Subject: Re: Lists question
From: (Rob Warnock)
Date: 1999/12/13
Newsgroups: comp.lang.scheme
Message-ID: <831ntk$>
Barry Margolin  <> wrote:
| Topharrier <> wrote:
| >(define rm '("room 1" 
| >    	  (("ball") ("string"))
| >	  (("east" tr) ("south" tr))))
| >
| >I need to be able to delete the objects from the second of the list
| >like ("ball")
| >
| >anyone knows how can I acomplish this?
| (define (delete item list predicate) ...)
| (define (delete-from-room object room)
|   (set-car! (cdr room)
|             (delete object (cdr room)
|                     (lambda (object entry) (string= object (car entry))))))
| (delete-from-room "ball" rm)

Well, in Scheme it's "string=?", not "string=", and even then:

	> (delete-from-room "ball" rm)
	error: "string=?": expects type <string> as 2nd arg; given ("ball")

Actually, I don't understand why the original data structure has the
objects in the "room" in individual lists, unless maybe it's to allow
multiple attributes on the objects or something, e.g.

	> (define rm '("room 1" 
		       (("ball" "red" "small") ("string" "long" "weak"))
		       (("east" tr) ("south" tr))))

In which case, you'd need a much more general (read: complex) matching
function for "delete-from-room", so it could accept requests like this:

	(delete-from-room '("ball" (or "red" "green")) rm)

Anyway, "Topharrier", Barry's code (even if it doesn't quite work)
is a *much* bigger "hint" than my verbiage...  Good luck.


Rob Warnock, 8L-846
Applied Networking
Silicon Graphics, Inc.		Phone: 650-933-1673
1600 Amphitheatre Pkwy.		FAX: 650-933-0511
Mountain View, CA  94043	PP-ASEL-IA