Topic: What should all lisp newbies know?

[Matt Stachoni]

Mine would be: Make sure your code takes into account what users may do when they use your routine.

As an example, take this into account:

Code: [Select]

(setq thing
  (entget
    (car
      (entsel "\nSelect item you want to delete: ")
    )
  )
)This will completely bomb out when the user doesn't select something - which will happen in the course of normal events.
It's better to wrap it into an (if..then..) expression:

Code: [Select]

(if (setq e (car (entsel "\nSelect the item you wish to delete:")))
  (setq e (entget e))
)
If the user doesn't select anything, (car nil) still returns nil, because nil is actually a list data type.