empty selection sets

[symysgames]

I often use selection sets in my programming but always encounter difficulties when they are empty. why is it so problematic? is it because of the nature of nil who is a atom and a list at the same time. Most of the time I make sure my selection set is never empty, I insert a dummy object and then remove once I don't need it, and I found it works ok . There are filters but I don't see how they can be of any use. For example if I have a selction set ss1 and then do (setq lg (sslength ss1)) if my selection set is empty it will not set lg to 0 but to nil. how can I overcome this without having to insert a dummy object.

[jammie]

Hi,

 

I'd sometimes create an empty selection set at the beginning of the code using SSADD with no other inputs

 

(setq ss1 (ssadd))

 

This produces a selection set of length 0.

 

Possibly, in the code you are working with, you may receive an error message similar to this

 

; error: bad argument type: lselsetp nil

 

If the selection set ss1 is not defined or nil, calling

 

(setq lg (sslength ss1)) 

 

Will cause an error in the code.

 

Regards

 

Jammie