Clearing the displayed value for a SELECT-OPTION

Question

How do I clear the displayed value of a SELECT-OPTION? 
I have 2 SELECT-OPTIONs on my screen (standard basic report program screen).  I use code like this to populate the drop-down boxes for each one. 
=====
AT SELECTION-SCREEN ON VALUE-REQUEST FOR s_prgrp-low.
  PERFORM fill_prgrp_values.
AT SELECTION-SCREEN ON VALUE-REQUEST FOR s_prctr-low.
  PERFORM fill_prctr_values.
=====
The value the user picks for the first SELECT-OPTION will affect what values I put in the drop-down list for the second SELECT-OPTION. 
If a user enters a value for the second SELECT-OPTION, and then goes back and changes the value of the first SELECT-OPTION, then I want to do two things:
1.  Create a new set of values for the drop-down list for the second SELECT-OPTION (no problem; working fine);
2.  Clear the displayed value from the second SELECT-OPTION that the user entered previously.  That value became invalid when the user picked a new value for the first SELECT-OPTION. 
How do I clear that second displayed value? 
I have tried CLEAR and REFRESH for the second variable using the formats s_prctr, s_prctr[], and s_prctr-low.  They will erase the values of the internal table or part(s) of it, but the displayed value stays on the screen. 
I need to clear out the displayed value so the user will either leave it blank or enter or select a new value. 
I am using F4IF_INT_TABLE_VALUE_REQUEST to build the drop-down lists, and it works fine, but I do not see any function module to clear the displayed value off the screen. 
Thanks for your help.

Answer 1

Sorry, but calling DYNP_VALUES_UPDATE did not work.  This is how I coded it. 
fld_reset_rcd-fieldname  = 'S_PRCTR'.
  fld_reset_rcd-stepl      = sy-stepl.
  CLEAR fld_reset_rcd-fieldvalue.      "  re-initialize s_prctr
  CLEAR fld_reset_rcd-fieldinp.        "  what goes in here?
  APPEND fld_reset_rcd TO fld_reset_tbl.
  CALL FUNCTION 'DYNP_VALUES_UPDATE'
    EXPORTING
      dyname = 'ZFI_GL_BALANCE_NGL'
      dynumb = '1000'
    TABLES
      dynpfields = fld_reset_tbl
<Added code tags>
I have discovered that CLEAR and REFRESH of s_prctr will clear it somewhat.  If I enter multiple values, ranges, etc., they will all be cleared, EXCEPT for the one single value that is displayed on the main screen.  It is not cleared and it remains if you push the button to display the pop-up to enter ranges, etc. 
To devrath.sampat  --  Thanks for your example for building the drop-down list, but that is not the problem I am having.  I am already able to build it just fine. 
To repeat my problem, if I: 
1.  first enter / select a value for the first SELECT-OPTION s_prgrp
2.  then enter / select a value for the second SELECT-OPTION s_prctr
3.  And finally go back and select a new value of the first SELECT-OPTION s_prgrp from its drop-down list,
     when I do, the program needs to clear the value displayed on the main screen for the second SELECT-OPTION s_prctr (any additional values, ranges, etc., are cleared by CLEAR and REFRESH, if I go look; but not the value shown on the main screen).
Edited by: Scott Crosby on Feb 14, 2012 4:20 PM
Edited by: Suhas Saha on Feb 15, 2012 12:03 PM