title: finding the missing variables in a df and print out the name of variable
body: <p>If I want to check whether my df have all the variables I need, how can I identify the missing variables and print the warning message.</p>
  <p>Currently I have:</p>
  <pre><code>  if (!all(c(&quot;test&quot;, &quot;result&quot;,&quot;Unit&quot;) %in% colnames(data))) {
        print(&quot;'test', 'result', 'Unit' are needed in this data set. Check data again.&quot;)
        
      } else{
        print(&quot;Everything is OK&quot;)
      } 
  </code></pre>
  <p>I would like to be able  to print out the missing one instead of <code>'test', 'result', 'Unit' are needed in this data set. Check data again.</code> Any suggestion?</p>