title: Replace all string instances of "NULL" with actual NULL or NA in a data frame
body: <p>I wondered if there was a one liner where I could replace all instances of string &quot;NULL&quot; in a DF across multiple fields with just NULL or NA?</p>
  <pre><code>exampledf &lt;- data.frame(
    a = c(&quot;NULL&quot;, &quot;1/1/20&quot;, &quot;2/28/20&quot;),
    b = c(&quot;NULL&quot;, &quot;blah&quot;, &quot;ha&quot;),
    c = c(&quot;NULL&quot;, &quot;NULL&quot;, &quot;cat&quot;)
  )
  </code></pre>
  <p>Is there a one liner that would replace &quot;NULL&quot; across the entire df with NULL or NA?</p>