title: converting columns to factor over list of dataframes
body: <p>I'm trying to convert several columns in a list of dataframes into factors.
  I've tried this, but it doesn't seem to convert the columns into factors:</p>
  
  <pre><code>factor_cols_REx &lt;- c('GESLACHT','GEVKL','BEROEP')
  for (i in (1:9)) {
    dataset_RE10_2014[[i]] &lt;- lapply(dataset_RE10_2014[[i]][factor_cols_REx],factor)
    dataset_RE10_2015[[i]] &lt;- lapply(dataset_RE10_2015[[i]][factor_cols_REx],factor)
  }
  </code></pre>
  
  <p>Any ideas on how to fix this?</p>