Is there a way to index a variable based on another variable?

Example:

date_w ap
------------------------------
1. 2012w42 12.99
2. 2012w43 12.31
3. 2012w44 12.14
------------------------------

I would like to refer to ap=12.31 as ap[_2012w43], or something similar. datew has %tw format.
Any ideas?
Thanks in advance!