event INITIALIZATION in sap abap reports

We have already seen how to fill default values for the selection criteria. But in many cases you need to calculate the value and then put it in selection criteria. For example, say, you are accepting date from user and you need to fill in the default value for lower range as sy-datum – 30 days and sy-datum for higher range. In this case you are calculating lower range and then filling the criteria. This can be done in INITIALIZATION event. Piece of code to do the above task would look like the following:
Tables: Sflight.
Select-options: fldate1 for sflight-fldate.
INITIALIZATION.
Data: date1 like SY-DATUM.
Date1 = sy-datum – 30.
Fldate1-low = date1.
Fldate1-high = sy-datum.
Append fldate1.
* Here appending is required because fldate1 is int’ table
This event is triggered when you execute your program for the first time i.e., before selection screen is displayed.