Re: English Idiom in Unix: Directory Recursively

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From	tar@sevak.isi.edu (Thomas A. Russ)
Newsgroups	comp.lang.python, comp.lang.lisp
Subject	Re: English Idiom in Unix: Directory Recursively
Date	18 May 2011 09:16:26 -0700
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Cross-posted to 2 groups.

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Hans Georg Schaathun <hg@schaathun.net> writes:

> ["Followup-To:" header set to comp.lang.python.]
> On 17 May 2011 23:42:20 -0700, Thomas A. Russ
>   <tar@sevak.isi.edu> wrote:
> :  Tree walks are the canonical example of what can't be done in an
> :  iterative fashion without the addition of an explicitly managed stack
> 
> Of course you can do it.  It isn't nice, but it is possible.
> I assume that you refer to depth first walks, as breadth first
> is more easily described by iteration on a queue in the first place.
> 
> Depth first can be achieved by looping over the nodes, with a
> state keeping references to the current and the previous node
> considered.

Well, unless you have a tree with backpointers, you have to keep the
entire parent chain of nodes visited.  Otherwise, you won't be able to
find the parent node when you need to backtrack.  A standard tree
representation has only directional links.

Consider:

A--+---B----+---D
   |        |
   |        +---E
   |        |
   |        +---F
   | 
   +---C

If all you keep is the current and previous node, then the only thing
you have reference do when doing the depth-first traverse is:
  1.  Current = A,  Previous = null
  2.  Current = B.  Previous = A
  3.  Current = D   Previous = B
  4.  Current = E   Previous = D
  5.  now what?  You can't get from E or D back to B.

> By comparing the previous node (pointer or ID) to the
> current node's parent and children one will know wherefrom the
> current node was entered, and can choose the next child in the
> list as the next node, or the parent if all children have been
> visited.  A visit action may be added in any or all times the
> node is visited.
> 
> This node requires no stack.  The only state space is constant,
> regardless of the size of the tree, requiring just the two pointers
> to previous and current.

This will only work if there is a backpointer to the parent.

So you have to add one extra pointer for each node back to its parent.
This extra pointer will be the size of the graph, rather than (on
average) log of the size of the graph stack frames.

-- 
Thomas A. Russ,  USC/Information Sciences Institute

Thread

English Idiom in Unix: Directory Recursively Xah Lee <xahlee@gmail.com> - 2011-05-17 15:26 -0700
  Re: English Idiom in Unix: Directory Recursively Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-17 17:20 -0600
    Re: English Idiom in Unix: Directory Recursively "Martin P. Hellwig" <martin.hellwig@gmail.com> - 2011-05-18 22:22 +0000
  Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-18 09:42 +1000
  Re: English Idiom in Unix: Directory Recursively "Steven W. Orr" <steveo@syslang.net> - 2011-05-17 21:16 -0400
  Re: English Idiom in Unix: Directory Recursively Roland Hutchinson <my.spamtrap@verizon.net> - 2011-05-18 04:51 +0000
    Re: English Idiom in Unix: Directory Recursively "Pascal J. Bourguignon" <pjb@informatimago.com> - 2011-05-18 07:19 +0200
      Re: English Idiom in Unix: Directory Recursively tar@sevak.isi.edu (Thomas A. Russ) - 2011-05-17 23:42 -0700
        Re: English Idiom in Unix: Directory Recursively Hans Georg Schaathun <hg@schaathun.net> - 2011-05-18 09:26 +0100
          Re: English Idiom in Unix: Directory Recursively tar@sevak.isi.edu (Thomas A. Russ) - 2011-05-18 09:16 -0700
            Re: English Idiom in Unix: Directory Recursively Hans Georg Schaathun <hg@schaathun.net> - 2011-05-18 19:11 +0100
              Re: English Idiom in Unix: Directory Recursively Raymond Wiker <raw@RAWMBP-2.local> - 2011-05-18 20:20 +0200
                Re: English Idiom in Unix: Directory Recursively Hans Georg Schaathun <hg@schaathun.net> - 2011-05-18 19:39 +0100
                Re: English Idiom in Unix: Directory Recursively Raymond Wiker <raw@RAWMBP-2.local> - 2011-05-18 21:09 +0200
                Re: English Idiom in Unix: Directory Recursively Hans Georg Schaathun <hg@schaathun.net> - 2011-05-18 21:02 +0100
                Re: English Idiom in Unix: Directory Recursively Raymond Wiker <raw@RAWMBP-2.local> - 2011-05-18 22:40 +0200
                Re: English Idiom in Unix: Directory Recursively Hans Georg Schaathun <hg@schaathun.net> - 2011-05-19 05:56 +0100
                Re: English Idiom in Unix: Directory Recursively tar@sevak.isi.edu (Thomas A. Russ) - 2011-05-19 16:14 -0700
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-19 04:41 +1000
            Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-19 04:12 +1000
            Re: English Idiom in Unix: Directory Recursively "Pascal J. Bourguignon" <pjb@informatimago.com> - 2011-05-18 20:57 +0200
              Re: English Idiom in Unix: Directory Recursively tar@sevak.isi.edu (Thomas A. Russ) - 2011-05-19 16:17 -0700
                Re: English Idiom in Unix: Directory Recursively "Pascal J. Bourguignon" <pjb@informatimago.com> - 2011-05-20 02:38 +0200
                Re: English Idiom in Unix: Directory Recursively Antti J Ylikoski <antti.ylikoski@tkk.fi> - 2011-05-20 12:00 +0300
        Re: English Idiom in Unix: Directory Recursively Peter Moylan <invalid@peter.pmoylan.org.invalid> - 2011-05-18 22:09 +1000
          Re: English Idiom in Unix: Directory Recursively rusi <rustompmody@gmail.com> - 2011-05-18 06:14 -0700
            Re: English Idiom in Unix: Directory Recursively Peter Moylan <invalid@peter.pmoylan.org.invalid> - 2011-05-19 11:06 +1000
              Re: English Idiom in Unix: Directory Recursively Jonathan de Boyne Pollard <J.deBoynePollard-newsgroups@NTLWorld.COM> - 2011-05-21 09:32 +0100
                Re: English Idiom in Unix: Directory Recursively Lars Enderin <lars.enderin@telia.com> - 2011-05-21 11:52 +0200
                Re: English Idiom in Unix: Directory Recursively Lars Enderin <lars.enderin@telia.com> - 2011-05-21 11:54 +0200
                Re: English Idiom in Unix: Directory Recursively Lars Enderin <lars.enderin@telia.com> - 2011-05-21 11:56 +0200
              Re: English Idiom in Unix: Directory Recursively Grant Edwards <invalid@invalid.invalid> - 2011-05-21 15:34 +0000
      Re: English Idiom in Unix: Directory Recursively Roland Hutchinson <my.spamtrap@verizon.net> - 2011-05-20 06:50 +0000
    Re: English Idiom in Unix: Directory Recursively rusi <rustompmody@gmail.com> - 2011-05-17 23:06 -0700
      Re: English Idiom in Unix: Directory Recursively Harrison Hill <harrishill@gmx.com> - 2011-05-17 23:50 -0700
        Re: English Idiom in Unix: Directory Recursively rusi <rustompmody@gmail.com> - 2011-05-18 00:16 -0700
        Re: English Idiom in Unix: Directory Recursively Peter Moylan <invalid@peter.pmoylan.org.invalid> - 2011-05-18 22:19 +1000
          Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 13:16 -0700
            Re: English Idiom in Unix: Directory Recursively Peter Moylan <invalid@peter.pmoylan.org.invalid> - 2011-05-30 22:58 +1000
        Re: English Idiom in Unix: Directory Recursively see@sig.for.address (Victor Eijkhout) - 2011-05-18 12:59 -0500
          Re: English Idiom in Unix: Directory Recursively Roland Hutchinson <my.spamtrap@verizon.net> - 2011-05-20 06:54 +0000
            Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-20 17:10 +1000
          Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 13:28 -0700
            Re: English Idiom in Unix: Directory Recursively Peter Moylan <invalid@peter.pmoylan.org.invalid> - 2011-05-30 23:02 +1000
          Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 13:28 -0700
        Re: English Idiom in Unix: Directory Recursively Glenn Knickerbocker <NotR@bestweb.net> - 2011-05-18 16:54 -0400
      Re: English Idiom in Unix: Directory Recursively Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-18 00:58 -0600
        Re: English Idiom in Unix: Directory Recursively rusi <rustompmody@gmail.com> - 2011-05-18 00:10 -0700
          Re: English Idiom in Unix: Directory Recursively Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-18 08:32 -0600
            Re: English Idiom in Unix: Directory Recursively rusi <rustompmody@gmail.com> - 2011-05-18 08:15 -0700
              Re: English Idiom in Unix: Directory Recursively Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-18 09:43 -0600
  Re: English Idiom in Unix: Directory Recursively Hans Georg Schaathun <hg@schaathun.net> - 2011-05-18 09:12 +0100
    Re: English Idiom in Unix: Directory Recursively Espen Vestre <espen@vestre.net> - 2011-05-18 10:20 +0200
    Re: English Idiom in Unix: Directory Recursively Rikishi42 <skunkworks@rikishi42.net> - 2011-05-19 23:21 +0200
      Re: English Idiom in Unix: Directory Recursively Hans Georg Schaathun <hg@schaathun.net> - 2011-05-20 05:28 +0100
        Re: English Idiom in Unix: Directory Recursively Rikishi42 <skunkworks@rikishi42.net> - 2011-05-23 20:48 +0200
      Re: English Idiom in Unix: Directory Recursively rusi <rustompmody@gmail.com> - 2011-05-19 22:13 -0700
        Re: English Idiom in Unix: Directory Recursively Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-20 06:55 +0000
          Re: English Idiom in Unix: Directory Recursively Hans Georg Schaathun <hg@schaathun.net> - 2011-05-20 09:48 +0100
            Re: English Idiom in Unix: Directory Recursively rusi <rustompmody@gmail.com> - 2011-05-20 10:21 -0700
          Re: English Idiom in Unix: Directory Recursively Rikishi42 <skunkworks@rikishi42.net> - 2011-05-23 20:56 +0200
            Re: English Idiom in Unix: Directory Recursively Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-24 00:18 +0000
              Re: English Idiom in Unix: Directory Recursively Rikishi42 <skunkworks@rikishi42.net> - 2011-05-25 00:06 +0200
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-25 10:40 +1000
                Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 14:18 -0700
                Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 14:19 -0700
                Re: English Idiom in Unix: Directory Recursively Xah Lee <xahlee@gmail.com> - 2011-05-24 23:05 -0700
                Re: English Idiom in Unix: Directory Recursively Thorsten Kampe <thorsten@thorstenkampe.de> - 2011-05-25 09:26 +0200
                Re: English Idiom in Unix: Directory Recursively Xah Lee <xahlee@gmail.com> - 2011-05-25 00:51 -0700
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-25 17:59 +1000
                Re: English Idiom in Unix: Directory Recursively Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-25 21:59 +0000
                Re: English Idiom in Unix: Directory Recursively Thorsten Kampe <thorsten@thorstenkampe.de> - 2011-05-26 10:48 +0200
                Re: English Idiom in Unix: Directory Recursively Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-26 10:06 +0000
                Re: English Idiom in Unix: Directory Recursively Thorsten Kampe <thorsten@thorstenkampe.de> - 2011-05-26 12:46 +0200
                Re: English Idiom in Unix: Directory Recursively harrismh777 <harrismh777@charter.net> - 2011-05-27 09:31 -0500
                Re: English Idiom in Unix: Directory Recursively Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-25 22:58 +0000
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-26 14:00 +1000
                Re: English Idiom in Unix: Directory Recursively Thorsten Kampe <thorsten@thorstenkampe.de> - 2011-05-26 10:59 +0200
                Re: English Idiom in Unix: Directory Recursively "Charles" <C.Sanders@DeleteThis.Bom.GOV.AU> - 2011-05-26 20:58 +1000
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-26 21:12 +1000
                Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 14:38 -0700
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-30 07:46 +1000
                Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 15:54 -0700
                Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 14:38 -0700
                Re: English Idiom in Unix: Directory Recursively Thorsten Kampe <thorsten@thorstenkampe.de> - 2011-05-26 13:20 +0200
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-26 21:28 +1000
                Re: English Idiom in Unix: Directory Recursively Xah Lee <xahlee@gmail.com> - 2011-05-26 14:51 -0700
                Re: English Idiom in Unix: Directory Recursively Rikishi42 <skunkworks@rikishi42.net> - 2011-05-28 21:36 +0200
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-29 05:58 +1000
                Re: English Idiom in Unix: Directory Recursively Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-29 00:49 +0000
                Re: English Idiom in Unix: Directory Recursively Rikishi42 <skunkworks@rikishi42.net> - 2011-05-30 23:04 +0200
                Re: English Idiom in Unix: Directory Recursively Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-30 23:17 +0000
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-29 07:28 +1000
                Re: English Idiom in Unix: Directory Recursively Rikishi42 <skunkworks@rikishi42.net> - 2011-05-30 23:10 +0200
                Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 13:58 -0700
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-30 07:27 +1000
          Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 13:39 -0700
  Re: English Idiom in Unix: Directory Recursively Mike Barnes <mikebarnes@bluebottle.com> - 2011-05-18 10:25 +0100
    Re: English Idiom in Unix: Directory Recursively Xah Lee <xahlee@gmail.com> - 2011-05-18 13:00 -0700
      Re: English Idiom in Unix: Directory Recursively Hans Georg Schaathun <hg@schaathun.net> - 2011-05-18 21:19 +0100
        Re: English Idiom in Unix: Directory Recursively Lanarcam <lanarcam1@yahoo.fr> - 2011-05-18 22:33 +0200
      Re: English Idiom in Unix: Directory Recursively Mike Barnes <mikebarnes@bluebottle.com> - 2011-05-18 22:00 +0100
      Re: English Idiom in Unix: Directory Recursively Jonathan de Boyne Pollard <J.deBoynePollard-newsgroups@NTLWorld.COM> - 2011-05-20 08:10 +0100
        Re: English Idiom in Unix: Directory Recursively Xah Lee <xahlee@gmail.com> - 2011-05-22 13:22 -0700
          Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-23 08:46 +1000
            Re: English Idiom in Unix: Directory Recursively Xah Lee <xahlee@gmail.com> - 2011-05-22 16:17 -0700
              Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-23 09:32 +1000
                Re: English Idiom in Unix: Directory Recursively Xah Lee <xahlee@gmail.com> - 2011-05-23 21:20 -0700
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-24 14:28 +1000
                Re: English Idiom in Unix: Directory Recursively Xah Lee <xahlee@gmail.com> - 2011-05-24 10:40 -0700
                Re: English Idiom in Unix: Directory Recursively Chris Angelico <rosuav@gmail.com> - 2011-05-25 08:14 +1000
                Re: English Idiom in Unix: Directory Recursively Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-25 10:15 +0000
      Re: English Idiom in Unix: Directory Recursively rantingrick <rantingrick@gmail.com> - 2011-05-29 13:32 -0700
    Re: English Idiom in Unix: Directory Recursively Jonathan de Boyne Pollard <J.deBoynePollard-newsgroups@NTLWorld.COM> - 2011-05-20 08:07 +0100
  Re: English Idiom in Unix: Directory Recursively John Nagle <nagle@animats.com> - 2011-05-18 13:07 -0700
  Re: English Idiom in Unix: Directory Recursively Jonathan de Boyne Pollard <J.deBoynePollard-newsgroups@NTLWorld.COM> - 2011-05-20 08:00 +0100
    Re: English Idiom in Unix: Directory Recursively David Schwartz <davids@webmaster.com> - 2011-05-27 22:56 -0700

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