Inserting into a list ?
in [Lisp]
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From: nicolas.edel on 18 Apr 2008 07:17 CL has find, position, remove, subsitute, and others. Why no insert (at least for list) ?
From: Stanisław Halik on 18 Apr 2008 07:48 thus spoke nicolas.edel(a)gmail.com: > CL has find, position, remove, subsitute, and others. Why no insert > (at least for list) ? Sure it has. See PUSH and tail-consing. Note that PUSH doesn't modify the original list, if it's passed as an argument, the change won't propagate by itself into the calling function. -- Nawet świnka wejdzie na drzewo kiedy ją chwalą.
From: Pascal J. Bourguignon on 18 Apr 2008 08:02 nicolas.edel(a)gmail.com writes: > CL has find, position, remove, subsitute, and others. Why no insert > (at least for list) ? Basically, because there is no list in lisp. See the lisp paradoxes in: http://www.informatimago.com/usenet.html Otherwise, insert could (very) naively be implemented as: (defun insert (value index list) (push value (nthcdr index list))) so the reason why there is no INSERT is the same as why there is no (SETF NTHCDR). See: http://groups.google.com/group/comp.lang.lisp/browse_thread/thread/e97408eed2a8d7d9/576860b23df03370?lnk=st&q= Namely: (insert 'head 0 list) cannot be defined. (let ((list (list 'neck 'body 'leg))) (let ((a list) (b list)) (insert 'head 0 list) ;; what would you get from: (print a) (print b) (print list) ;; ? (values))) Ok, so you could define an insert similar to delete: (defun insert (value index sequence) (etypecase sequence (null (list value)) (list (if (zerop index) (cons value sequence) (progn (push value (cdr (nthcdr (1- index) sequence))) sequence))) (vector '\...))) C/USER[32]> (let ((list (list 1 2 3 4 5 6 7))) (setf list (insert pi 3 list)) (print list) (setf list (insert -1 0 list)) (print list) (values)) (1 2 3 3.1415926535897932385L0 4 5 6 7) (-1 1 2 3 3.1415926535897932385L0 4 5 6 7) But this is about as far as you can go, without a true list ADT. If you really need to insert elements inside a list, then you should define a list data type to let you do so easily, efficiently, and meaningfully. Definitely with identity, perhaps with double-links, etc. -- __Pascal Bourguignon__
From: Pascal J. Bourguignon on 18 Apr 2008 08:11 StanisÅaw Halik <sthalik+usenet(a)tehran.lain.pl> writes: > thus spoke nicolas.edel(a)gmail.com: > >> CL has find, position, remove, subsitute, and others. Why no insert >> (at least for list) ? > > Sure it has. See PUSH and tail-consing. Note that PUSH doesn't modify > the original list, Not always true. PUSH modifies the place you give it. If this places is a cell in the list, PUSH will modify that list. See my other answer. > if it's passed as an argument, the change won't > propagate by itself into the calling function. -- __Pascal Bourguignon__
From: nicolas.edel on 18 Apr 2008 16:12
> Basically, because there is no list in lisp. > See the lisp paradoxes in:http://www.informatimago.com/usenet.html There is no list *but* you may use 'list with the typep function ? Hum ... I'll have follow the white^^link and better understand your meaning ;) > so the reason why there is no INSERT is the same as why there is no > (SETF NTHCDR). See:http://groups.google.com/group/comp.lang.lisp/browse_thread/thread/e9... Ok, I'll investigate on this to understand the issues. > Ok, so you could define an insert similar to delete: > [...] Well, I know it's not that hard to define the functions I need by myself, but I was wondering why the standard lacks them. I'll follow the links and read them twice. Thanks. -Nicolas |