Re: replace element in list

[Eric Abrahamsen <eric@ericabrahamsen.net>]

edgar@openmail.cc writes:

> Hello,
>
> I want to share and know if there is a better way (more efficient or
> clearer) to replace something within a list (I don't know LISP).  My
> code is like this, and it works for the given example.
>
> (defun my-list-replace (obj orig new)
>     "Replaces an element in a list with something else"
>     (let* (
>            ;; Position of the thing we need to remove
>            (pos (cl-position orig obj :test 'equal))
>            ;; If pos is nil, reset to zero
>            (pos (if pos pos 0))
>            ;; The length of the original object
>            (objlen (length obj))
>            ;; The elements before the element to remove
>            (head (butlast obj (- objlen pos)))
>            ;; The elements after the element to remove
>            (trail (nthcdr (+ 1 pos) obj)))
>       ;; Join (1) the sub-list before the element to be replaced
>       ;; with (2) the new element and (3) the rest of the list
>       (append head (append new trail))
>       ))

You'll probably get a bunch of suggestions, but mine is to use `setf'
and `nth'. You can do:

(let ((orig '(("a" . "b") ("c" "d")))
      (obj  '("c" "d"))
      (new '(":)")))
  (setf (nth (cl-position obj orig :test #'equal) orig) new)
  orig)

Hope that's useful.

Eric