Init & at selection screen o/p

Question

hi !
i need the difference between
initialization and at selection-screen output   events.
please explain with an example.
points will be awarded!

Answer 1

At selection screen o/pevent is executed at PBO of the selection screen every time the user presses ENTER - in contrast to INITIALIZATION . Therefore, this event is not suitable for setting selection screen default values. Also, since AT SELECTION-SCREEN OUTPUT is first executed after the variant is imported (if a variant is used) and after adopting any values specified under SUBMIT in the WITH clause, changing the report parameters or the selection options in AT SELECTION-SCREEN OUTPUT would destroy the specified values.
Here, however, you can use LOOP AT SCREEN or MODIFY SCREEN to change the input/output attributes of selection screen fields.
Example
Output all fields of the SELECT-OPTION NAME highlighted:
    SELECT-OPTIONS NAME FOR SY-REPID MODIF ID XYZ.
    AT SELECTION-SCREEN OUTPUT.
      LOOP AT SCREEN.
        CHECK SCREEN-GROUP1 = 'XYZ'.
        SCREEN-INTENSIFIED = '1'.
        MODIFY SCREEN.
      ENDLOOP.
Executed before the selection screen is displayed.
The parameters (PARAMETERS ) and selection criteria (SELECT-OPTIONS ) defined in the program already contain default values (if specified). You can assign different values here and also change the database-specific selections.
In contrast to R/2 , this event is also executed during background processing.
Example
Define the last day of the previous month as the key date:
    PARAMETERS QUAL_DAY TYPE D DEFAULT SY-DATUM.
    INITIALIZATION.
      QUAL_DAY+6(2) = '01'.
      QUAL_DAY      = QUAL_DAY - 1.
Here, the default value of QUAL_DAY is the current date, e.g. 05.04.88 ( QUAL_DAY = '19880405'). Two subseqent statements set the date first to the beginning of the month, e.g. 01.04.88 ( QUAL_DAY = '19880401') and then, by subtracting one day, to the last day of the previous month, e.g. 31.03.88 ( QUAL_DAY = '19880331').
Note
In more precise terms, INITIALIZATION is executed in the following steps:
Specify default values for the selections. Execute the event INITIALIZATION. Import variant (if used to start the report). On SUBMIT , the values specified for each WHERE clause are also transferred, if necessary. Execute the event AT SELECTION-SCREEN OUTPUT , if it occurs in the report (unlike INITIALIZATION , this event is always executed for PBO of a selection screen). Display selection screen. Transport the screen fields containing user input to the report fields. Continue with START-OF-SELECTION .