Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ukma!husc6!rice!titan!dorai
From: dorai@titan.rice.edu (Dorai Sitaram)
Newsgroups: comp.lang.lisp
Subject: Re: lisp problem
Keywords: help
Message-ID: <3079@kalliope.rice.edu>
Date: 11 Apr 89 05:15:27 GMT
References: <10053@ihlpl.ATT.COM>
Sender: usenet@rice.edu
Reply-To: dorai@titan.rice.edu (Dorai Sitaram)
Organization: Rice University, Houston
Lines: 34

In article <10053@ihlpl.ATT.COM> canoura@ihlpl.ATT.COM (jlcanoura) writes:
$PLEASE, does anybody know how to solve this problem in LISP.
$
$By using recursion and the Append function, Given a list of
$nondistinct elements and a positive number K where K is <=
$the size of the list, I need to write a function which returns
$a list, the element of whose are the combinations of the 
$element of the given list taken K at a time. 
$e.g.(COMB '(n1 n2 ....n) K)
$
$Examples. ( COMB is the name of the function)
$
$(COMB '(1 2 3 ) 2) should return  
$  ((1 2) (1 3) (2 3))
$
$(COMB '(1 2 3) 3) should return
$  ((1 2 3))
$
$(COMB '(1 2 3) 1) should return
$  ((1) (2) (3))

(defun comb (s k)
  (cond ((zerop k) (list nil))
	((null s) nil)
	(t (let ((a (car s)) (s1 (cdr s)))
	     (append (mapcar #'(lambda (c) (cons a c)) (comb s1 (1- k)))
		     (comb s1 k))))))

Regards,

--dorai
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We must believe in free will. We have no choice.       --Isaac Bashevis Singer
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