Lisp If Not Nil
I Am So Sorry for Having to Ask This Question but I'm Trying to Do a Simple If Statement That Checks If a Variable Is Nil or Not. (Defun Test (Input) (Let...
I am so sorry for having to ask this question but I'm trying to do a simple if statement that checks if a variable is nil or not.
(defun test (input)
(let ((testvar (first input)))
(if (not nil testvar)
(do this)
(do that))))
Could anyone explain the proper syntax to me?
3 Answers
Since nil is equivalent to the boolean value false, there is no need to compare to it explicitly.
A simple
(if testvar
(...)
(...))
will do the job. You only need not if you want to check for the opposite, e.g. if you want to check that a variable is not nil:
(if (not testvar)
(...)
(...))
Apart from that, there is also a predicate function called null that you might use. It is basically meant for checking whether a given list is empty, but since the empty list is equivalent to nil, it will work (as the examples on the linked page point out):
(null '()) => T
(null nil) => T
(null t) => NIL
(null 1) => NIL
Anyway, this basically only moves the problem one layer up ;-)
Do you want to check if the variable is nil or if it is not nil?
For not nil: (if v ... ...)
For nil: (if (not v) ... ...)
There are (in CL) many variations which are all logically the same but may indicate intent better: (if (null v) ... ...) if the same as the second case above but might indicate to the reader that you are looking for `()‘ instead of logical falsity (ie an empty list). And there are plenty of other variations.
jkiiski was right:
Just (if (not testvar) ...). Or put the true branch first and do (if testvar ...)
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