What is the best way to conditionally apply a class?

Question

Lets say you have an array that is rendered in a ul with an li for each element and a property on the controller called selectedIndex.

Answer 1

The was I recently did that was doing this:

The isShowing value is a value that is located on my controller that gets toggled with the click of a button and the parts between the single parenthesis are classes I created in my css file.

EDIT: I would also like to add that codeschool.com has a free course that is sponsored by google on AngularJS that goes over all of this stuff and then some. There is no need to pay for anything, just signup for an account and get going! Best of luck to you all!