what's the typical way to add an item to the end of the list?
I have a list (1 2 3) and want to add 4 to it (where 4 is the result of an evaluation (+ 2 2))
(setf nlist '(1 2 3))
(append nlist (+ 2 2))
This says that append expects a list, not a number. How would I accomplish this?
This function might be useful in some situations, it transparently appends a single element to a list, i.e. it modifies the list but returns the appended element (enclosed in a list):
(defun attach1 (lst x)
(setf (cdr (last lst)) (cons x nil)))
;; (attach1 nlist (+ 2 2)) ; append without wrapping element to be added in a list
Cons-ing at the end of a list can be achieved with this function:
(defun cons-last (lst x)
(let ((y (copy-list lst))) (setf (cdr (last y)) (cons x nil)) y))
;; (cons-last nlist (+ 2 2))
If you want to add an item onto the end of a given list without changing that list, then as previously suggested you can use a function like
(defun annex (lst item)
"Returns a new list with item added onto the end of the given list."
(nconc (copy-list lst) (list item)))
This returns a new extended list, while preserving the input list. However, if you want to modify the input list to include the added item, then you can use a macro like
(define-modify-macro pushend (item)
(lambda (place item)
(nconc place (list item)))
"Push item onto end of a list: (pushend place item).")
Pushend operates like push, but "pushes" the item onto the end of the given list. Also note the argument order is the reverse of push.