Question

I’ve an HTML page with a number of tables, one for each user in the database.
The first four columns of each table are radio buttons, with the values: G, A, X and blank respectively.

What I want to do is set the radio button to e.g. ‘A’ for all rows that are currently blank.

Here was where I started:

var st='A';
$('#user123 input').val([st]);

This works, but it also sets to A the rows that were already ‘G’ or ‘X’.

Here is what I’ve tried that didn’t work:

$('#user123 input[value=]').siblings().val([st]);

(nothing gets set)

$('#user123 input[value=]').parent().children().val([st]);

(the blank radio button gets cleared, none get set)

$('#user123 input[value=]').val([st]);

(the blank radio button gets cleared, none get set)

$('#user123 input').not('[value=G]').val([st]);

(planning to add exceptions for A and X later, but it already doesn’t work – all radio buttons get set to ‘A’, not just the blank ones)

I’m really close aren’t I? But I just cannot find the magic incantation I need!

Here is a cut-down version of the table HTML:

<table id="user123">
<tr>
<td><input name="r520" value="G" type="radio"></td>
<td><input name="r520" value="A" type="radio"></td>
<td><input name="r520" value="X" type="radio"></td>
<td><input name="r520" value="" type="radio" checked></td>
<td>2011-06-01</td>
</tr>
<tr>
<td><input name="r767" value="G" type="radio"></td>
<td><input name="r767" value="A" type="radio"></td>
<td><input name="r767" value="X" type="radio"></td>
<td><input name="r767" value="" type="radio" checked></td>
<td>2011-06-03</td>
</tr>
</table>

UPDATE
Thanks for replies; here is the solution I went with (mainly inspired by michelpm’s):

var st='A';
$('#user123 tr').each(function() {
    if($("input:radio[value=]:checked",this).length){
        $("input:radio[value="+st+"]",this).attr("checked",true);  
        }
    });

Answer 1

If by blank you meant no radio is checked in that row:

http://jsfiddle.net/michelpm/GzPrd/

$("#user123 tr").each(function() {
    if($(":radio:checked", this).length == 0) {
        $(":radio[value=A]", this).attr("checked", true);        
    }
});

EDIT:

If you want so bad to do that in one line this one may do the trick…

http://jsfiddle.net/michelpm/GzPrd/2/

function select(value) {
     return function() {
        $(":radio[name=" + this.name + "][value=" + value + "]")
            .attr("checked", true);
    }; 
}

$("#user123 :radio:not([value]):checked").each(select('A'));