how can i get index of element a

Question

<table id="table_vendor">
<thead>
  <tr>  
    <th>vendor id</th>
  </tr>
</thead> 
<tbody>
<tr>
    <td class="vendor_id"><a href="#" name="adwa" class="adwa">0001</a></td>
</tr>
<tr>
    <td class="vendor_id"><a href="#" name="adwa" class="adwa">0002</a></td>
</tr>
<tr>
    <td class="vendor_id"><a href="#" name="adwa" class="adwa">0003</a></td>
</tr>
<tr>
    <td class="vendor_id"><a href="#" name="adwa" class="adwa">0004</a></td>
</tr>
</tbody>
</table>

and jquery

$("tr td a").click(function(){

                var index = $(this).index();
        alert(index);
});

i want index of element a but when i click it show 0

I'm sorry for my English.

https://jsfiddle.net/lorizzimp/ktg22seu/