Emacs -- How to replace nth element of a list with a let-bound variable

Question

I have not found any examples of how to replace the nth element of a list without first adding every element (one-by-one) with the function add-to-ordered-list -- e.g., (add-to-ordered-list 'the-list 'a 1). That requires subsequently deleting the element -- e.g., (delq a the-list). The third step in the process is to add the new element -- e.g., (add-to-ordered-list 'the-list "HELLO-WORLD!" 1). As the following function named variable-list-example uses 26 elements, I'm looking for a better way than first adding all 26 elements (one-by-one) to essentially assign each element a position number. The function add-to-ordered-list uses a number assignment that is one digit different than the standard nth element approach where the first element has a value of 0, whereas add-to-ordered-list uses the value of 1 for the first element. Here is the link to the documentation for add-to-ordered-list: http://www.gnu.org/software/emacs/manual/html_node/elisp/List-Variables.html

In addition, using the combination '( at the beginning of a list seems to prevent using a let-bound variable inside the list -- e.g., the let-bound variable appears in the list as my-variable instead of HELLO-WORLD!. The only work-around I have found is the above-mentioned example using add-to-ordered-list -- which is a lengthy three-step process (i.e., adding each element one-by-one at the outset).

To resolve both issues, I am looking for some assistance please to replace the 17th element of the-list (which is the letter r) with my-variable (which is HELLO-WORLD!).

---

(defun variable-list-example ()
(interactive)
  (let* (
      (my-variable "HELLO-WORLD!")
      (the-list '(a b c d e f g h i j k l m n o p q r s t u v w x y z)))
    ;; replace the 17th element of `the-list` with `my-variable`
    (message "Presto -- this is your new list:  %s" the-list)
  ))

---

THE LIST

'(a b c d e f g h i j k l m n o p q r s t u v w x y z)

---

nth ELEMENT -- CHEAT SHEET / LEGEND

element 0:   a
element 1:   b
element 2:   c
element 3:   d
element 4:   e
element 5:   f
element 6:   g
element 7:   h
element 8:   i
element 9:   j
element 10:  k
element 11:  l
element 12:  m
element 13:  n
element 14:  o
element 15:  p
element 16:  q
element 17:  r
element 18:  s
element 19:  t
element 20:  u
element 21:  v
element 22:  w
element 23:  x
element 24:  y
element 25:  z

Answer 1

You need to use setcar and nthcdr:

(setcar (nthcdr 17 the-list) my-variable)

The more common-lisp-like approach is to use Generalized Variables:

(setf (nth 17 the-list) my-variable)