at selection-screen

hi, iam new to this abap.

my query is..

i have designed the selection screen with four fields all with select options.

in selection screen i hav to check the validity of the parameters and display error message if invalid input are made

and also..

and also display information message and leave processing the list if no material data could be found according to the selection criteria

these is mentioned in the functional spec

where should i write the code for the above 2 conditions

let me know as soon

thanks and regards.

kiran rathod