How about
$([your selector]).attr('colspan',3);
I would imagine that to work but have no way to test at the moment. Using .attr() would be the usual jQuery way of setting attributes of elements in the wrapped set.
As has been mentioned in another answer, in order to get this to work would require removing the td elements that have no text in them from the DOM. It may be easier to do this all server side
EDIT:
As was mentioned in the comments, there is a bug in attempting to set colspan using attr() in IE, but the following works in IE6 and FireFox 3.0.13.
Working Demo
notice the use of the attribute colSpan and not colspan - the former works in both IE and Firefox, but the latter does not work in IE. Looking at jQuery 1.3.2 source, it would appear that attr() attempts to set the attribute as a property of the element if
- it exists as a property on the element (
colSpan exists as a property and defaults to 1 on <td> HTMLElements in IE and FireFox)
- the document is not xml and
- the attribute is none of href, src or style
using colSpan as opposed to colspan works with attr() because the former is a property defined on the element whereas the latter is not.
the fall-through for attr() is to attempt to use setAttribute() on the element in question, setting the value to a string, but this causes problems in IE (bug #1070 in jQuery)
// convert the value to a string (all browsers do this but IE) see #1070
elem.setAttribute( name, "" + value );
In the demo, for each row, the text in each cell is evaluated. If the text is a blank string, then the cell is removed and a counter incremented. The first cell in the row that does not have class="colTime" has a colspan attribute set to the value of the counter + 1 (for the span it occupies itself).
After this, for each row, the text in the cell with class="colspans" is set to the colspan attribute values of each cell in the row from left to right.
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<title>Sandbox</title>
<meta http-equiv="Content-type" content="text/html; charset=utf-8" />
<style type="text/css" media="screen">
body { background-color: #000; font: 16px Helvetica, Arial; color: #fff; }
td { text-align: center; }
</style>
</head>
<body>
<table class="tblSimpleAgenda" cellpadding="5" cellspacing="0">
<tbody>
<tr>
<th align="left">Time</th>
<th align="left">Room 1</th>
<th align="left">Room 2</th>
<th align="left">Room 3</th>
<th align="left">Colspans (L -> R)</th>
</tr>
<tr valign="top">
<td class="colTime">09:00 – 10:00</td>
<td class="col1"></td>
<td class="col2">Meeting 2</td>
<td class="col3"></td>
<td class="colspans">holder</td>
</tr>
<tr valign="top">
<td class="colTime">10:00 – 10:45</td>
<td class="col1">Meeting 1</td>
<td class="col2">Meeting 2</td>
<td class="col3">Meeting 3</td>
<td class="colspans">holder</td>
</tr>
<tr valign="top">
<td class="colTime">11:00 – 11:45</td>
<td class="col1">Meeting 1</td>
<td class="col2">Meeting 2</td>
<td class="col3">Meeting 3</td>
<td class="colspans">holder</td>
</tr>
<tr valign="top">
<td class="colTime">11:00 – 11:45</td>
<td class="col1">Meeting 1</td>
<td class="col2">Meeting 2</td>
<td class="col3"></td>
<td class="colspans">holder</td>
</tr>
</tbody>
</table>
</body>
</html>
jQuery code
$(function() {
$('table.tblSimpleAgenda tr').each(function() {
var tr = this;
var counter = 0;
$('td', tr).each(function(index, value) {
var td = $(this);
if (td.text() == "") {
counter++;
td.remove();
}
});
if (counter !== 0) {
$('td:not(.colTime):first', tr)
.attr('colSpan', '' + parseInt(counter + 1,10) + '');
}
});
$('td.colspans').each(function(){
var td = $(this);
var colspans = [];
td.siblings().each(function() {
colspans.push(($(this).attr('colSpan')) == null ? 1 : $(this).attr('colSpan'));
});
td.text(colspans.join(','));
});
});
This is just a demonstration to show that attr() can be used, but to be aware of it's implementation and the cross-browser quirks that come with it. I've also made some assumptions about your table layout in the demo (i.e. apply the colspan to the first "non-Time" cell in each row), but hopefully you get the idea.