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From: sondeen@isi.edu (Jeff Sondeen)
Newsgroups: comp.lang.perl
Subject: Problem permutating perl program... help please!
Message-ID: <16402@venera.isi.edu>
Date: 16 Jan 91 22:25:24 GMT
Reply-To: sondeen@venera.isi.edu (Jeff Sondeen)
Distribution: comp
Organization: Information Sciences Institute, Univ. of So. California
Lines: 48

Could someone explain why the following permutating perl program doesn't
work?  The idea is to generate all permutations of a list 1..$siz (for
example, input of 3 should yield (1,2,3) (1,3,2) (2,3,1) (2,1,3) (3,1,2) and
(3,2,1), but it doesn't.  It just does (1,2,3), as follows

perl permute.p 3                       
len:3   i:1     ins: 1 2 3      outs:  
len:2   i:1     ins: 2 3        outs: 1
len:1   i:1     ins: 3  outs: 1 2      
1 2 3                                  
len:1   i:1     ins: 3  outs: 1 2      
len:2   i:2     ins: 3 2        outs: 1
len:3   i:1     ins: 2 3 1      outs:  

It might be that somehow the local variable $i is getting clobbered
(acting like a global???)

perl program follows: ------------------------------------------------------

$siz = @ARGV[0];

@terms = (1..$siz);
do permute($siz,@terms,());

exit;

sub permute {
	local($len,@outs) = @_;
	local(@ins) = splice(@outs,0,$len);
	local($i,$out);

	if ($len > 0) {
		for $i (1..$len) {
print "len:$len	i:$i	ins: @ins	outs: @outs\n";
			$out = shift(@ins);
			do permute($len-1,@ins,@outs,$out);
			push(@ins,$out);
print "len:$len	i:$i	ins: @ins	outs: @outs\n";
		}
	} else {
		print "@outs\n";
	}
}
-- 

/jeff	sondeen@isi.edu				"engineers were discouraged
		from bringing problems to the attention of their supervisors"
	-- John Magnus, final report, Hubble Space Telescope investigation