  <td style="text-align:left;"> exercise </td>
   <td style="text-align:left;"> read_csv(
  simple_csv, 
  col_types = list(x = col_double())
) </td>
  </tr>
  <tr>
   <td style="text-align:left;"> controlling-column-types-5 </td>
   <td style="text-align:left;"> exercise </td>
   <td style="text-align:left;"> df &lt;- read_csv(
  simple_csv, 
  col_types = list(x = col_double())
)

problems(df) </td>
  </tr>
  <tr>
   <td style="text-align:left;"> controlling-column-types-6 </td>
   <td s