what is the 'cons' to add an item to the e

Question

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?

Answer 1

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))

Answer 2

You could use append, but beware that it can lead to bad performance if used in a loop or on very long lists.

(append '(1 2 3) (list (+ 2 2)))

If performance is important, the usual idiom is building lists by prepending (using cons), then reverse (or nreverse).

Answer 3

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.

Answer 4

If you are trying to add two lists for example (1 2 3) + (1 2 3) here is the code (recursive)

(defun add-to-all (x y)
    (T (appendl (+ (first x) (first y)) (add-to-all (tail x) (tail y)) ))
)

If you are trying to add an item to the end of the second list, for example 3 + (1 2 3)

(defun add-to-all (x y)
  (cond ((null? y) nil)
    (T (appendl (+ (first x) (first y)) (add-to-all (tail x) (tail y)) ))
  )
)

Answer 5

(append l (list e)) ; e is the element that you want to add at the tail of a list

Answer 6

You haven't specified the kind of Lisp, so if you use Emacs Lisp and dash list manipulation library, it has a function -snoc that returns a new list with the element added to the end. The name is reversed "cons".

(-snoc '(1 2) 3) ; (1 2 3)