Defining dynamic selector with variable

Defining dynamic selector with variable

How can I dynamically specify a class value to use in a selector?  For instance, I have a number, and I need to identify an element that ends with that number.  In this case, there are four divs that have class values of

my_id_0
my_id_1
my_id_2
my_id_3

I have a 0 (obtained previously in the code) stored in the value tabId, so I want to get the values of the class attribute for <div id="my_id_0">.  How do I create my selector?  I tried

$("div# parent_element_id #my_id_" + tabId).attr("class");

but it comes back as undefined.  Do I need to define it as a variable and put it in the selector that way?  Or is there another way?

Thanks.