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empty selection sets


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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.

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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.





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