Re: Replace element in list

[Andreas Röhler]

Am 02.09.19 um 15:43 schrieb tomas@tuxteam.de:
> On Mon, Sep 02, 2019 at 09:29:39AM -0400, Stefan Monnier wrote:
> > > is there a recommended way to replace element x at index i of somelist
> > > y by newelement?
> > What Thomas is hinting at with his "copy-list" and "functional sector"
> > is that if you need to do that, there's a problem upstream.
> > E.g. could you use a struct instead of a list?
> Thanks for putting my mumbling into civilised words :-)
>
> Cheers
> -- t


Hmm, what means struct here?

Tried a little program to answer a quiz in

https://www.futurelearn.com/courses/how-computers-work/4/steps/551852/questions/3:

;; If the head is in state A at the position shown below, what would the 
final output of this Turing machine be?

;; |0|0|0|0|1|1|0|0|0|
;;                ^

Started that way:

[leaving out here z_B - z_E]

(defun z_A (zeichen pos)

  ;; Char: 1 (49, #o61, #x31)
  (when (eq 49 (aref zeichen pos))
    (aset zeichen pos 48)
    (z_B zeichen (1- pos))))

(defun zeichenmachine ()
  (interactive)
  (let ((zeichen "000011000")
    (pos 5))
    (message "%s" zeichen)
    (z_A zeichen pos)))

But that was easier to read:

[leaving out here state_B - state_E]

(defun state_A (elist pos)
  (if (eq 1 (nth pos elist))
      (progn
    (setf (nth pos elist) 0)
    (state_B elist (1- pos)))
    (message "Fertig %s" elist)))

(defun statemachine ()
  (interactive)
  (let ((elist (list 0 0 0 0 1 1 0 0 0))
    (pos 5))
    (state_A elist pos)))