Selection screen - i want to block the input in range-high but i get '0'

Question

i have selection screen :
SL_MONAT   FOR BKPF-MONAT NO-EXTENSION ,
i want to block the high value input  , so i use  :
  LOOP AT SCREEN.
  IF SCREEN-NAME = 'SL_MONAT-HIGH'.
  screen-input = '0'.
endif.
  endloop .
  When the screen display, the field is block but there is value '0' (zero) in the field.
why?
  i just want to block the high option (by condition)  , with out any value inside  .
  when I delete the u201C loop at screenu201D  , no value display inside  .

Answer 1

REPORT  ZSRK_073                                .
TABLES : BKPF.
SELECT-OPTIONS : SL_MONAT FOR BKPF-MONAT NO-EXTENSION .
AT SELECTION-SCREEN OUTPUT.
  LOOP AT SCREEN.
    IF SCREEN-NAME CS 'SL_MONAT-HIGH'.
      SCREEN-INPUT = 0.
      SCREEN-OUTPUT = 0.
      MODIFY SCREEN.
    ENDIF.
  ENDLOOP.