  
  <p>The last line of code gives me the following error: <code>invalid 'replacement' argument</code>.  Is replacement with NULL possible?  Reason I ask, is cause I'm trying to upload a large dataset into postgres and some integer variables are being rejected due to having blanks and NA values.  Thanks.  </p>