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 ((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 ...)