How To Form A Method To Change HTML TD Attributes When Passed Element ID?

I have a ton of TD attributes that have to be changed based on the
data in the DB. All TD elements are ID and runat=server.

Changing the attributes of a SINGLE TD is simple:

<td id=thisTD runat=server>

thisTD.Attributes.Clear();
thisTD.Attributes.Add("Class",reader["thisTD_Class"].toString());

Now, say we have 100 TD elements with IDs TD_1 to TD_100

I have not been able to find a way to (essentially) say:

for(int i=0;i<100;i++){
"TD_"+i.Attributes.Clear();
"TD_"+i.Attributes.Add("Class",reader["TDClass"+i].toString());
}

in J-Script, I could do it by Eval() or about eight ways through the
DOM. It seems crazy to me that if I can access the Attributes of a
specific element directly by ID (thisTD.Attributes), I couldn't also
access it by reference.

But I have been able to find NOTHING to indicate how this may be done.
Obviously, my problem is not limited to a single attribute, or else I
would have just typed them all out by now. Trouble is, there is a lot
of logic that must be applied to the data before setting the
attribute. Without being able to iterate or loop by reference, I will
be typing for ages just to change some freaking classes.