er_data_changed->modify_cell

Dear Friends,

Can someone help me with populating MP_MOD_ROWS parameter of class CL_ALV_CHANGED_DATA_PROTOCOL. I want to pass values from F4 help on OO ALV grid to the program to be updated. I use method er_data_changed->modify_cell but within tis method I found out that mp_mod_rows is empty. So I receive a dump. How can I declare and fill mp_mod_rows in my subroutine before calling modify_cell?

Thx in advance,

Ali