i want to increment folder value by some LUA script
 
newvalue = incrementation + get_folder_value("folder",x[i])
set_folder_value("folder",x[i], newvalue)
 
It works only when get_folder_value("folder",x[i]) is defined. It does not work 1st time, when the old value = null, and the procedure just stops.
 
For a while i used workaround by auto definition folder:=0 for all new items.  
But this way seems not intelligent - i feel there should be an easy way to say something like IF NULL THEN 0 ELSE increment...

elegant and simple hack! thank you!
 
while you are online, i'd like to ask a bit another question, where i still want to know a type of value.
 
This is what i want:
   I'd like to set an itemIDkey.
   I select an item, launch LUA,
   if it is a new "clear" item, then set itemIDkey = (max( of all) + 1) ,
   if itemIDkey is already defined, then leave it as is.
 
This is what i have so far:
Code:

x = get_folder_items("itemIDkey") 
max=table.maxn(x) 
m=0
	for i=1,max do 
	m = math.max(m, get_folder_value("itemIDkey",x[i]) )
	end
set_folder_value("itemIDkey",get_select_items()[1],m+1) 
 

 
How can i know if the value is defined without the risk to be thrown out by get_folder_value attempt for undefined value?

i'm sorry, but the desired string does not work
 
Code:

msgbox("before")
avalue = 0 +  ("0" .. get_folder_value("folder",x[i]))
msgbox("after") 

 
the message "before" appears fine, the message "after" - does not.

got it. The doubledot ("..") means text concatenation, and i tried to understand it like logical function.
Thanks!