Trying to create a copy of a list. I have used copy-list but this modifies the original list therefore I cannot use copy-list and copy-tree isn't working. Any suggestions would be appreciated

(defun switch-var (var list_a)
 (let ((temp (copy-list list_a)))
 (setf (cdr (assoc var temp)) (not (cdr (assoc var temp))))temp))

For instance in lisp I will create a list then call switch-var

(setf *list_a* '((A NIL) (B T) (C T) (D NIL)))

* (switch-var ’b *list_a*)

;I will get this which is ok
((A NIL) (B NIL) (C T) (D NIL))

;but if i call it again

* (switch-var ’b *list_a*)

;I will get this which is not ok
((A NIL) (B T) (C T) (D NIL))

;so techincally I do not want to modify the original list_a
;in the function I just want to modify the temp

It would be better to use a loop to copy and modify the list in a single pass. Or map, but I prefer loop.

(defparameter *foo* '((a . nil)
                      (b . t)
                      (c . t)
                      (d . nil)))

(defun switch-variable (var list)
  (loop
     for (name . val) in list
     collecting (cons name
                      (if (eql name var)
                          (not val)
                          val))))

(switch-variable 'b *foo*)
;=> ((A) (B) (C . T) (D))
(switch-variable 'b *foo*)
;=> ((A) (B) (C . T) (D))

Using cons cells instead of lists for the variables is more efficient. It won't show the NIL when printing, but that's just visual (the value is still NIL).