How to use a variable within the context.expand string (groovy)

I'd like to pass in a variable to the following command within a groovy script, in order to take advantage of the 1.7 functionality with respect to iterations on data sources:

context.expand('${Roster#startDate::0}')

where "0" would be replaced by an integer value (String or int, doesn't matter as long as it works). 

This is probably a basic groovy question but I can't seem the get the right combination.

Thanks!