how to create an auto increment field

[shifting_18]

HI! I was about to create an auto increment field for the message control Id. I should use a channel map here right? I can't make it work because ++ operator applies to integer only. I use parseInt like parseInt("controlid") with "controlid" as variable name.
BUt it keeps on saying controlid is not defined.
What is the right way to do this?
i need to work this out.. please,, thank you!!!!