title: Subset a dataframe using a string of column names
body: <p>I need to subset a dataframe (df) by a string of columns names that I have created - not sure how to inject this into a subet..?</p>
  
  <p>for example </p>
  
  <pre><code>colstoKeep is a character string:
  "col1", "col2", "col3", "col4"
  </code></pre>
  
  <p>how do I push this into a subset function</p>
  
  <pre><code>df&lt;- df[colstoKeep]
  </code></pre>
  
  <p>I'm sure this is easy.? because the above doesn't work.</p>