is there a way to replace the elements in a list?

[yesimthetaxman]

I don't actually mean destructively replace, I mean lets say you pass the list (a b c d e), I want to return the list (8 8 8 8 8)... so basically I want a list returned with the same number of elements but instead of the original items, it'd be an item of my choosing. I am able to do this using the replace function, but I am wondering if there is a way to implement this using something like mapcar or reduce.

[Duke]

I don't actually mean destructively replace, I mean lets say you pass the list (a b c d e), I want to return the list (8 8 8 8 8)... so basically I want a list returned with the same number of elements but instead of the original items, it'd be an item of my choosing. I am able to do this using the replace function, but I am wondering if there is a way to implement this using something like mapcar or reduce.

Are you doing the L-99? (You probably should; you'd be able to answer this question yourself by #15 or so.)

One way to do it is with a recursive function. The form to do the rather silly thing you ask looks like this:

Code: Select all

CL-PIXIE> (defun dumb-replace (input-list substitution)
            (labels ((rec (lst acc)
                       (if (null lst)
                           acc
                           (rec (cdr lst) (cons substitution acc)))))
              (rec input-list nil)))
DUMB-REPLACE
CL-PIXIE> (dumb-replace (list 'a 'b 'c 'd) 'bork)
(BORK BORK BORK BORK)


...But it can change to suit a variety of problems, as you'll see if you're doing the L-99.

[marcoxa]

I don't actually mean destructively replace, I mean lets say you pass the list (a b c d e), I want to return the list (8 8 8 8 8)... so basically I want a list returned with the same number of elements but instead of the original items, it'd be an item of my choosing. I am able to do this using the replace function, but I am wondering if there is a way to implement this using something like mapcar or reduce.

Code: Select all

(defun transfomgrify (list &optional (e 42))
   (map 'list (constantly e) list))

Cheers

[gugamilare]

@marcoxa and Duke: I think you guys misunderstood him, he wants to destructively replace the elements.

@yesimthetaxman: What you want is the function mapl:

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(defun my-replace (elt list)
  (mapl #'(lambda (sublist)
            (setf (car sublist) elt))
    list))

Note: I didn't test the code.

[I X Code X 1]

@marcoxa and Duke: I think you guys misunderstood him, he wants to destructively replace the elements.

I don't actually mean destructively replace.

I don't think he wants it to be destructively replaced.

[gugamilare]

[marcoxa]

I don't actually mean destructively replace, I mean lets say you pass the list (a b c d e), I want to return the list (8 8 8 8 8)... so basically I want a list returned with the same number of elements but instead of the original items, it'd be an item of my choosing. I am able to do this using the replace function, but I am wondering if there is a way to implement this using something like mapcar or reduce.

I gave you the answer using MAP. The version with MAPCAR is

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(defun transmogrify (l &optional (x 42))
    (mapcar (constantly x) l))


A destructive version is

Code: Select all

(defun transmogrify/d (l &optional (x 42))
   (map-into l (constantly x) l))


Code: Select all

CL-USER 2 > (defvar ll (list 1 2 3 4))
LL

CL-USER 3 > (transmogrify/d ll)
(42 42 42 42)

CL-USER 4 > ll
(42 42 42 42)


Using the MAP version will hint you at generalizing the function so that it works with strings and vectors.

Cheers

[yesimthetaxman]

thank youuuuuuuuu