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From: montnaro@sprite.crd.ge.com (Skip Montanaro)
Newsgroups: gnu.emacs.lisp.manual
Subject: Correction to ELisp address, union/intersect examples
Message-ID: <MONTNARO.89Apr28125102@sprite.crd.ge.com>
Date: 28 Apr 89 16:51:02 GMT
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Reply-To: <montanaro@sprite.crd.ge.com> (Skip Montanaro)
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Organization: GE Corporate Research & Development, Schenectady, NY
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In article <BOB.89Apr24150043@allosaur.cis.ohio-state.edu> bob@allosaur.cis.ohio-state.edu (Bob Sutterfield) writes:

   OK, I think we're up to date again now... I just put the new Emacs
   Lisp manual in place on osu-cis and updated GNU.how-to-get.

In what I think is the latest INFO-ized version of the ELisp manual, the
comment/corretion address is incorrect. Maybe we're "not quite up-to-date".
:-)

I quote from file elisp-1:

    Mail comments and corrections to: gnu.emacs.lisp.manual@prep.ai.mit.edu

     -Bil Lewis       31-Oct-87
     -Dan LaLiberte 01-Apr-89

According to the aliases file on rice-chex, it should be

    gnu-manual@prep.ai.mit.edu

Perhaps the address was correct in the texinfo source, but got mangled
somehow in the texinfo->info process? 



Changing the subject entirely, here are sample union/intersect functions.
They were solicited in the Lisp manual section on using lists as sets. The
functions below are not too robust, since they don't check their args for
list-hood or nil-ness.  They're just part of a throwaway prototype, but
demonstrate the idea.

(defun union (a b)
  "merge lists A and B together, eliminating duplicates (boolean set or)."
  (let ((l nil)
	(both (append a b)))
    (while both
      (setq l (append l (list (car both))))
      (setq both (delete (car both) both)))
    l))

(defun intersect (a b)
  "merge lists A and B together, retaining only elements they have in common
(boolean set and)."
  (let ((l nil))
    (while a
      (if (contains (car a) b)
	  (progn
	    (setq b (delete (car a) b))
	    (setq l (append l (list (car a))))))
      (setq a (cdr a)))
    l))

They both use delete (could use delq if eql comparison is good enough for you):

(defun delete (e l)
  "Delete E from list L. Uses 'equal for comparison."
  (let ((n nil))
    (while l
      (if (not (equal e (car l)))
	  (setq n (append n (list (car l)))))
      (setq l (cdr l)))
    n))

--
Skip Montanaro (montanaro@sprite.crd.ge.com)