Topic: [challenge] A26 : Add the N'th element into a list at position

[JohnK]

Add the N'th element into a list at a position.

(add-nth <list> <item> <position>)

Example:
(add-nth '(a c d) 'B 1)
> (A B C D)
(add-nth '(a c d) 'B 10)
> (A C D B)


EDIT: Changed from 1 based indexing to 0 based indexing and added condition for larger position.

[Marc'Antonio Alessi]

I have this old one (similar):

Code: [Select]

; Function: ALE_AppendBeforeNth
;
; Version 1.00 - 15/11/2005
;
; Description:
;   returns a copy of the list with a new item added before
;   the item in the nth position (NthPos)
;
; Arguments:
;   NthPos = Integer - nth like
;   NewItm = An atom or list
;   In_Lst = A list
;   InRLst = Original list reversed
;
; Return Values:
;   A list with a new item added before the item in the nth position
;
; Examples:
;   (setq alist '((0 . A) (1 . B) (2 . C) (3 . D)))
;
;   (ALE_AppendBeforeNth 0 "NEW" alist (reverse alist))
;   Returns: ("NEW" (0 . A) (1 . B) (2 . C) (3 . D))
;
;   (ALE_AppendBeforeNth 1 "NEW" alist (reverse alist))
;   Returns: ((0 . A) "NEW" (1 . B) (2 . C) (3 . D))
;
;   (ALE_AppendBeforeNth 3 "NEW" alist (reverse alist))
;   Returns: ((0 . A) (1 . B) (2 . C) (3 . D) "NEW")
;
;   (ALE_AppendBeforeNth 4 "NEW" alist (reverse alist))
;   Returns: ((0 . A) (1 . B) (2 . C) (3 . D))
;
;   (ALE_AppendBeforeNth 1 '(9 . Z) alist (reverse alist))
;   Returns: ((0 . A) (9 . Z) (1 . B) (2 . C) (3 . D))
;
;   (ALE_AppendBeforeNth 1 nil alist (reverse alist))
;   Returns: ((0 . A) nil (1 . B) (2 . C) (3 . D))
;
(defun ALE_AppendBeforeNth (NthPos NewItm In_Lst InRLst / LstLng NthItm)
  (cond
    ( (null In_Lst) nil )
    ( (zerop NthPos) (cons NewItm In_Lst) )
    ( (null (setq NthItm (nth NthPos In_Lst))) In_Lst )
    ( (zerop (setq LstLng (- (length In_Lst) (1+ NthPos))))
      (append In_Lst (list NewItm))
    )
    ( T
      (while
        (/=
          NthPos
          (length (setq InRLst (cdr (member NthItm InRLst))))
        )
      )
      (while
        (/=
          LstLng
          (length (setq In_Lst (cdr (member NthItm In_Lst))))
        )
      )
      (append (reverse InRLst) (list NewItm) (cons NthItm In_Lst))
    )
  )
)

[ribarm]

Recursive :

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth ( l e p )
2.   (if (and l (/= p 1))
3.     (cons (car l) (add-nth (cdr l) e (1- p)))
4.     (if l
5.       (cons e (cons (car l) (add-nth (cdr l) e (1- p))))
6.     )
7.   )
8. )
9.  

Iterative - straight forward :

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth ( l e p / r )
2.   (repeat (1- p)
3.     (setq r (cons (car l) r) l (cdr l))
4.   )
5.   (setq r (cons e r))
6.   (repeat (length l)
7.     (setq r (cons (car l) r) l (cdr l))
8.   )
9.   (reverse r)
10. )
11.  

Iteration with applying anonymous (lambda) function by using (mapcar) function on main list and applying concatenation (append) function to resulting list with lists as elements :

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth ( l e p )
2.   (apply (function append) (mapcar (function (lambda ( x ) (if (zerop (setq p (1- p))) (list e x) (list x)))) l))
3. )
4.  

: (add-nth '(a c d) 'B 2)
(A B C D)

[bruno_vdh]

(add-nth '(a c d) 'B 2)
> (A B C D)

Hi, what should the following data return, knowing that nth starts at zero

Code: [Select]

(add-nth '(a c d) 'b 1)
(add-nth '(a c d) 'b 0)
(add-nth '(a c d) 'b 10)
Waiting for a first quick proposal

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth (l x n)
2.   (if (> n 1)
3.     (cons (car l) (add-nth (cdr l) x (1- n)))
4.     (cons x l)
5.   )
6. )

[well20152016]

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth (l b n / lst)
2.   (setq i 0)
3.   (foreach a l
4.     (if (equal (setq i (+ 1 i)) n ) (setq lst (cons b lst) lst (cons a lst)) (setq lst (cons a lst)))
5.     )
6. (reverse lst))

[Lee Mac]

Code - Auto/Visual Lisp: [Select]

1. (defun addnth ( x n l )
2.     (cond
3.         (   (not l) (list x))
4.         (   (< 1 n) (cons (car l) (addnth x (1- n) (cdr l))))
5.         (   (cons x l))
6.     )
7. )

Code - Auto/Visual Lisp: [Select]

1. _$ (addnth 'b 2 '(a c d))
2. (A B C D)
3. _$ (addnth 'b 10 '(a c d))
4. (A C D B)

Odd that you would opt for the index to be one-based...

[dexus]

My attempt was very similar to ribarm's iterative version.

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth-dexus (l i n / r)
2.   (repeat (min (1- n) (length l))
3.     (setq r (cons (car l) r)
4.           l (cdr l))
5.   )
6.   (setq r (cons i r))
7.   (while l
8.     (setq r (cons (car l) r)
9.           l (cdr l))
10.   )
11.   (reverse r)
12. )

[JohnK]

I'm sorry I didn't notice that I made this 1 based indexing; I am beyond the challenges I had an opportunity to workout before hand so I probably typed the declaration up without thinking twice about the index.

We can make this one 0 based index and I think "(add-nth '(a c d) 'b 10)" should attach to the end so it should return (a c d b).

[bruno_vdh]

I'm sorry I didn't notice that I made this 1 based indexing; I am beyond the challenges I had an opportunity to workout before hand so I probably typed the declaration up without thinking twice about the index.

We can make this one 0 based index and I think "(add-nth '(a c d) 'b 10)" should attach to the end so it should return (a c d b).

No worries

For brevity:

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth (l x n)
2.   (if (and l (> n 0))
3.     (cons (car l) (add-nth (cdr l) x (1- n )))
4.     (cons x l)))

For fantasy:

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth (l x n)
2.   (if (/= 0 n nil l)
3.     (cons (car l) (add-nth (cdr l) x (1- n )))
4.     (cons x l)))

For possible optimization:

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth (l x n / f)
2.   (defun f (l n)
3.     (if (> n 0)
4.       (cons (car l) (f (cdr l) (1- n)))
5.       (cons x l)))
6.   (f l (min n (length l)))
7. )
8.  

The results:

Code: [Select]

_$ (add-nth nil 'b 5)
(B)
_$ (add-nth '(a c d) 'b 0)
(B A C D)
_$ (add-nth '(a c d) 'b 1)
(A B C D)
_$ (add-nth '(a c d) 'b 10)
(A C D B)

[JohnK]

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth-se7en (aList item pos / add-nth)
2.   (defun add-nth ( aList item index cntr )
3.     (if aList
4.       (if (= cntr index)
5.         (cons item (add-nth aList item index (1+ cntr)))
6.         (cons (car aList) (add-nth (cdr aList) item index (1+ cntr))))) )
7.   (if (>= pos (length aList))
8.     (reverse (cons item (reverse aList)))
9.     (add-nth aList item pos 0)
10.     )
11.   )

bruno_vdh, I really need to spend some time and study your code; you have some very nice routines!

[Lee Mac]

Another, disqualified -

Code - Auto/Visual Lisp: [Select]

1. (defun addnth ( x n l / b m )
2.     (setq m (vl-member-if-not '(lambda ( a ) (if (< 0 (setq n (1- n))) (setq b (cons a b)))) l))
3.     (append (reverse b) (cons x m))
4. )

[bruno_vdh]

Another, disqualified -

Code - Auto/Visual Lisp: [Select]

1. (defun addnth ( x n l / b m )
2.     (setq m (vl-member-if-not '(lambda ( a ) (if (< 0 (setq n (1- n))) (setq b (cons a b)))) l))
3.     (append (reverse b) (cons x m))
4. )

Hi, maybe…

Code: [Select]

(if (> 1 (setq n (1- n)))

We can make this one 0 based index

[bruno_vdh]

bruno_vdh, I really need to spend some time and study your code; you have some very nice routines!

Thank you, always for the game.

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth (l x n / m)  
2.   (repeat (min n (length l)) (setq m (cons (car l) m) l (cdr l)))
3.   (append (reverse m) (cons x l))
4. )

Code - Auto/Visual Lisp: [Select]

1. (defun add-nth (l x n)
2.   (mapcar '(lambda (a b)
3.              (cond ((= -1 (setq n (1- n))) x)
4.                    ((minusp n) a)
5.                    (b)
6.              )
7.            )
8.           (cons x l)
9.           (reverse (cons x (reverse l)))
10.   )
11. )

[Lee Mac]

Another, disqualified -

Code - Auto/Visual Lisp: [Select]

1. (defun addnth ( x n l / b m )
2.     (setq m (vl-member-if-not '(lambda ( a ) (if (< 0 (setq n (1- n))) (setq b (cons a b)))) l))
3.     (append (reverse b) (cons x m))
4. )

Hi, maybe…

Code: [Select]

(if (> 1 (setq n (1- n)))

We can make this one 0 based index

Oh I didn't realise it had changed...

[JohnK]

Yes, it was a mistake on my part. Sorry.