how to click an input where a td is empty? - xpath

Question

I have this table like this
var tdCnt=0;
$('table tr').each(function(){
$('<input />', {
type : 'checkbox',
id : 'td' + tdCnt,
class : 'dt-checkboxes',
value : name
}).appendTo($(this).find("td").eq(0));
tdCnt++;
});
.dt-checkboxes {
float :left;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<table>
<tr>
<td>1</td>
<td>2</td>
<td>3</td>
<td></td>
<td>5</td>
<td>6</td>
</tr>
<tr>
<td>7</td>
<td>8</td>
<td>9</td>
<td></td>
<td>11</td>
<td>12</td>
</tr>
<tr>
<td>13</td>
<td>14</td>
<td>15</td>
<td>16</td>
<td>17</td>
<td>18</td>
</tr>
<tr>
<td>19</td>
<td>20</td>
<td>21</td>
<td>22</td>
<td>23</td>
<td>24</td>
</tr>
<tr>
<td>25</td>
<td>26</td>
<td>27</td>
<td>28</td>
<td>29</td>
<td>30</td>
</tr>
</table>
I just want to click the inputs where the 4th td is empty how can I achieve that?
this is the code im using, but it keeps clicking the ones that are not empty
(//input[#type='checkbox' and //td[4]/descendant::div[string-length()=0]])[$counter]
or this one
(//input[#type='checkbox' and //td[4][not(text())]])[2]
and neither works, it keeps clicking the ones that are not empty how can I achieve this?
Thank you for all your help

Answer 1

To complete, 3 ways to select an empty td element (in your case : the fourth) :
//tr/td[4][.=""]
//tr/td[4][string-length()=0]
//tr/td[4][not(text())]
Regarding your sample data, you have one checkbox per tr element. To select 'inputs where the 4th td is empty', you can use :
//tr/td[4][.=""]/preceding::input[1][#type='checkbox']
//tr/td[4][.=""]/../td/input[#type='checkbox']
The first will look for the empty td element, then its first preceding input element containing a specific attribute.
The second will look for the empty td element, then the child of its parent (ie : the input element).
or if I fix your second try :
//input[#type='checkbox'][ancestor::tr[1]/td[4][.=""]]
Look anywhere for an input element whith a specific attribute. And where the fourth td element, child of its first tr ancestor, is empty.
Output : 2 inputs nodes