I want to write a function in lisp that reverses all elements from the list using map functions but I don't have any idea how to start this.. I think I have to use the built in reverse function somehow.. For example if I have the list (1 2 3 (4 5 6 (7 8 9))) I would get (((9 8 7) 6 5 4) 3 2 1) or if I had the list(1 2 3 (4 5) (6 7)) I would get ((7 6) (5 4) 3 2 1) .. Any help is appreciated!
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From your comments below, it seems that you are describing a recursive problem, for which the simplest solution is a recursive function (that is, a function that calls itself). I have tagged the question with "recursion" for that reason.Ryan C. Thompson– Ryan C. Thompson2010-12-07 07:43:16 +00:00Commented Dec 7, 2010 at 7:43
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3 Answers
Just a quick answer, not sure about efficiency/elegancy:
(defun reverse-deeply (list)
(mapcar #'(lambda (li)
(cond
((consp li) (reverse-deeply li))
(t li)))
(reverse list)))
1 Comment
Ryan C. Thompson
The key is the recursion. This should do it.
(defun reverse-list (list)
(let ((result nil))
(dolist (e list result)
(push e result))))
Comments
Here is a version that works for me in Common-Lisp.
(defun reverse-list (list)
(if (atom list)
list ;; Not actually a list, return the atom
(reverse (mapcar #'reverse-list list)))
;; Testing it out
(reverse-list '((1 2 3) (4 5 (3 6))))
Output:
(((6 3) 5 4) (3 2 1))
Mapcar is a function that takes another function as its first parameter and a list as its second parameter. It then calls that function on each element of the list. It returns a list of all of the answers. So after I use 'mapcar' to reverse all of the sublists, I call 'reverse' again to reverse the bigger list.
The function that it calls on each sublist is 'reverse-list'. This checks if the list is an atom. If it is, then it returns itself. If it's a list, then it calls mapcar again on each element in the list, then reverses the result.
6 Comments
SnailBoy
This is not quite what I had in mind.. this function reverses the lists but the sublists order have to be reversed also..
khachik
If someone doesn't know how
mapcar works, it may be confusing, since there are two ways to achieve the result: a) reverse the top-level container list; b) reverse member-lists :)
Jonathan Sternberg
That better? Sorry, I must have misunderstood what you were asking for.
khachik
@SnailBoy
(reverse (mapcar #'reverse '((1 2 3) (4 5 6)))) isn't obvious?SnailBoy
This also reverses only 2 levels of the list .. I need it to reverse all elements of the list including sublists sublists of sublist .. sublists of sublists of sublist and so on ..
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