Re: [External] : Re: Easy to add with push but not to the end of a list

[Emanuel Berg]

Drew Adams wrote:

>> Why is the below O(N), because of `last'?
>> 
>> (defun push-last (elem lst)
>>   (let ((elem-lst (list elem)))
>>     (if lst
>>         (setcdr (last lst) elem-lst)
>>       (setq lst elem-lst) )
>>     lst) )
>
> Yes.
>
> (append lst (list elem)) also traverses the list
> usually.

Now it works at least, O(n) or not ...

(defmacro push-last (elem lst)
  (if (and (symbolp lst)
           (not (symbol-value lst)) )
      (list 'setq 'lst `(list ,elem))
    (list 'nconc lst `(list ,elem)) ))

;; (setq lst ())         ; ()
;; (push-last 1 lst)     ; (1)
;; (push-last 1 lst)     ; (1 1)
;; lst                   ; (1 1)
;;
;; (setq lst '(1 2 3 4)) ; (1 2 3 4)
;; (push-last 5 lst)     ; (1 2 3 4 5)
;; (push-last 5 lst)     ; (1 2 3 4 5)
;; lst                   ; (1 2 3 4 5 5)

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