There are a few ways to do it, each with a different level of impact to your code. The way that requires no changes except a new line of code to define buttonnumber is:
buttonnumber = ismember(app.grpRadioButtons.Buttons,app.grpRadioButtons.SelectedObject);
Obviously, you'll have to put in your own appropriate tag for the radio button group. Also, I personally wouldn't use the variable name buttonnumber since this is strictly a logical output (which works exactly as-is to index into data). If you truly (superfluously) want the numeric value, throw a find on it.
