Replace an item in a list in Common Lisp?

Question

I have a list of things (I\'ll call it L), an index(N) and a new thing(NEW). If I want to replace the thing in L at N with NEW, what is the best way to do this? Should I get the

Answer 1

quickly you can do it with JS on list-replace

Answer 2

(setf (nth N L) NEW)

should do the trick.

Answer 3

The obvious solution is slow and uses memory, as noted by others. If possible, you should try to defer replacing the element(s) until you need to perform another element-wise operation on the list, e.g. (loop for x in list do ...).

That way, you'll amortize away the consing (memory) and the iteration (cpu).

Answer 4

I just try to fix hazzen's code:

(define (replace-nth list n elem)
  (cond 
    ((null? list) ())
    ((eq? n 0) (cons elem list))
    (#t (cons(car list) (replace-nth (cdr list) (- n 1) elem)))))

> (replace-nth (list 3 2 9 2) 2 8)
(3 2 8 9 2)

This code inserted new element in the list. If we want to replace an element:

(define (replace-nth list n elem)
  (cond 
    ((null? list) ())
    ((eq? n 0) (cons elem (cdr list)))
    (#t (cons(car list) (replace-nth (cdr list) (- n 1) elem)))))

> (replace-nth (list 3 2 9 2) 2 8)
(3 2 8 2)

0 <= n <= length(list) - 1