How to check if a number is undefined

I am attempting to make a physics engine in desmos (weehoo) using gravity with a list of points and I need to check for if a value in the list is undefined. How would I go about doing this?

Do you need to know which one specifically is undefined? Or do you just need to exclude it from the list? Within a graph, you can exclude undefined items in a list by defining a new list in this manner:

Thank you for your response.