Macro codes for a selecting range automatic selection

Eagle warrior Posts 1 Registration date Sunday September 29, 2013 Status Member Last seen September 29, 2013 - Sep 29, 2013 at 01:50 AM
Hi Experts,
i am new to excel. Need a urgent help. I am preparing excel macros, where I have sheet with Column A as key field with 275 rows to work with, it varies each time.
so I need 2 things from here:

1) if the range of A columns increases, macros should automatically count the total rows and
2) Populate value "ABCD" in Blank Field of C column

After this, once "ABCD" is populated in blank fields of C column>
1) for these rows only. A+C for these fields should be concatenated and filled in Column P and copied to Q same fields