Re: How could I modify list element?

[Niels Giesen <niels.giesen@gmail.com>]

richardeng <richardeng@foxmail.com> writes:

> Hi all,
>     setcar/setcdr is not convenient.
>     In a long list, ex.
>     (setq a '(a b c d e f g))
>     I want to change 'e to 'E.
>     I need a function: (set-list-elt list old-elt new-elt)
>
>     How? translate list to vector, modify, then turn it back???
Numerous ways, first the destructive ones:

(setcar (member 'e a) 'E)

(setcar (nthcdr 4 a) 'E)

If (require 'cl) is no problem for you, nth is setfable

(setf (nth 4 a) 'E)

A DIY recursive function can do it too:
(defun set-nth (n list new)
  (cond 
   ((or (< n 0) (>= n (length list)))
    (error "out of bounds"))
   ((= n 0)
    (setcar list new))
   (t (cons (car list) (set-nth (1- n) (cdr list) new)))))

... Or a purely functional approach, to leave the original list intact:
(defun replace-nth (n list new)
  (cond 
   ((or (< n 0) (>= n (length list)))
    (error "out of bounds"))
   ((= n 0)
    (cons new (cdr list)))
   (t (cons (car list) (replace-nth (1- n) (cdr list) new)))))