title: assigning unique value to new column based on multiple columns/rows
body: <p>I have a large dataset with unique values for individuals that have some repeats.  Each of these repeated rows are important data that I need to keep.  However, I want to assign a unique ID to each individual.</p>
  <p>Below is example raw data:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>pit</th>
  <th>let</th>
  <th>ret</th>
  <th>otherdata</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>19000</td>
  <td>1</td>
  <td>2</td>
  <td>xx</td>
  </tr>
  <tr>
  <td>19030</td>
  <td>3</td>
  <td>4</td>
  <td>sy</td>
  </tr>
  <tr>
  <td>19530</td>
  <td>5</td>
  <td>6</td>
  <td>gh</td>
  </tr>
  <tr>
  <td>19030</td>
  <td>3</td>
  <td>4</td>
  <td>sh</td>
  </tr>
  <tr>
  <td>19530</td>
  <td>5</td>
  <td>6</td>
  <td>lh</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>What I want is:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>pit</th>
  <th>let</th>
  <th>ret</th>
  <th>ID</th>
  <th>otherdata</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>19000</td>
  <td>1</td>
  <td>2</td>
  <td>1</td>
  <td>xx</td>
  </tr>
  <tr>
  <td>19030</td>
  <td>3</td>
  <td>4</td>
  <td>2</td>
  <td>sy</td>
  </tr>
  <tr>
  <td>19530</td>
  <td>5</td>
  <td>6</td>
  <td>3</td>
  <td>gh</td>
  </tr>
  <tr>
  <td>19030</td>
  <td>3</td>
  <td>4</td>
  <td>2</td>
  <td>sh</td>
  </tr>
  <tr>
  <td>19530</td>
  <td>5</td>
  <td>6</td>
  <td>3</td>
  <td>lh</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>I'm not sure how to get the unique number to be used for each set of the three columns and retain all the rows.</p>
  <p>Thanks</p>