Page 1 of 1
is there a way to replace the elements in a list?
Posted: Tue Jul 12, 2011 10:55 pm
by 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.
Re: is there a way to replace the elements in a list?
Posted: Tue Jul 12, 2011 11:10 pm
by Duke
yesimthetaxman wrote: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.
Re: is there a way to replace the elements in a list?
Posted: Wed Jul 13, 2011 12:25 am
by marcoxa
yesimthetaxman wrote: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
Re: is there a way to replace the elements in a list?
Posted: Wed Jul 13, 2011 10:34 am
by 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:
Code: Select all
(defun my-replace (elt list)
(mapl #'(lambda (sublist)
(setf (car sublist) elt))
list))
Note: I didn't test the code.
Re: is there a way to replace the elements in a list?
Posted: Wed Jul 13, 2011 10:55 am
by I X Code X 1
gugamilare wrote:@marcoxa and Duke: I think you guys misunderstood him, he wants to destructively replace the elements.
yesimthetaxman wrote:I don't actually mean destructively replace.
I don't think he wants it to be destructively replaced.
Re: is there a way to replace the elements in a list?
Posted: Wed Jul 13, 2011 11:59 am
by gugamilare
Re: is there a way to replace the elements in a list?
Posted: Thu Jul 14, 2011 5:17 am
by marcoxa
yesimthetaxman wrote: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
Code: Select all
(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
Re: is there a way to replace the elements in a list?
Posted: Thu Jul 14, 2011 10:53 pm
by yesimthetaxman
thank youuuuuuuuu