Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!convex!usenet
From: tchrist@convex.COM (Tom Christiansen)
Newsgroups: comp.lang.perl
Subject: Re: Problem permutating perl program... help please!
Message-ID: <1991Jan16.233408.3059@convex.com>
Date: 16 Jan 91 23:34:08 GMT
References: <16402@venera.isi.edu>
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Reply-To: tchrist@convex.COM (Tom Christiansen)
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Organization: CONVEX Software Development, Richardson, TX
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From the keyboard of sondeen@venera.isi.edu (Jeff Sondeen):
: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:  

On my system it does the whole thing, so I wonder whether you
are running an older version of perl that has the iterator-
during-recursion bug.  I'm running PL44, and it works fine.

A couple things I noticed that won't make any difference, but
did still look odd anyway.

:$siz = @ARGV[0];

You actually mean $ARGV[0], since you want a scalar, not an 
array here.

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

Not sure what the null list there is for.  Perl isn't like
C which needs a null at the end of a list for variatic functions.
In fact, as far as I can tell, empty lists are squished away 
entirely.  This is demo'd by this code chunk, which things
they're all just 4 anyway.

    do count(1..4);
    do count(1..4, ());
    do count((), 1..4, ());
    do count(@x, 1..4, ());

    sub count {
	print "found ",1+$#_, " items\n";
    }

--tom
--
"Hey, did you hear Stallman has replaced /vmunix with /vmunix.el?  Now
 he can finally have the whole O/S built-in to his editor like he
 always wanted!" --me (Tom Christiansen <tchrist@convex.com>)