Topic: Replace item in list with new item

[JohnK]

well, this has been a fun challenge. I'll have to check these things out later.

heh, not really useful though!

[VovKa]

the best performance is achieved using the loops. of course it's fun to play with mapcar, but should someone ever risk using positional-list-pop on huge lists?

[daron]

I think they're useful.

[CAB]

My version:

Code: [Select]

  ;;  CAB 11.16.07
  ;;  Remove based on pointer list
  (defun RemoveNlst (nlst lst)
    (setq i -1)
    (vl-remove-if '(lambda (x) (not (null (vl-position (setq i (1+ i)) nlst))))
      lst
    )
  ) 
 

Code: [Select]

  (setq result (RemoveNlst '(1 4) '(0 "A" 2 3 "B" 5 6 7 8 9)))
  ;;  removes A & B

[CAB]

One more:

Code: [Select]

;;  CAB 12/27/2006
;;  (RemoveNth 3 '(0 1 2 3 4 5))
;;  (0 1 2 4 5)
(defun removeNth (i lst)
  (setq i (1+ i))
  (vl-remove-if '(lambda(x) (zerop (setq i (1- i)))) lst)
)

[JohnK]

the best performance is achieved using the loops. of course it's fun to play with mapcar, but should someone ever risk using positional-list-pop on huge lists?

My thoughts exactly!

[JohnK]

A `pop' procedure only removes the first item in a list. It does ONLY that because its only useful for when you dont have the list in your possession (You only have the address for the variable or in some cases the variable name --since we in Autolisp cant really deal in address we have to use variable names).  Didnt you notice that you only pass the variable NAME to my procedure and that variable gets redefined...out of scope no less?

The concept is that if you need to modify that list then you would just go get that list and modify it as you need to... We are playing with variable scope not lists. (SET, SETQ, EVAL) not list modifiers (SUBST, MAPCAR, etc.)

If we want to rehash a remove nth procedure we can do that but a `pop' is NOT a remove nth procedure.

[daron]

Didn't know that. I think removeNth has been hashed out enough. I guess that's really what I've been looking for, just didn't know there was a difference.

[CAB]

 Sorry, I didn't get it.

Code: [Select]

(defun poplst (lst) (cdr lst))

[JohnK]

the variable isnt redefined. (Use SET, EVAL)

Command: *Cancel*

Command: (setq mylist '(1 2 3 4))
(1 2 3 4)

Command: (poplst mylist)
(2 3 4)

Command: !mylist
(1 2 3 4)

[JohnK]

Code: [Select]

(defun list-push (sym lst)
  (set lst (cons sym (eval lst))) )

(defun list-pop (lst / ~tval )
  (setq ~tval (eval lst))
  (set lst (cdr ~tval))
  (car ~tval) )