Input object–must be two-dimensional (e.g., matrix, data.frame)
–over which the second dimension (columns) will be iterated</p></td>
    </tr>
    <tr>
      <th>.f</th>
      <td><p>Action to apply to each element of <code>.data</code>. The action can be
  articulated in one of the four following ways:</p><ol>
<li><p>supplying a function object (e.g., <code>mean</code>)</p></li>
<li><p>defining a function (in-line; e.g., <code>function(x) mean(x)</code>)</p></li>
<li><p>specifying a formula-like call w