Add or remove element in the middle of list (Scheme)

Question

Is it possible to add or remove elements in the middle of a linked list in Scheme? I can't seem to think of a way doing this with car/cdr/cons, but I reccon there must be a way to this.

If I have a list '(1 3 5 6), and I need to put in 4 between 5 and 6 for example, is this doable?

score 0 · Accepted Answer · edited May 23 '17 at 11:43

0

Adding an element at a given position was done in a previous answer. Removing an element at a given position is similar:

(define (delete-at k lst)
  (cond ((null? lst)
         '())
        ((zero? k)
         (cdr lst))
        (else
         (cons (car lst)
               (delete-at (sub1 k) (cdr lst))))))

edited May 23 '17 at 11:43

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answered Mar 25 '15 at 20:00

Óscar López

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Will this give a new list, or alter the existing one? – user16655 Mar 26 '15 at 13:18
@user16655 this (as is the case with most list procedures in Scheme) will return a new list – Óscar López Mar 26 '15 at 14:49

score 0 · Answer 2 · answered Mar 25 '15 at 20:05

Here are two versions:

; list-insert : value index list -> list
;   return list where the ith element is x
(define (list-insert x i xs)
  (if (= i 0)
      (cons x xs)
      (cons (first xs) (list-insert x (- i 1) (rest xs)))))

(define (list-insert/version2 x i xs)
  (define-values (before after) (split-at xs i))
  (append before (list x) after))

(list-insert/version2 'x 2 '(a b c d))
(list-insert          'x 2 '(a b c d))

Both versions will allocate a new list. For large lists it will become inefficient.

Add or remove element in the middle of list (Scheme)

2 Answers2